# NuttX Operating System User's Manual by Gregory Nutt Last Updated: March 13, 2009

# 1.0 Introduction

This manual provides general usage information for the NuttX RTOS from the perspective of the firmware developer.

## 1.1 Document Overview

This user's manual is divided into three sections plus a index:

- Section 1.0, Introduction: This section provides an overview of the NuttX user's manual.
- Section 2.0, OS Interfaces: This section details the program interfaces provided by NuttX. This section is divided into several paragraphs that describe different groups of OS interfaces:
  - Paragraph 2.1 Task Control Interfaces
  - Paragraph 2.2 Task Scheduling Interfaces
  - Paragraph 2.3 Task Switching Interfaces
  - Paragraph 2.4 Named Message Queue Interfaces
  - Paragraph 2.5 Counting Semaphore Interfaces
  - Paragraph 2.6 Watchdog Timer Interfaces
  - Paragraph 2.7 Clocks and Timers
  - Paragraph 2.8 Signal Interfaces
  - Paragraph 2.9 Pthread Interfaces
  - Paragraph 2.10 Environment Variables
  - Paragraph 2.11 File System Interfaces
  - Paragraph 2.12 Network Interfaces
- Section 3.0, OS Data Structures: This section documents the data structures that are used at the NuttX interface.
  - Paragraph 3.1 Scalar Types
  - Paragraph 3.2 Hidden Interface Structures
  - Paragraph 3.3 Access to the `errno` Variable
  - Paragraph 3.4 User Interface Structures
- Index

## 1.2 Intended Audience and Scope

The intended audience for this document are firmware developers who are implementing applications on NuttX. Specifically, this documented is limited to addressing only NuttX RTOS APIs that are available to the application developer. As such, this document does not focus on any technical details of the organization or implementation of NuttX. Those technical details are provided in the NuttX Porting Guide.

Information about configuring and building NuttX is also needed by the application developer. That information can also be found in the NuttX Porting Guide.

# 2.0 OS Interfaces

This section describes each C-callable interface to the NuttX Operating System. The description of each interface is presented in the following format:

Function Prototype: The C prototype of the interface function is provided.

Description: The operation performed by the interface function is discussed.

Input Parameters: All input parameters are listed along with brief descriptions of each input parameter.

Returned Values: All possible values returned by the interface function are listed. Values returned as side-effects (through pointer input parameters or through global variables) will be addressed in the description of the interface function.

Assumptions/Limitations: Any unusual assumptions made by the interface function or any non-obvious limitations to the use of the interface function will be indicated here.

POSIX Compatibility: Any significant differences between the NuttX interface and its corresponding POSIX interface will be noted here.

NOTE: In order to achieve an independent name space for the NuttX interface functions, differences in function names and types are to be expected and will not be identified as differences in these paragraphs.

## 2.1 Task Control Interfaces

Tasks. NuttX is a flat address OS. As such it does not support processes in the way that, say, Linux does. NuttX only supports simple threads running within the same address space. However, the programming model makes a distinction between tasks and pthreads:

- tasks are threads which have a degree of independence
- pthreads share some resources.

File Descriptors and Streams. This applies, in particular, in the area of opened file descriptors and streams. When a task is started using the interfaces in this section, it will be created with at most three open files.

If CONFIG_DEV_CONSOLE is defined, the first three file descriptors (corresponding to stdin, stdout, stderr) will be duplicated for the the new task. Since these file descriptors are duplicated, the child task can free close them or manipulate them in any way without effecting the parent task. File-related operations (open, close, etc.) within a task will have no effect on other tasks. Since the three file descriptors are duplicated, it is also possible to perform some level of redirection.

pthreads, on the other hand, will always share file descriptors with the parent thread. In this case, file operations will have effect only all pthreads the were started from the same parent thread.

The following task control interfaces are provided by Nuttx:

- 2.1.1 task_create
- 2.1.2 task_init
- 2.1.3 task_activate
- 2.1.4 task_delete
- 2.1.5 exit
- 2.1.6 task_restart
- 2.1.7 getpid

### 2.1.1 task_create

Function Prototype:

       #include <sched.h>
       int task_create(char *name, int priority, int stack_size, main_t entry, const char *argv[]);

Description: This function creates and activates a new task with a specified priority and returns its system-assigned ID.

The entry address entry is the address of the "main" function of the task. This function will be called once the C environment has been set up. The specified function will be called with four arguments. Should the specified routine return, a call to exit() will automatically be made.

Note that an arbitrary number of arguments may be passed to the spawned functions. The maximum umber of arguments is an OS configuration parameter (`CONFIG_MAX_TASK_ARGS`).

The arguments are copied (via `strdup`) so that the life of the passed strings is not dependent on the life of the caller to `task_create()`.

The newly created task does not inherit scheduler characteristics from the parent task: The new task is started at the default system priority and with the SCHED_FIFO scheduling policy. These characteristcs may be modified after the new task has been started.

The newly created task does inherit the first three file descriptors (corresponding to stdin, stdout, and stderr) and redirection of standard I/O is supported.

Input Parameters:

- name. Name of the new task
- priority. Priority of the new task
- stack_size. size (in bytes) of the stack needed
- entry. Entry point of a new task
- argv. A pointer to an array of input parameters. Up to `CONFIG_MAX_TASK_ARG` parameters may be provided. If fewer than `CONFIG_MAX_TASK_ARG` parameters are passed, the list should be terminated with a NULL argv[] value. If no parameters are required, argv may be NULL.

Returned Values:

- Returns the non-zero task ID of the new task or ERROR if memory is insufficient or the task cannot be created (`errno` is not set).

Assumptions/Limitations:

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following similar interface:

       int taskSpawn(char *name, int priority, int options, int stackSize, FUNCPTR entryPt,
                     int arg1, int arg2, int arg3, int arg4, int arg5,
                     int arg6, int arg7, int arg8, int arg9, int arg10);

The NuttX task_create() differs from VxWorks' taskSpawn() in the following ways:

- Interface name
- Various differences in types of arguments
- There is no options arguement.
- A variable number of parameters can be passed to a task (VxWorks supports ten).

### 2.1.2 task_init

Function Prototype:

       #include <sched.h>
       STATUS task_init(_TCB *tcb, char *name, int priority, uint32 *stack, uint32 stack_size,
                        maint_t entry, const char *argv[]);

Description:

This function initializes a Task Control Block (TCB) in preparation for starting a new thread. It performs a subset of the functionality of `task_create()` (see above).

Unlike task_create(), task_init() does not activate the task. This must be done by calling task_activate().

Input Parameters:

- tcb. Address of the new task's TCB
- name. Name of the new task (not used)
- priority. Priority of the new task
- stack. Start of the pre-allocated stack
- stack_size. size (in bytes) of the pre-allocated stack
- entry. Entry point of a new task
- argv. A pointer to an array of input parameters. Up to `CONFIG_MAX_TASK_ARG` parameters may be provided. If fewer than `CONFIG_MAX_TASK_ARG` parameters are passed, the list should be terminated with a NULL argv[] value. If no parameters are required, argv may be NULL.

Returned Values:

- OK, or ERROR if the task cannot be initialized.

  This function can only failure is it is unable to assign a new, unique task ID to the TCB (`errno` is not set).

Assumptions/Limitations:

- task_init() is provided to support internal OS functionality. It is not recommended for normal usage. task_create() is the preferred mechanism to initialize and start a new task.

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following similar interface:

       STATUS taskInit(WIND_TCB *pTcb, char *name, int priority, int options, uint32 *pStackBase, int stackSize,
                       FUNCPTR entryPt, int arg1, int arg2, int arg3, int arg4, int arg5,
                       int arg6, int arg7, int arg8, int arg9, int arg10);

The NuttX task_init() differs from VxWorks' taskInit() in the following ways:

- Interface name
- Various differences in types or arguments
- There is no options argument.
- A variable number of parameters can be passed to a task (VxWorks supports ten).

### 2.1.3 task_activate

Function Prototype:

        #include <sched.h>
        STATUS task_activate( _TCB *tcb );

Description: This function activates tasks created by task_init(). Without activation, a task is ineligible for execution by the scheduler.

Input Parameters:

- tcb. The TCB for the task for the task (same as the task_init argument).

Returned Values:

- OK, or ERROR if the task cannot be activated (`errno` is not set).

Assumptions/Limitations:

- task_activate() is provided to support internal OS functionality. It is not recommended for normal usage. task_create() is the preferred mechanism to initialize and start a new task.

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following similar interface:

        STATUS taskActivate( int tid );

The NuttX task_activate() differs from VxWorks' taskActivate() in the following ways:

- Function name
- With VxWork's taskActivate, the pid argument is supposed to be the pointer to the WIND_TCB cast to an integer.

### 2.1.4 task_delete

Function Prototype:

        #include <sched.h>
        STATUS task_delete( pid_t pid );

Description: This function causes a specified task to cease to exist -- its stack and TCB will be deallocated. This function is the companion to task_create().

Input Parameters:

- pid. The task ID of the task to delete. An ID of zero signifies the calling task.

Returned Values:

- OK, or ERROR if the task cannot be deleted. This function can fail if the provided pid does not correspond to a task (`errno` is not set)

Assumptions/Limitations:

task_delete() must be used with caution: If the task holds resources (for example, allocated memory or semaphores needed by other tasks), then task_delete() can strand those resources.

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following similar interface:

        STATUS taskDelete( int tid );

The NuttX task_delete() differs from VxWorks' taskDelete() in the following ways:

- No support is provided for calling the tasks deletion routines (because taskDeleteHookAdd() is not supported).
- Deletion of self is not supported. Use _exit();

### 2.1.5 exit

Function Prototype:

        #include <sched.h>
        void exit( int code );

        #include <nuttx/unistd.h>
        void _exit( int code );

Description: This function causes the calling task to cease to exist -- its stack and TCB will be deallocated. exit differs from _exit in that it flushs streams, closes file descriptors and will execute any function registered with atexit().

Input Parameters:

- code. (ignored)

Returned Values: None.

Assumptions/Limitations:

POSIX Compatibility: This is equivalent to the ANSI interface:

        void exit( int code );

And the unix interface:

        void _exit( int code );

The NuttX exit() differs from ANSI exit() in the following ways:

- The code parameter is ignored.

### 2.1.6 task_restart

Function Prototype:

        #include <sched.h>
        STATUS task_restart( pid_t pid );

Description: This function "restarts" a task. The task is first terminated and then reinitialized with same ID, priority, original entry point, stack size, and parameters it had when it was first started.

Input Parameters:

- pid. The task ID of the task to delete. An ID of zero signifies the calling task.

Returned Values:

- OK, or ERROR if the task ID is invalid or the task could not be restarted. This function can fail if: (1) A pid of zero or the pid of the calling task is provided (functionality not implemented) (2) The pid is not associated with any task known to the system.

Assumptions/Limitations:

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following similar interface:

        STATUS taskRestart (int tid);

The NuttX task_restart() differs from VxWorks' taskRestart() in the following ways:

- Restart of the currently running task is not supported.
- The VxWorks description says that the ID, priority, etc. take the value that they had when the task was terminated.

### 2.1.7 getpid

Function Prototype:

        #include <unistd.h>
        pid_t getpid( void );

Description: This function returns the task ID of the calling task. The task ID will be invalid if called at the interrupt level.

Input Parameters: None.

Returned Values:

- The task ID of the calling task.

Assumptions/Limitations:

POSIX Compatibility: Compatible with the POSIX interface of the same name.

## 2.2 Task Scheduling Interfaces

By default, NuttX performs strict priority scheduling: Tasks of higher priority have exclusive access to the CPU until they become blocked. At that time, the CPU is available to tasks of lower priority. Tasks of equal priority are scheduled FIFO.

Optionally, a Nuttx task or thread can be configured with round-robin scheduler. This is similar to priority scheduling except that tasks with equal priority and share CPU time via time-slicing. The time-slice interval is a constant determined by the configuration setting `CONFIG_RR_INTERVAL`.

The OS interfaces described in the following paragraphs provide a POSIX- compliant interface to the NuttX scheduler:

- 2.2.1 sched_setparam
- 2.2.2 sched_getparam
- 2.2.3 sched_setscheduler
- 2.2.4 sched_getscheduler
- 2.2.5 sched_yield
- 2.2.6 sched_get_priority_max
- 2.2.7 sched_get_priority_min
- 2.2.8 sched_get_rr_interval

### 2.2.1 sched_setparam

Function Prototype:

        #include <sched.h>
        int sched_setparam(pid_t pid, const struct sched_param *param);

Description: This function sets the priority of the task specified by pid input parameter.

NOTE: Setting a task's priority to the same value has the similar effect to `sched_yield()`: The task will be moved to after all other tasks with the same priority.

Input Parameters:

- `pid`. The task ID of the task. If `pid` is zero, the priority of the calling task is set.
- `param`. A structure whose member `sched_priority` is the integer priority. The range of valid priority numbers is from `SCHED_PRIORITY_MIN` through `SCHED_PRIORITY_MAX`.

Returned Values: On success, sched_setparam() returns 0 (OK). On error, -1 (ERROR) is returned, and `errno` is set appropriately.

- `EINVAL`. The parameter `param` is invalid or does not make sense for the current scheduling policy.
- `EPERM`. The calling task does not have appropriate privileges.
- `ESRCH`. The task whose ID is `pid` could not be found.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- The range of priority values for the POSIX call is 0 to 255.

### 2.2.2 sched_getparam

Function Prototype:

        #include <sched.h>
        int sched_getparam (pid_t pid, struct sched_param *param);

Description: This function gets the scheduling priority of the task specified by pid.

Input Parameters:

- `pid`. The task ID of the task. If pid is zero, the priority of the calling task is returned.
- `param`. A structure whose member `sched_priority` is the integer priority. The task's priority is copied to the `sched_priority` element of this structure.

Returned Values:

- 0 (OK) if successful, otherwise -1 (ERROR).

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.2.3 sched_setscheduler

Function Prototype:

        #include <sched.h>
        int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param);

Description: sched_setscheduler() sets both the scheduling policy and the priority for the task identified by pid. If pid equals zero, the scheduler of the calling thread will be set. The parameter 'param' holds the priority of the thread under the new policy.

Input Parameters:

- pid. The task ID of the task. If pid is zero, the priority of the calling task is set.
- policy. Scheduling policy requested (either SCHED_FIFO or SCHED_RR).
- `param`. A structure whose member sched_priority is the integer priority. The range of valid priority numbers is from SCHED_PRIORITY_MIN through SCHED_PRIORITY_MAX.``

Returned Values: On success, sched_setscheduler() returns OK (zero). On error, ERROR (-1) is returned, and `errno` is set appropriately:

- EINVAL The scheduling policy is not one of the recognized policies.
- ESRCH The task whose ID is pid could not be found.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.2.4 sched_getscheduler

Function Prototype:

        #include <sched.h>
        int sched_getscheduler (pid_t pid);

Description: sched_getscheduler() returns the scheduling policy currently applied to the task identified by pid. If pid equals zero, the policy of the calling process will be retrieved. * * Inputs: * * Return Value: This function returns the current scheduling policy.

Input Parameters:

- pid. The task ID of the task to query. If pid is zero, the calling task is queried.

Returned Values:

- On success, sched_getscheduler() returns the policy for the task (either SCHED_FIFO or SCHED_RR). On error, ERROR (-1) is returned, and `errno` is set appropriately:
  - ESRCH The task whose ID is pid could not be found.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- Does not report errors via `errno`.

### 2.2.5 sched_yield

Function Prototype:

        #include <sched.h>
        int sched_yield( void );

Description: This function forces the calling task to give up the CPU (only to other tasks at the same priority).

Input Parameters: None.

Returned Values:

- 0 (OK) or -1 (ERROR)

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.2.6 sched_get_priority_max

Function Prototype:

        #include <sched.h>
        int sched_get_priority_max (int policy)

Description: This function returns the value of the highest possible task priority for a specified scheduling policy.

Input Parameters:

- policy. Scheduling policy requested.

Returned Values:

- The maximum priority value or -1 (ERROR).

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.2.7 sched_get_priority_min

Function Prototype:

        #include <sched.h>
        int sched_get_priority_min (int policy);

Description: This function returns the value of the lowest possible task priority for a specified scheduling policy.

Input Parameters:

- policy. Scheduling policy requested.

Returned Values:

- The minimum priority value or -1 (ERROR)

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.2.8 sched_get_rr_interval

Function Prototype:

        #include <sched.h>
        int sched_get_rr_interval (pid_t pid, struct timespec *interval);

Description: sched_rr_get_interval() writes the timeslice interval for task identified by pid into the timespec structure pointed to by interval. If pid is zero, the timeslice for the calling process is written into 'interval. The identified process should be running under the SCHED_RR scheduling policy.'

Input Parameters:

- pid. The task ID of the task. If pid is zero, the priority of the calling task is returned.
- interval. A structure used to return the time slice.

Returned Values: On success, sched_rr_get_interval() returns OK (0). On error, ERROR (-1) is returned, and `errno` is set to:

- EFAULT Cannot copy to interval
- EINVAL Invalid pid.
- ENOSYS The system call is not yet implemented.
- ESRCH The process whose ID is pid could not be found.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

## 2.3 Task Switching Interfaces

- 2.3.1 sched_lock
- 2.3.2 sched_unlock
- 2.3.3 sched_lockcount

### 2.3.1 sched_lock

Function Prototype:

        #include <sched.h>
        STATUS sched_lock( void );

Description: This function disables context switching by Disabling addition of new tasks to the ready-to-run task list. The task that calls this function will be the only task that is allowed to run until it either calls sched_unlock (the appropriate number of times) or until it blocks itself.

Input Parameters: None.

Returned Values:

- OK or ERROR.

Assumptions/Limitations:

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the comparable interface:

        STATUS taskLock( void );

### 2.3.2 sched_unlock

Function Prototype:

        #include <sched.h>
        STATUS sched_unlock( void );

Description: This function decrements the preemption lock count. Typically this is paired with sched_lock() and concludes a critical section of code. Preemption will not be unlocked until sched_unlock() has been called as many times as sched_lock(). When the lockCount is decremented to zero, any tasks that were eligible to preempt the current task will execute.

Input Parameters: None.

Returned Values:

- OK or ERROR.

Assumptions/Limitations:

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the comparable interface:

        STATUS taskUnlock( void );

### 2.3.3 sched_lockcount

Function Prototype:

        #include <sched.h>
        sint32 sched_lockcount( void )

Description: This function returns the current value of the lockCount. If zero, preemption is enabled; if non-zero, this value indicates the number of times that sched_lock() has been called on this thread of execution.

Input Parameters: None.

Returned Values:

- The current value of the lockCount.

Assumptions/Limitations:

POSIX Compatibility: None.

## 2.4 Named Message Queue Interfaces

NuttX supports POSIX named message queues for intertask communication. Any task may send or receive messages on named message queues. Interrupt handlers may send messages via named message queues.

- 2.4.1 mq_open
- 2.4.2 mq_close
- 2.4.3 mq_unlink
- 2.4.4 mq_send
- 2.4.5 mq_timedsend
- 2.4.6 mq_receive
- 2.4.7 mq_timedreceive
- 2.4.8 mq_notify
- 2.4.9 mq_setattr
- 2.4.10 mq_getattr

### 2.4.1 mq_open

Function Prototype:

        #include <mqueue.h>
        mqd_t mq_open( const char *mqName, int oflags, ... );

Description: This function establish a connection between a named message queue and the calling task. After a successful call of mq_open(), the task can reference the message queue using the address returned by the call. The message queue remains usable until it is closed by a successful call to mq_close().

Input Parameters:

- mqName. Name of the queue to open
- oflags. Open flags. These may be any combination of:
  - O_RDONLY. Open for read access.
  - O_WRONLY. Open for write access.
  - O_RDWR. Open for both read & write access.
  - O_CREAT. Create message queue if it does not already exist.
  - O_EXCL. Name must not exist when opened.
  - O_NONBLOCK. Don't wait for data.
- ... Optional parameters. When the O_CREAT flag is specified, POSIX requires that a third and fourth parameter be supplied:
  - mode. The mode parameter is of type mode_t. In the POSIX specification, this mode value provides file permission bits for the message queue. This parameter is required but not used in the present implementation.
  - attr. A pointer to an mq_attr that is provided to initialize. the message queue. If attr is NULL, then the messages queue is created with implementation-defined default message queue attributes. If attr is non-NULL, then the message queue mq_maxmsg attribute is set to the corresponding value when the queue is created. The mq_maxmsg attribute determines the maximum number of messages that can be queued before addition attempts to send messages on the message queue fail or cause the sender to block; the mq_msgsize attribute determines the maximum size of a message that can be sent or received. Other elements of attr are ignored (i.e, set to default message queue attributes).

Returned Values:

- A message queue descriptor or -1 (ERROR)

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- The mq_msgsize attributes determines the maximum size of a message that may be sent or received. In the present implementation, this maximum message size is limited at 22 bytes.

### 2.4.2 mq_close

Function Prototype:

        #include <mqueue.h>
        int mq_close( mqd_t mqdes );

Description: This function is used to indicate that the calling task is finished with the specified message queued mqdes. The mq_close() deallocates any system resources allocated by the system for use by this task for its message queue.

If the calling task has attached a notification request to the message queue via this mqdes (see mq_notify()), this attachment will be removed and the message queue is available for another task to attach for notification.

Input Parameters:

- mqdes. Message queue descriptor.

Returned Values:

- 0 (OK) if the message queue is closed successfully, otherwise, -1 (ERROR).

Assumptions/Limitations:

- The behavior of a task that is blocked on either a `mq_send()` or `mq_receive()` is undefined when `mq_close()` is called.
- The result of using this message queue descriptor after successful return from mq_close() is undefined.

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.4.3 mq_unlink

Function Prototype:

        #include <mqueue.h>
        int mq_unlink( const char *mqName );

Description: This function removes the message queue named by "mqName." If one or more tasks have the message queue open when mq_unlink() is called, removal of the message queue is postponed until all references to the message queue have been closed.

Input Parameters:

- mqName. Name of the message queue

Returned Values: None.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.4.4 mq_send

Function Prototype:

        #include <mqueue.h>
        int mq_send(mqd_t mqdes, const void *msg, size_t msglen, int prio);

Description: This function adds the specified message, `msg`, to the message queue, `mqdes`. The `msglen` parameter specifies the length of the message in bytes pointed to by `msg`. This length must not exceed the maximum message length from the `mq_getattr()`.

If the message queue is not full, `mq_send()` will place the `msg` in the message queue at the position indicated by the `prio` argument. Messages with higher priority will be inserted before lower priority messages The value of `prio` must not exceed `MQ_PRIO_MAX`.

If the specified message queue is full and `O_NONBLOCK` is not set in the message queue, then `mq_send()` will block until space becomes available to the queue the message.

If the message queue is full and `NON_BLOCK` is set, the message is not queued and `ERROR` is returned.

Input Parameters:

- `mqdes`. Message queue descriptor.
- `msg`. Message to send.
- `msglen`. The length of the message in bytes.
- `prio`. The priority of the message.

Returned Values: On success, `mq_send()` returns 0 (`OK`); on error, -1 (`ERROR`) is returned, with `errno` set to indicate the error:

- `EAGAIN`. The queue was empty, and the `O_NONBLOCK` flag was set for the message queue description referred to by `mqdes`.
- `EINVAL`. Either `msg` or `mqdes` is `NULL` or the value of `prio` is invalid.
- `EPERM`. Message queue opened not opened for writing.
- `EMSGSIZE`. `msglen` was greater than the `maxmsgsize` attribute of the message queue.
- `EINTR`. The call was interrupted by a signal handler.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### mq_timedsend

Function Prototype:

        #include <mqueue.h>
        int mq_timedsend(mqd_t mqdes, const char *msg, size_t msglen, int prio,
                         const struct timespec *abstime);

Description: This function adds the specified message, `msg`, to the message queue, `mqdes`. The `msglen` parameter specifies the length of the message in bytes pointed to by `msg`. This length must not exceed the maximum message length from the `mq_getattr()`.

If the message queue is not full, `mq_timedsend()` will place the `msg` in the message queue at the position indicated by the `prio` argument. Messages with higher priority will be inserted before lower priority messages The value of `prio` must not exceed `MQ_PRIO_MAX`.

If the specified message queue is full and `O_NONBLOCK` is not set in the message queue, then `mq_send()` will block until space becomes available to the queue the message or until a timeout occurs.

`mq_timedsend()` behaves just like `mq_send()`, except that if the queue is full and the `O_NONBLOCK` flag is not enabled for the message queue description, then `abstime` points to a structure which specifies a ceiling on the time for which the call will block. This ceiling is an absolute timeout in seconds and nanoseconds since the Epoch (midnight on the morning of 1 January 1970).

If the message queue is full, and the timeout has already expired by the time of the call, `mq_timedsend()`returns immediately.``

Input Parameters:

- `mqdes`. Message queue descriptor.
- `msg`. Message to send.
- `msglen`. The length of the message in bytes.
- `prio`. The priority of the message.

Returned Values: On success, `mq_send()` returns 0 (`OK`); on error, -1 (`ERROR`) is returned, with `errno` set to indicate the error:

- `EAGAIN`. The queue was empty, and the `O_NONBLOCK` flag was set for the message queue description referred to by `mqdes`.
- `EINVAL`. Either `msg` or `mqdes` is `NULL` or the value of `prio` is invalid.
- `EPERM`. Message queue opened not opened for writing.
- `EMSGSIZE`. `msglen` was greater than the `maxmsgsize` attribute of the message queue.
- `EINTR`. The call was interrupted by a signal handler.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.4.5 mq_receive

Function Prototype:

        #include <mqueue.h>
        ssize_t mq_receive(mqd_t mqdes, void *msg, size_t msglen, int *prio);

Description: This function receives the oldest of the highest priority messages from the message queue specified by `mqdes`. If the size of the buffer in bytes, `msgLen`, is less than the `mq_msgsize` attribute of the message queue, `mq_receive()` will return an error. Otherwise, the selected message is removed from the queue and copied to `msg`.

If the message queue is empty and `O_NONBLOCK` was not set, `mq_receive()` will block until a message is added to the message queue. If more than one task is waiting to receive a message, only the task with the highest priority that has waited the longest will be unblocked.

If the queue is empty and `O_NONBLOCK` is set, `ERROR` will be returned.

Input Parameters:

- `mqdes`. Message Queue Descriptor.
- `msg`. Buffer to receive the message.
- `msglen`. Size of the buffer in bytes.
- `prio`. If not NULL, the location to store message priority.

Returned Values:. One success, the length of the selected message in bytes is returned. On failure, -1 (`ERROR`) is returned and the `errno` is set appropriately:

- `EAGAIN` The queue was empty and the `O_NONBLOCK` flag was set for the message queue description referred to by `mqdes`.
- `EPERM` Message queue opened not opened for reading.
- `EMSGSIZE` `msglen` was less than the `maxmsgsize` attribute of the message queue.
- `EINTR` The call was interrupted by a signal handler.
- `EINVAL` Invalid `msg` or `mqdes`

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.4.6 mq_timedreceive

Function Prototype:

        #include <mqueue.h>
        ssize_t mq_timedreceive(mqd_t mqdes, void *msg, size_t msglen,
                                int *prio, const struct timespec *abstime);

Description: This function receives the oldest of the highest priority messages from the message queue specified by `mqdes`. If the size of the buffer in bytes, `msgLen`, is less than the `mq_msgsize` attribute of the message queue, `mq_timedreceive()` will return an error. Otherwise, the selected message is removed from the queue and copied to `msg`.

If the message queue is empty and `O_NONBLOCK` was not set, `mq_timedreceive()` will block until a message is added to the message queue (or until a timeout occurs). If more than one task is waiting to receive a message, only the task with the highest priority that has waited the longest will be unblocked.

`mq_timedreceive()` behaves just like `mq_receive()`, except that if the queue is empty and the `O_NONBLOCK flag is not enabled for the message queue description, then `abstime` points to a structure which specifies a ceiling on the time for which the call will block. This ceiling is an absolute timeout in seconds and nanoseconds since the Epoch (midnight on the morning of 1 January 1970).```

If no message is available, and the timeout has already expired by the time of the call, `mq_timedreceive()` returns immediately.

Input Parameters:

- `mqdes`. Message Queue Descriptor.
- `msg`. Buffer to receive the message.
- `msglen`. Size of the buffer in bytes.
- `prio`. If not NULL, the location to store message priority.
- `abstime`. The absolute time to wait until a timeout is declared.

Returned Values:. One success, the length of the selected message in bytes is returned. On failure, -1 (`ERROR`) is returned and the `errno` is set appropriately:

- `EAGAIN`: The queue was empty and the `O_NONBLOCK` flag was set for the message queue description referred to by `mqdes`.
- `EPERM`: Message queue opened not opened for reading.
- `EMSGSIZE`: `msglen` was less than the `maxmsgsize` attribute of the message queue.
- `EINTR`: The call was interrupted by a signal handler.
- `EINVAL`: Invalid `msg` or `mqdes` or `abstime`
- `ETIMEDOUT`: The call timed out before a message could be transferred.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.4.7 mq_notify

Function Prototype:

        #include <mqueue.h>
        int mq_notify(mqd_t mqdes, const struct sigevent *notification);

Description: If the "notification" input parameter is not NULL, this function connects the task with the message queue such that the specified signal will be sent to the task whenever the message changes from empty to non-empty. One notification can be attached to a message queue.

If "notification" is NULL, the attached notification is detached (if it was held by the calling task) and the queue is available to attach another notification.

When the notification is sent to the registered task, its registration will be removed. The message queue will then be available for registration.

Input Parameters:

- mqdes. Message queue descriptor
- notification. Real-time signal structure containing:
  - sigev_notify. Should be SIGEV_SIGNAL (but actually ignored)
  - sigev_signo. The signo to use for the notification
  - sigev_value. Value associated with the signal

Returned Values: None.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- The notification signal will be sent to the registered task even if another task is waiting for the message queue to become non-empty. This is inconsistent with the POSIX specification which states, "If a process has registered for notification of message arrival at a message queue and some process is blocked in mq_receive waiting to receive a message when a message arrives at the queue, the arriving message shall satisfy the appropriate mq_receive() ... The resulting behavior is as if the message queue remains empty, and no notification shall be sent."

### 2.4.8 mq_setattr

Function Prototype:

        #include <mqueue.h>
        int mq_setattr( mqd_t mqdes, const struct mq_attr *mqStat,
                         struct mq_attr *oldMqStat);

Description: This function sets the attributes associated with the specified message queue "mqdes." Only the "O_NONBLOCK" bit of the "mq_flags" can be changed.

If "oldMqStat" is non-null, mq_setattr() will store the previous message queue attributes at that location (just as would have been returned by mq_getattr()).

Input Parameters:

- mqdes. Message queue descriptor
- mqStat. New attributes
- oldMqState. Old attributes

Returned Values:

- 0 (OK) if attributes are set successfully, otherwise -1 (ERROR).

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.4.9 mq_getattr

Function Prototype:

        #include <mqueue.h>
        int mq_getattr( mqd_t mqdes, struct mq_attr *mqStat);

Description: This functions gets status information and attributes associated with the specified message queue.

Input Parameters:

- mqdes. Message queue descriptor
- mqStat. Buffer in which to return attributes. The returned attributes include:
  - mq_maxmsg. Max number of messages in queue.
  - mq_msgsize. Max message size.
  - mq_flags. Queue flags.
  - mq_curmsgs. Number of messages currently in queue.

Returned Values:

- 0 (OK) if attributes provided, -1 (ERROR) otherwise.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

## 2.5 Counting Semaphore Interfaces

Semaphores. Semaphores are the basis for synchronization and mutual exclusion in NuttX. NuttX supports POSIX semaphores.

Semaphores are the preferred mechanism for gaining exclusive access to a resource. sched_lock() and sched_unlock() can also be used for this purpose. However, sched_lock() and sched_unlock() have other undesirable side-affects in the operation of the system: sched_lock() also prevents higher-priority tasks from running that do not depend upon the semaphore-managed resource and, as a result, can adversely affect system response times.

Priority Inversion. Proper use of semaphores avoids the issues of `sched_lock()`. However, consider the following example:

1. Some low-priority task, Task C, acquires a semphore in order to get exclusive access to a protected resource.
2. Task C is suspended to allow some high-priority task,
Task A, to execute.
3. Task A attempts to acquire the semaphore held by Task C and gets blocked until Task C relinquishes the semaphore.
4. Task C is allowed to execute again, but gets suspended by some medium-priority Task B.

At this point, the high-priority Task A cannot execute until Task B (and possibly other medium-priority tasks) completes and until Task C relinquishes the semaphore. In effect, the high-priority task, Task A behaves as though it were lower in priority than the low-priority task, Task C! This phenomenon is called priority inversion.

Some operating systems avoid priority inversion by automatically increasing the priority of the low-priority Task C (the operable buzz-word for this behavior is priority inheritance). NuttX supports this behavior, but only if `CONFIG_PRIORITY_INHERITANCE` is defined in your OS configuration file. If `CONFIG_PRIORITY_INHERITANCE` is not defined, then it is left to the designer to provide implementations that will not suffer from priority inversion. The designer may, as examples:

- Implement all tasks that need the semphore-managed resources at the same priority level,
- Boost the priority of the low-priority task before the semaphore is acquired, or
- Use sched_lock() in the low-priority task.

Priority Inheritance. As mentioned, NuttX does support priority inheritance provided that `CONFIG_PRIORITY_INHERITANCE` is defined in your OS configuration file. However, the implementation and configuration of the priority inheritance feature is sufficiently complex that more needs to be said. How can a feature that can be described by a single, simple sentence require such a complex implementation:

- `CONFIG_SEM_PREALLOCHOLDERS`. First of all, in NuttX priority inheritance is implement on POSIX counting semaphores. The reason for this is that these semaphores are the most primitive waiting mechanism in NuttX; Most other waiting facilities are based on semaphores. So if priority inheritance is implemented for POSIX counting semaphores, then most NuttX waiting mechanisms will have this capability.

  Complexity arises because counting semaphores can have numerous holders of semaphore counts. Therefore, in order to implement priority inheritance across all holders, then internal data structures must be allocated to manage the various holders associated with a semaphore. The setting `CONFIG_SEM_PREALLOCHOLDERS` defines the maximum number of different threads (minus one per semaphore instance) that can take counts on a semaphore with priority inheritance support. This setting defines the size of a single pool of preallocated structures. It may be set to zero if priority inheritance is disabled OR if you are only using semaphores as mutexes (only one holder) OR if no more than two threads participate using a counting semaphore.

  The cost associated with setting `CONFIG_SEM_PREALLOCHOLDERS` is slightly increased code size and around 6-12 bytes times the value of `CONFIG_SEM_PREALLOCHOLDERS`.

- `CONFIG_SEM_NNESTPRIO`: In addition, there may be multiple threads of various priorities that need to wait for a count from the semaphore. These, the lower priority thread holding the semaphore may have to be boosted numerous time and, to make things more complex, will have to keep track of all of the boost priorities values in in order to correctly restore the priorities after a count has been handed out to the higher priority thread. The `CONFIG_SEM_NNESTPRIO` defines the size of an array, one array per active thread. This setting is the maximum number of higher priority threads (minus 1) than can be waiting for another thread to release a count on a semaphore. This value may be set to zero if no more than one thread is expected to wait for a semaphore.

  The cost associated with setting `CONFIG_SEM_NNESTPRIO` is slightly increased code size and (`CONFIG_SEM_PREALLOCHOLDERS` + 1) times the maximum number of active threads.

- Increased Susceptibility to Bad Thread Behavior. These various structures tie the semaphore implementation more tightly to the behavior of the implementation. For examples, if a thread executes while holding counts on a semaphore, or if a thread exits without call `sem_destroy()` then. Or what if the thread with the boosted priority reprioritizes itself? The NuttX implement of priority inheritance attempts to handle all of these types of corner cases, but it is very likely that some are missed. The worst case result is that memory could by stranded within the priority inheritance logic.

POSIX semaphore interfaces:

- 2.5.1 sem_init
- 2.5.2 sem_destroy
- 2.5.3 sem_open
- 2.5.4 sem_close
- 2.5.5 sem_unlink
- 2.5.6 sem_wait
- 2.5.7 sem_trywait
- 2.5.8 sem_post
- 2.5.9 sem_getvalue

### 2.5.1 sem_init

Function Prototype:

        #include <semaphore.h>
        int sem_init ( sem_t *sem, int pshared, unsigned int value );

Description: This function initializes the UN-NAMED semaphore sem. Following a successful call to sem_init(), the semaphore may be used in subsequent calls to sem_wait(), sem_post(), and sem_trywait(). The semaphore remains usable until it is destroyed.

Only sem itself may be used for performing synchronization. The result of referring to copies of sem in calls to sem_wait(), sem_trywait(), sem_post(), and sem_destroy(), is not defined.

Input Parameters:

- sem. Semaphore to be initialized
- pshared. Process sharing (not used)
- value. Semaphore initialization value

Returned Values:

- 0 (OK), or -1 (ERROR) if unsuccessful.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- pshared is not used.

### 2.5.2 sem_destroy

Function Prototype:

        #include <semaphore.h>
        int sem_destroy ( sem_t *sem );

Description: This function is used to destroy the un-named semaphore indicated by sem. Only a semaphore that was created using sem_init() may be destroyed using sem_destroy(). The effect of calling sem_destroy() with a named semaphore is undefined. The effect of subsequent use of the semaphore sem is undefined until sem is re-initialized by another call to sem_init().

The effect of destroying a semaphore upon which other tasks are currently blocked is undefined.

Input Parameters:

- sem. Semaphore to be destroyed.

Returned Values:

- 0 (OK), or -1 (ERROR) if unsuccessful.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.5.3 sem_open

Function Prototype:

        #include <semaphore.h>
        sem_t *sem_open ( const char *name, int oflag, ...);

Description: This function establishes a connection between named semaphores and a task. Following a call to sem_open() with the semaphore name, the task may reference the semaphore associated with name using the address returned by this call. The semaphore may be used in subsequent calls to sem_wait(), sem_trywait(), and sem_post(). The semaphore remains usable until the semaphore is closed by a successful call to sem_close().

If a task makes multiple calls to sem_open() with the same name, then the same semaphore address is returned (provided there have been no calls to sem_unlink()).

Input Parameters:

- name. Semaphore name
- oflag. Semaphore creation options. This may one of the following bit settings:
  - oflag = 0: Connect to the semaphore only if it already exists.
  - oflag = O_CREAT: Connect to the semaphore if it exists, otherwise create the semaphore.
  - oflag = O_CREAT with O_EXCL (O_CREAT|O_EXCL): Create a new semaphore unless one of this name already exists.
- ... Optional parameters. NOTE: When the O_CREAT flag is specified, POSIX requires that a third and fourth parameter be supplied:
  - mode. The mode parameter is of type mode_t. This parameter is required but not used in the present implementation.
  - value. The value parameter is type unsigned int. The semaphore is created with an initial value of value. Valid initial values for semaphores must be less than or equal to SEM_VALUE_MAX (defined in `include/limits.h`).

Returned Values:

- A pointer to sem_t or -1 (ERROR) if unsuccessful.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- Treatment of links/connections is highly simplified. It is just a counting semaphore.

### 2.5.4 sem_close

Function Prototype:

        #include <semaphore.h>
        int sem_close ( sem_t *sem );

Description: This function is called to indicate that the calling task is finished with the specified named semaphore, sem. The sem_close() deallocates any system resources allocated by the system for this named semaphore.

If the semaphore has not been removed with a call to sem_unlink(), then sem_close() has no effect on the named semaphore. However, when the named semaphore has been fully unlinked, the semaphore will vanish when the last task closes it.

Care must be taken to avoid risking the deletion of a semaphore that another calling task has already locked.

Input Parameters:

- sem. Semaphore descriptor

Returned Values:

- 0 (OK), or -1 (ERROR) if unsuccessful.

Assumptions/Limitations:

- Care must be taken to avoid deletion of a semaphore that another task has already locked.
- sem_close() must not be called with an un-named semaphore.

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.5.5 sem_unlink

Function Prototype:

        #include <semaphore.h>
        int sem_unlink ( const char *name );

Description: This function will remove the semaphore named by the input name parameter. If one or more tasks have the semaphore named by name oepn when sem_unlink() is called, destruction of the semaphore will be postponed until all references have been destroyed by calls to sem_close().

Input Parameters:

- name. Semaphore name

Returned Values:

- 0 (OK), or -1 (ERROR) if unsuccessful.

Assumptions/Limitations:

- Care must be taken to avoid deletion of a semaphore that another task has already locked.
- sem_unlink() must not be called with an un-named semaphore.

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- Treatment of links/connections is highly simplified. It is just a counting semaphore.
- Calls to sem_open() to re-create or re-connect to the semaphore may refer to the same semaphore; POSIX specifies that a new semaphore with the same name should be created after sem_unlink() is called.

### 2.5.6 sem_wait

Function Prototype:

        #include <semaphore.h>
        int sem_wait ( sem_t *sem );

Description: This function attempts to lock the semaphore referenced by sem. If the semaphore as already locked by another task, the calling task will not return until it either successfully acquires the lock or the call is interrupted by a signal.

Input Parameters:

- sem. Semaphore descriptor.

Returned Values:

- 0 (OK), or -1 (ERROR) is unsuccessful

If sem_wait returns -1 (ERROR) then the cause of the failure will be indicated by the thread-specific `errno`. The following lists the possible values for `errno`:

- EINVAL: Indicates that the sem input parameter is not valid.
- EINTR: Indicates that the wait was interrupt by a signal received by this task. In this case, the semaphore has not be acquired.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.5.7 sem_trywait

Function Prototype:

        #include <semaphore.h>
        int sem_trywait ( sem_t *sem );

Description: This function locks the specified semaphore only if the semaphore is currently not locked. In any event, the call returns without blocking.

Input Parameters:

- sem. The semaphore descriptor

Returned Values:

- 0 (OK) or -1 (ERROR) if unsuccessful

If sem_wait returns -1 (ERROR) then the cause of the failure will be indicated by the thread-specific `errno`. The following lists the possible values for `errno`:

- EINVAL: Indicates that the sem input parameter is not valid.
- EAGAIN: Indicates that the semaphore was not acquired.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.5.8 sem_post

Function Prototype:

        #include <semaphore.h>
        int sem_post ( sem_t *sem );

Description: When a task has finished with a semaphore, it will call sem_post(). This function unlocks the semaphore referenced by sem by performing the semaphore unlock operation.

If the semaphore value resulting from this operation is positive, then no tasks were blocked waiting for the semaphore to become unlocked; The semaphore value is simply incremented.

If the value of the semaphore resulting from this operation is zero, then on of the tasks blocked waiting for the semaphore will be allowed to return successfully from its call to sem_wait().

NOTE: sem_post() may be called from an interrupt handler.

Input Parameters:

- sem. Semaphore descriptor

Returned Values:

- 0 (OK) or -1 (ERROR) if unsuccessful.

Assumptions/Limitations: This function cannot be called from an interrupt handler. It assumes the currently executing task is the one that is performing the unlock.

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.5.9 sem_getvalue

Function Prototype:

        #include <semaphore.h>
        int sem_getvalue ( sem_t *sem, int *sval );

Description: This function updates the location referenced by sval argument to have the value of the semaphore referenced by sem without effecting the state of the semaphore. The updated value represents the actual semaphore value that occurred at some unspecified time during the call, but may not reflect the actual value of the semaphore when it is returned to the calling task.

If sem is locked, the value return by sem_getvalue() will either be zero or a negative number whose absolute value represents the number of tasks waiting for the semaphore.

Input Parameters:

- sem. Semaphore descriptor
- sval. Buffer by which the value is returned

Returned Values:

- 0 (OK) or -1 (ERROR) if unsuccessful.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

## 2.6 Watchdog Timer Interfaces

NuttX provides a general watchdog timer facility. This facility allows the NuttX user to specify a watchdog timer function that will run after a specified delay. The watchdog timer function will run in the context of the timer interrupt handler. Because of this, a limited number of NuttX interfaces are available to he watchdog timer function. However, the watchdog timer function may use `mq_send()`, `sigqueue()`, or `kill()` to communicate with NuttX tasks.

- 2.6.1 wd_create
- 2.6.2 wd_delete
- 2.6.3 wd_start
- 2.6.4 wd_cancel
- 2.6.5 wd_gettime

### 2.6.1 wd_create

Function Prototype:

        #include <wdog.h>
        WDOG_ID wd_create (void);

Description: The wd_create function will create a watchdog by allocating the appropriate resources for the watchdog.

Input Parameters: None.

Returned Values:

- Pointer to watchdog that may be used as a handle in subsequent NuttX calls (i.e., the watchdog ID), or NULL if insufficient resources are available to create the watchdogs.

Assumptions/Limitations:

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following comparable interface:

        WDOG_ID wdCreate (void);

Differences from the VxWorks interface include:

- The number of available watchdogs is fixed (configured at initialization time).

### 2.6.2 wd_delete

Function Prototype:

        #include <wdog.h>
        STATUS wd_delete (WDOG_ID wdog);

Description: The wd_delete function will deallocate a watchdog. The watchdog will be removed from the timer queue if has been started.

Input Parameters:

- wdog. The watchdog ID to delete. This is actually a pointer to a watchdog structure.

Returned Values:

- OK or ERROR

Assumptions/Limitations: It is the responsibility of the caller to assure that the watchdog is inactive before deleting it.

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following comparable interface:

        STATUS wdDelete (WDOG_ID wdog);

Differences from the VxWorks interface include:

- Does not make any checks to see if the watchdog is being used before de-allocating it (i.e., never returns ERROR).

### 2.6.3 wd_start

Function Prototype:

        #include <wdog.h>
        STATUS wd_start( WDOG_ID wdog, int delay, wdentry_t wdentry,
                         intt argc, ....);

Description: This function adds a watchdog to the timer queue. The specified watchdog function will be called from the interrupt level after the specified number of ticks has elapsed. Watchdog timers may be started from the interrupt level.

Watchdog times execute in the context of the timer interrupt handler.

Watchdog timers execute only once.

To replace either the timeout delay or the function to be executed, call wd_start again with the same wdog; only the most recent wd_start() on a given watchdog ID has any effect.

Input Parameters:

- wdog. Watchdog ID
- delay. Delay count in clock ticks
- wdentry. Function to call on timeout
- argc. The number of uint32 parameters to pass to wdentry.
- .... uint32 size parameters to pass to wdentry

Returned Values:

- OK or ERROR

Assumptions/Limitations: The watchdog routine runs in the context of the timer interrupt handler and is subject to all ISR restrictions.

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following comparable interface:

        STATUS wdStart (WDOG_ID wdog, int delay, FUNCPTR wdentry, int parameter);

Differences from the VxWorks interface include:

- The present implementation supports multiple parameters passed to wdentry; VxWorks supports only a single parameter. The maximum number of parameters is determined by

### 2.6.4 wd_cancel

Function Prototype:

        #include <wdog.h>
        STATUS wd_cancel (WDOG_ID wdog);

Description: This function cancels a currently running watchdog timer. Watchdog timers may be canceled from the interrupt level.

Input Parameters:

- wdog. ID of the watchdog to cancel.

Returned Values:

- OK or ERROR

Assumptions/Limitations:

POSIX Compatibility: This is a NON-POSIX interface. VxWorks provides the following comparable interface:

        STATUS wdCancel (WDOG_ID wdog);

### 2.6.5 wd_gettime

Function Prototype:

        #include <wdog.h>
        Sint wd_gettime(WDOG_ID wdog);

Description: This function returns the time remaining before the the specified watchdog expires.

Input Parameters:

- `wdog`. Identifies the watchdog that the request is for.

Returned Value: The time in system ticks remaining until the watchdog time expires. Zero means either that wdog is not valid or that the wdog has already expired.

## 2.7 Clocks and Timers

- 2.7.1 clock_settime
- 2.7.2 clock_gettime
- 2.7.3 clock_getres
- 2.7.4 mktime
- 2.7.5 gmtime_r
- 2.7.6 localtime_r
- 2.7.7 timer_create
- 2.7.8 timer_delete
- 2.7.9 timer_settime
- 2.7.10 timer_gettime
- 2.7.11 timer_getoverrun

### 2.7.1 clock_settime

Function Prototype:

        #include <time.h>
        int clock_settime(clockid_t clockid, const struct timespec *tp);

Description:

Input Parameters:

- `parm`.

Returned Values:

If successful, the clock_settime() function will return zero (OK). Otherwise, an non-zero error number will be returned to indicate the error:

- `To be provided`.

### 2.7.2 clock_gettime

Function Prototype:

        #include <time.h>
        int clock_gettime(clockid_t clockid, struct timespec *tp);

Description:

Input Parameters:

- `parm`.

Returned Values:

If successful, the clock_gettime() function will return zero (OK). Otherwise, an non-zero error number will be returned to indicate the error:

- `To be provided`.

### 2.7.3 clock_getres

Function Prototype:

        #include <time.h>
        int clock_getres(clockid_t clockid, struct timespec *res);

Description:

Input Parameters:

- `parm`.

Returned Values:

If successful, the clock_getres() function will return zero (OK). Otherwise, an non-zero error number will be returned to indicate the error:

- `To be provided`.

### 2.7.4 mktime

Function Prototype:

        #include <time.h>
        time_t mktime(struct tm *tp);

Description:

Input Parameters:

- `parm`.

Returned Values:

If successful, the mktime() function will return zero (OK). Otherwise, an non-zero error number will be returned to indicate the error:

- `To be provided`.

### 2.7.5 gmtime_r

Function Prototype:

        #include <time.h>
        struct tm *gmtime_r(const time_t *clock, struct tm *result);

Description:

Input Parameters:

- `parm`.

Returned Values:

If successful, the gmtime_r() function will return zero (OK). Otherwise, an non-zero error number will be returned to indicate the error:

- `To be provided`.

### 2.7.6 localtime_r

        #include <time.h>
        #define localtime_r(c,r) gmtime_r(c,r)

### 2.7.7 timer_create

Function Prototype:

        #include <time.h>
        int timer_create(clockid_t clockid, struct sigevent *evp, timer_t *timerid);

Description: The `timer_create()` function creates per-thread timer using the specified clock, `clock_id`, as the timing base. The `timer_create()` function returns, in the location referenced by `timerid`, a timer ID of type timer_t used to identify the timer in timer requests. This timer ID is unique until the timer is deleted. The particular clock, `clock_id`, is defined in `<time.h>`. The timer whose ID is returned will be in a disarmed state upon return from `timer_create()`.````

The `evp` argument, if non-NULL, points to a `sigevent` structure. This structure is allocated by the called and defines the asynchronous notification to occur. If the `evp` argument is NULL, the effect is as if the `evp` argument pointed to a `sigevent` structure with the `sigev_notify` member having the value `SIGEV_SIGNAL`, the `sigev_signo` having a default signal number, and the `sigev_value` member having the value of the timer ID.

Each implementation defines a set of clocks that can be used as timing bases for per-thread timers. All implementations shall support a `clock_id` of `CLOCK_REALTIME`.

Input Parameters:

- `clockid`. Specifies the clock to use as the timing base. Must be `CLOCK_REALTIME`.
- `evp`. Refers to a user allocated sigevent structure that defines the asynchronous notification. evp may be NULL (see above).
- `timerid`. The pre-thread timer created by the call to timer_create().

Returned Values:

If the call succeeds, `timer_create()` will return 0 (`OK`) and update the location referenced by `timerid` to a `timer_t`, which can be passed to the other per-thread timer calls. If an error occurs, the function will return a value of -1 (`ERROR`) and set `errno` to indicate the error.

- `EAGAIN`. The system lacks sufficient signal queuing resources to honor the request.
- `EAGAIN`. The calling process has already created all of the timers it is allowed by this implementation.
- `EINVAL`. The specified clock ID is not defined.
- `ENOTSUP`. The implementation does not support the creation of a timer attached to the CPU-time clock that is specified by clock_id and associated with a thread different thread invoking timer_create().

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- Only `CLOCK_REALTIME` is supported for the `clockid` argument.

### 2.7.8 timer_delete

Function Prototype:

        #include <time.h>
        int timer_delete(timer_t timerid);

Description: The `timer_delete()` function deletes the specified timer, `timerid`, previously created by the `timer_create()` function. If the timer is armed when `timer_delete()` is called, the timer will be automatically disarmed before removal. The disposition of pending signals for the deleted timer is unspecified.

Input Parameters:

- `timerid`. The pre-thread timer, previously created by the call to timer_create(), to be deleted.

Returned Values:

If successful, the timer_delete() function will return zero (OK). Otherwise, the function will return a value of -1 (ERROR) and set `errno` to indicate the error:

- `EINVAL`. The timer specified timerid is not valid.

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.7.9 timer_settime

Function Prototype:

        #include <time.h>
        int timer_settime(timer_t timerid, int flags, const struct itimerspec *value,
                          struct itimerspec *ovalue);

Description: The `timer_settime()` function sets the time until the next expiration of the timer specified by `timerid` from the `it_value` member of the value argument and arm the timer if the `it_value` member of value is non-zero. If the specified timer was already armed when `timer_settime()` is called, this call will reset the time until next expiration to the value specified. If the `it_value` member of value is zero, the timer will be disarmed. The effect of disarming or resetting a timer with pending expiration notifications is unspecified.

If the flag `TIMER_ABSTIME` is not set in the argument flags, `timer_settime()` will behave as if the time until next expiration is set to be equal to the interval specified by the `it_value` member of value. That is, the timer will expire in `it_value` nanoseconds from when the call is made. If the flag `TIMER_ABSTIME` is set in the argument flags, `timer_settime()` will behave as if the time until next expiration is set to be equal to the difference between the absolute time specified by the `it_value` member of value and the current value of the clock associated with `timerid`. That is, the timer will expire when the clock reaches the value specified by the `it_value` member of value. If the specified time has already passed, the function will succeed and the expiration notification will be made.

The reload value of the timer will be set to the value specified by the `it_interval` member of value. When a timer is armed with a non-zero `it_interval`, a periodic (or repetitive) timer is specified.

Time values that are between two consecutive non-negative integer multiples of the resolution of the specified timer will be rounded up to the larger multiple of the resolution. Quantization error will not cause the timer to expire earlier than the rounded time value.

If the argument `ovalue` is not NULL, the t`imer_settime()` function will store, in the location referenced by `ovalue`, a value representing the previous amount of time before the timer would have expired, or zero if the timer was disarmed, together with the previous timer reload value. Timers will not expire before their scheduled time.

NOTE:At present, the `ovalue` argument is ignored.

Input Parameters:

- `timerid`. The pre-thread timer, previously created by the call to timer_create(), to be be set.
- `flags`. Specifie characteristics of the timer (see above)
- `value`. Specifies the timer value to set
- `ovalue`. A location in which to return the time remaining from the previous timer setting (ignored).

Returned Values:

If the timer_gettime() succeeds, a value of 0 (OK) will be returned. If an error occurs, the value -1 (ERROR) will be returned, and `errno` set to indicate the error.

- `EINVAL`. The timerid argument does not correspond to an ID returned by timer_create() but not yet deleted by timer_delete().
- `EINVAL`. A value structure specified a nanosecond value less than zero or greater than or equal to 1000 million, and the it_value member of that structure did not specify zero seconds and nanoseconds.

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- The `ovalue` argument is ignored.

### 2.7.10 timer_gettime

Function Prototype:

        #include <time.h>
        int timer_gettime(timer_t timerid, struct itimerspec *value);

Description: The `timer_gettime()` function will store the amount of time until the specified timer, `timerid`, expires and the reload value of the timer into the space pointed to by the `value` argument. The `it_value` member of this structure will contain the amount of time before the timer expires, or zero if the timer is disarmed. This value is returned as the interval until timer expiration, even if the timer was armed with absolute time. The `it_interval` member of `value` will contain the reload value last set by `timer_settime()`.

Due to the asynchronous operation of this function, the time reported by this function could be significantly more than that actual time remaining on the timer at any time.

Input Parameters:

- `timerid`. Specifies pre-thread timer, previously created by the call to t`imer_create()`, whose remaining count will be returned.

Returned Values:

If successful, the timer_gettime() function will return zero (OK). Otherwise, an non-zero error number will be returned to indicate the error:

- `EINVAL`. The `timerid` argument does not correspond to an ID returned by `timer_create()` but not yet deleted by `timer_delete()`.

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.7.11 timer_getoverrun

Function Prototype:

        #include <time.h>
        int timer_getoverrun(timer_t timerid);

Description: Only a single signal will be queued to the process for a given timer at any point in time. When a timer for which a signal is still pending expires, no signal will be queued, and a timer overrun will occur. When a timer expiration signal is delivered to or accepted by a process, if the implementation supports the Realtime Signals Extension, the `timer_getoverrun()` function will return the timer expiration overrun count for the specified timer. The overrun count returned contains the number of extra timer expirations that occurred between the time the signal was generated (queued) and when it was delivered or accepted, up to but not including an implementation-defined maximum of `DELAYTIMER_MAX`. If the number of such extra expirations is greater than or equal to `DELAYTIMER_MAX`, then the overrun count will be set to `DELAYTIMER_MAX`. The value returned by `timer_getoverrun()` will apply to the most recent expiration signal delivery or acceptance for the timer. If no expiration signal has been delivered for the timer, or if the Realtime Signals Extension is not supported, the return value of `timer_getoverrun()` is unspecified.

NOTE: This interface is not currently implemented in NuttX.

Input Parameters:

- `timerid`. Specifies pre-thread timer, previously created by the call to `timer_create()`, whose overrun count will be returned.

Returned Values: If the `timer_getoverrun()` function succeeds, it will return the timer expiration overrun count as explained above. `timer_getoverrun()` will fail if:

- `EINVAL`. The `timerid` argument does not correspond to an ID returned by `timer_create()` but not yet deleted by `timer_delete()`.

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the full POSIX implementation include:

- This interface is not currently implemented by NuttX.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

## 2.8 Signal Interfaces

NuttX provides signal interfaces for tasks. Signals are used to alter the flow control of tasks by communicating asynchronous events within or between task contexts. Any task or interrupt handler can post (or send) a signal to a particular task. The task being signaled will execute task-specified signal handler function the next time that the task has priority. The signal handler is a user-supplied function that is bound to a specific signal and performs whatever actions are necessary whenever the signal is received.

There are no predefined actions for any signal. The default action for all signals (i.e., when no signal handler has been supplied by the user) is to ignore the signal. In this sense, all NuttX are real time signals.

Tasks may also suspend themselves and wait until a signal is received.

The following signal handling interfaces are provided by NuttX:

- 2.8.1 sigemptyset
- 2.8.2 sigfillset
- 2.8.3 sigaddset
- 2.8.4 sigdelset
- 2.8.5 sigismember
- 2.8.6 sigaction
- 2.8.7 sigprocmask
- 2.8.8 sigpending
- 2.8.9 sigsuspend
- 2.8.10 sigwaitinfo
- 2.8.11 sigtimedwait
- 2.8.12 sigqueue
- 2.8.13 kill

### 2.8.1 sigemptyset

Function Prototype:

        #include <signal.h>
        int sigemptyset(sigset_t *set);

Description: This function initializes the signal set specified by set such that all signals are excluded.

Input Parameters:

- set. Signal set to initialize.

Returned Values:

- 0 (OK), or -1 (ERROR) if the signal set cannot be initialized.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.2 sigfillset

Function Prototype:

        #include <signal.h>
        int sigfillset(sigset_t *set);

Description: This function initializes the signal set specified by set such that all signals are included.

Input Parameters:

- set. Signal set to initialize

Returned Values:

- 0 (OK), or -1 (ERROR) if the signal set cannot be initialized.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.3 sigaddset

Function Prototype:

        #include <signal.h>
        int sigaddset(sigset_t *set, int signo);

Description: This function adds the signal specified by signo to the signal set specified by set.

Input Parameters:

- set. Signal set to add signal to
- signo. Signal to add

Returned Values:

- 0 (OK), or -1 (ERROR) if the signal number is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.4 sigdelset

Function Prototype:

        #include <signal.h>
        int sigdelset(sigset_t *set, int signo);

Description: This function deletes the signal specified by signo from the signal set specified by set.

Input Parameters:

- set. Signal set to delete the signal from
- signo. Signal to delete

Returned Values:

- 0 (OK), or -1 (ERROR) if the signal number is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.5 sigismember

Function Prototype:

        #include <signal.h>
        int  sigismember(const sigset_t *set, int signo);

Description: This function tests whether the signal specified by signo is a member of the set specified by set.

Input Parameters:

- set. Signal set to test
- signo. Signal to test for

Returned Values:

- 1 (TRUE), if the specified signal is a member of the set,
- 0 (OK or FALSE), if it is not, or
- -1 (ERROR) if the signal number is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.6 sigaction

Function Prototype:

        #include <signal.h>
        int sigaction( int signo, const struct sigaction *act,
                       struct sigaction *oact );

Description: This function allows the calling task to examine and/or specify the action to be associated with a specific signal.

The structure sigaction, used to describe an action to be taken, is defined to include the following members:

- sa_u.sa_handler. A pointer to a signal-catching function.
- sa_u.sa_sigaction. An alternative form for the signal catching function.
- sa_mask. Additional set of signals to be blocked during execution of the signal-catching function.
- sa_flags: Special flags to affect behavior of a signal.

If the argument act is not NULL, it points to a structure specifying the action to be associated with the specified signal. If the argument oact is not NULL, the action previously associated with the signal is stored in the location pointed to by the argument oact. If the argument act is NULL, signal handling is unchanged by this function call; thus, the call can be used to enquire about the current handling of a given signal.

When a signal is caught by a signal-catching function installed by the sigaction() function, a new signal mask is calculated and installed for the duration of the signal-catching function. This mask is formed by taking the union of the current signal mask and the value of the sa_mask for the signal being delivered, and then including the signal being delivered. If and when the signal handler returns, the original signal mask is restored.

Signal catching functions execute in the same address environment as the task that called sigaction() to install the signal-catching function.

Once an action is installed for a specific signal, it remains installed until another action is explicitly requested by another call to sigaction().

Input Parameters:

- sig. Signal of interest
- act. Location of new handler
- oact. Location to store old handler

Returned Values:

- 0 (OK), or -1 (ERROR) if the signal number is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the POSIX implementation include:

- Special values of sa_handler in the struct sigaction act input not handled (SIG_DFL, SIG_IGN).
- All sa_flags in struct sigaction of act input are ignored (all treated like SA_SIGINFO).

### 2.8.7 sigprocmask

Function Prototype:

        #include <signal.h>
        int sigprocmask(int how, const sigset_t *set, sigset_t *oset);

Description: This function allows the calling task to examine and/or change its signal mask. If the set is not NULL, then it points to a set of signals to be used to change the currently blocked set. The value of how indicates the manner in which the set is changed.

If there are any pending unblocked signals after the call to sigprocmask(), those signals will be delivered before sigprocmask() returns.

If sigprocmask() fails, the signal mask of the task is not changed.

Input Parameters:

- how. How the signal mast will be changed:
  - SIG_BLOCK. The resulting set is the union of the current set and the signal set pointed to by the set input parameter.
  - SIG_UNBLOCK. The resulting set is the intersection of the current set and the complement of the signal set pointed to by the set input parameter.
  - SIG_SETMASK. The resulting set is the signal set pointed to by the set input parameter.
- set. Location of the new signal mask
- oset. Location to store the old signal mask

Returned Values:

- 0 (OK), or -1 (ERROR) if how is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.8 sigpending

Function Prototype:

        #include <signal.h>
        int sigpending( sigset_t *set );

Description: This function stores the returns the set of signals that are blocked for delivery and that are pending for the calling task in the space pointed to by set.

If the task receiving a signal has the signal blocked via its sigprocmask, the signal will pend until it is unmasked. Only one pending signal (for a given signo) is retained by the system. This is consistent with POSIX which states: "If a subsequent occurrence of a pending signal is generated, it is implementation defined as to whether the signal is delivered more than once."

Input Parameters:

- set. The location to return the pending signal set.

Returned Values:

- 0 (OK) or -1 (ERROR)

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.9 sigsuspend

Function Prototype:

        #include <signal.h>
        int sigsuspend( const sigset_t *set );

Description: The sigsuspend() function replaces the signal mask with the set of signals pointed to by the argument set and then suspends the task until delivery of a signal to the task.

If the effect of the set argument is to unblock a pending signal, then no wait is performed.

The original signal mask is restored when sigsuspend() returns.

Waiting for an empty signal set stops a task without freeing any resources (a very bad idea).

Input Parameters:

- set. The value of the signal mask to use while suspended.

Returned Values:

- -1 (ERROR) always

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the POSIX specification include:

- POSIX does not indicate that the original signal mask is restored.
- POSIX states that sigsuspend() "suspends the task until delivery of a signal whose action is either to execute a signal-catching function or to terminate the task." Only delivery of the signal is required in the present implementation (even if the signal is ignored).

### 2.8.10 sigwaitinfo

Function Prototype:

        #include <signal.h>
        int sigwaitinfo(const sigset_t *set, struct siginfo *info);

Description: This function is equivalent to sigtimedwait() with a NULL timeout parameter. (see below).

Input Parameters:

- set. The set of pending signals to wait for.
- info. The returned signal values

Returned Values:

- Signal number that cause the wait to be terminated, otherwise -1 (ERROR) is returned.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.8.11 sigtimedwait

Function Prototype:

        #include <signal.h>
        int sigtimedwait( const sigset_t *set, struct siginfo *info,
                          const struct timespec *timeout );

Description: This function selects the pending signal set specified by the argument set. If multiple signals are pending in set, it will remove and return the lowest numbered one. If no signals in set are pending at the time of the call, the calling task will be suspended until one of the signals in set becomes pending OR until the task interrupted by an unblocked signal OR until the time interval specified by timeout (if any), has expired. If timeout is NULL, then the timeout interval is forever.

If the info argument is non-NULL, the selected signal number is stored in the si_signo member and the cause of the signal is store in the si_code member. The content of si_value is only meaningful if the signal was generated by sigqueue(). The following values for si_code are defined in signal.h:

- SI_USER. Signal sent from kill, raise, or abort
- SI_QUEUE. Signal sent from sigqueue
- SI_TIMER. Signal is result of timer expiration
- SI_ASYNCIO. Signal is the result of asynch IO completion
- SI_MESGQ. Signal generated by arrival of a message on an empty message queue.

Input Parameters:

- set. The set of pending signals to wait for.
- info. The returned signal values
- timeout. The amount of time to wait

Returned Values:

- Signal number that cause the wait to be terminated, otherwise -1 (ERROR) is returned.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the POSIX interface include:

- Values for si_codes differ
- No mechanism to return cause of ERROR. (It can be inferred from si_code in a non-standard way).
- POSIX states that "If no signal is pending at the time of the call, the calling task shall be suspended until one or more signals in set become pending or until it is interrupted by an unblocked, caught signal." The present implementation does not require that the unblocked signal be caught; the task will be resumed even if the unblocked signal is ignored.

### 2.8.12 sigqueue

Function Prototype:

        #include <signal.h>
        int sigqueue (int tid, int signo, union sigval value);

Description: This function sends the signal specified by signo with the signal parameter value to the task specified by tid.

If the receiving task has the signal blocked via its sigprocmask, the signal will pend until it is unmasked. Only one pending signal (for a given signo) is retained by the system. This is consistent with POSIX which states: "If a subsequent occurrence of a pending signal is generated, it is implementation defined as to whether the signal is delivered more than once."

Input Parameters:

- tid. ID of the task to receive signal
- signo. Signal number
- value. Value to pass to task with signal

Returned Values:

- On success (at least one signal was sent), zero (OK) is returned. On error, -1 (ERROR) is returned, and `errno` is set appropriately.
  - `EGAIN`. The limit of signals which may be queued has been reached.
  - `EINVAL`. signo was invalid.
  - `EPERM`. The task does not have permission to send the signal to the receiving process.
  - `ESRCH`. No process has a PID matching pid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the POSIX interface include:

- Default action is to ignore signals.
- Signals are processed one at a time in order
- POSIX states that, "If signo is zero (the null signal), error checking will be performed but no signal is actually sent." There is no null signal in the present implementation; a zero signal will be sent.

### 2.8.13 kill

Function Prototype:

       #include <sys/types.h>
       #include <signal.h>
       int kill(pid_t pid, int sig);

Description: The kill() system call can be used to send any signal to any task.

If the receiving task has the signal blocked via its sigprocmask, the signal will pend until it is unmasked. Only one pending signal (for a given signo) is retained by the system. This is consistent with POSIX which states: "If a subsequent occurrence of a pending signal is generated, it is implementation defined as to whether the signal is delivered more than once."

Input Parameters:

- pid. The id of the task to receive the signal. The POSIX `kill()` specification encodes process group information as zero and negative pid values. Only positive, non-zero values of pid are supported by this implementation. ID of the task to receive signal
- signo. The signal number to send. If signo is zero, no signal is sent, but all error checking is performed.

Returned Values:

- OK or ERROR

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Differences from the POSIX interface include:

- Default action is to ignore signals.
- Signals are processed one at a time in order
- Sending of signals to 'process groups' is not supported in NuttX.

## 2.9 Pthread Interfaces

NuttX does not support processes in the way that, say, Linux does. NuttX only supports simple threads or tasks running within the same address space. For the most part, threads and tasks are interchangeable and differ primarily only in such things as the inheritance of file descriptors. Basically, threads are initialized and uninitialized differently and share a few more resources than tasks.

The following pthread interfaces are supported in some form by NuttX:

- 2.9.1 pthread_attr_init
- 2.9.2 pthread_attr_destroy
- 2.9.3 pthread_attr_setschedpolicy
- 2.9.4 pthread_attr_getschedpolicy
- 2.9.5 pthread_attr_setschedparam
- 2.9.6 pthread_attr_getschedparam
- 2.9.7 pthread_attr_setinheritsched
- 2.9.8 pthread_attr_getinheritsched
- 2.9.9 pthread_attr_setstacksize
- 2.9.10 pthread_attr_getstacksize
- 2.9.11 pthread_create
- 2.9.12 pthread_detach
- 2.9.13 pthread_exit
- 2.9.14 pthread_cancel
- 2.9.15 pthread_setcancelstate
- 2.9.16 pthread_testcancelstate
- 2.9.17 pthread_join
- 2.9.18 pthread_yield
- 2.9.19 pthread_self
- 2.9.20 pthread_getschedparam
- 2.9.21 pthread_setschedparam
- 2.9.22 pthread_key_create
- 2.9.23 pthread_setspecific
- 2.9.24 pthread_getspecific
- 2.9.25 pthread_key_delete
- 2.9.26 pthread_mutexattr_init
- 2.9.27 pthread_mutexattr_destroy
- 2.9.28 pthread_mutexattr_getpshared
- 2.9.29 pthread_mutexattr_setpshared
- 2.9.30 pthread_mutexattr_gettype
- 2.9.31 pthread_mutexattr_settype
- 2.9.32 pthread_mutex_init
- 2.9.33 pthread_mutex_destroy
- 2.9.34 pthread_mutex_lock
- 2.9.35 pthread_mutex_trylock
- 2.9.36 pthread_mutex_unlock
- 2.9.37 pthread_condattr_init
- 2.9.38 pthread_condattr_destroy
- 2.9.39 pthread_cond_init
- 2.9.40 pthread_cond_destroy
- 2.9.41 pthread_cond_broadcast
- 2.9.42 pthread_cond_signal
- 2.9.43 pthread_cond_wait
- 2.9.44 pthread_cond_timedwait
- 2.9.45 pthread_barrierattr_init
- 2.9.46 pthread_barrierattr_destroy
- 2.9.47 pthread_barrierattr_setpshared
- 2.9.48 pthread_barrierattr_getpshared
- 2.9.49 pthread_barrier_init
- 2.9.50 pthread_barrier_destroy
- 2.9.51 pthread_barrier_wait
- 2.9.52 pthread_once
- 2.9.53 pthread_kill
- 2.9.54 pthread_sigmask

No support for the following pthread interfaces is provided by NuttX:

- `pthread_atfork`. register fork handlers.
- `pthread_attr_getdetachstate`. get and set the detachstate attribute.
- `pthread_attr_getguardsize`. get and set the thread guardsize attribute.
- `pthread_attr_getinheritsched`. get and set the inheritsched attribute.
- `pthread_attr_getscope`. get and set the contentionscope attribute.
- `pthread_attr_getstack`. get and set stack attributes.
- `pthread_attr_getstackaddr`. get and set the stackaddr attribute.
- `pthread_attr_setdetachstate`. get and set the detachstate attribute.
- `pthread_attr_setguardsize`. get and set the thread guardsize attribute.
- `pthread_attr_setscope`. get and set the contentionscope attribute.
- `pthread_attr_setstack`. get and set stack attributes.
- `pthread_attr_setstackaddr`. get and set the stackaddr attribute.
- `pthread_barrier_destroy`. destroy and initialize a barrier object.
- `pthread_barrier_init`. destroy and initialize a barrier object.
- `pthread_barrier_wait`. synchronize at a barrier.
- `pthread_cleanup_pop`. establish cancellation handlers.
- `pthread_cleanup_push`. establish cancellation handlers.
- `pthread_condattr_getclock`. set the clock selection condition variable attribute.
- `pthread_condattr_getpshared`. get the process-shared condition variable attribute.
- `pthread_condattr_setclock`. set the clock selection condition variable attribute.
- `pthread_condattr_setpshared`. set the process-shared condition variable attribute.
- `pthread_getconcurrency`. get and set the level of concurrency.
- `pthread_getcpuclockid`. access a thread CPU-time clock.
- `pthread_mutex_getprioceiling`. get and set the priority ceiling of a mutex.
- `pthread_mutex_setprioceiling`. get and set the priority ceiling of a mutex.
- `pthread_mutex_timedlock`. lock a mutex.
- `pthread_mutexattr_getprioceiling`. get and set the prioceiling attribute of the mutex attributes object.
- `pthread_mutexattr_getprotocol`. get and set the protocol attribute of the mutex attributes object.
- `pthread_mutexattr_setprioceiling`. get and set the prioceiling attribute of the mutex attributes object.
- `pthread_mutexattr_setprotocol`. get and set the protocol attribute of the mutex attributes object.
- `pthread_rwlock_destroy`. destroy and initialize a read-write lock object.
- `pthread_rwlock_init`. destroy and initialize a read-write lock object.
- `pthread_rwlock_rdlock`. lock a read-write lock object for reading.
- `pthread_rwlock_timedrdlock`. lock a read-write lock for reading.
- `pthread_rwlock_timedwrlock`. lock a read-write lock for writing.
- `pthread_rwlock_tryrdlock`. lock a read-write lock object for reading.
- `pthread_rwlock_trywrlock`. lock a read-write lock object for writing.
- `pthread_rwlock_unlock`. unlock a read-write lock object.
- `pthread_rwlock_wrlock`. lock a read-write lock object for writing.
- `pthread_rwlockattr_destroy`. destroy and initialize the read-write lock attributes object.
- `pthread_rwlockattr_getpshared`. get and set the process-shared attribute of the read-write lock attributes object.
- `pthread_rwlockattr_init`. destroy and initialize the read-write lock attributes object.
- `pthread_rwlockattr_setpshared`. get and set the process-shared attribute of the read-write lock attributes object.
- `pthread_setcanceltype`. set cancelability state.
- `pthread_setconcurrency`. get and set the level of concurrency.
- `pthread_spin_destroy`. destroy or initialize a spin lock object.
- `pthread_spin_init`. destroy or initialize a spin lock object.
- `pthread_spin_lock`. lock a spin lock object.
- `pthread_spin_trylock`. lock a spin lock object.
- `pthread_spin_unlock`. unlock a spin lock object.
- `pthread_testcancel`. set cancelability state.

### 2.9.1 pthread_attr_init

Function Prototype:

        #include <pthread.h>
        int pthread_attr_init(pthread_attr_t *attr);

Description: Initializes a thread attributes object (attr) with default values for all of the individual attributes used by the implementation.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_init() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.2 pthread_attr_destroy

Function Prototype:

        #include <pthread.h>
        int pthread_attr_destroy(pthread_attr_t *attr);

Description: An attributes object can be deleted when it is no longer needed.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_destroy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.3 pthread_attr_setschedpolicy

Function Prototype:

        #include <pthread.h>
        int pthread_attr_setschedpolicy(pthread_attr_t *attr, int policy);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_setschedpolicy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.4 pthread_attr_getschedpolicy

Function Prototype:

        #include <pthread.h>
        int pthread_attr_getschedpolicy(pthread_attr_t *attr, int *policy);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_getschedpolicy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.5 pthread_attr_getschedpolicy

Function Prototype:

       #include <pthread.h>
        int pthread_attr_setschedparam(pthread_attr_t *attr,
    				      const struct sched_param *param);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_getschedpolicy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.6 pthread_attr_getschedparam

Function Prototype:

       #include <pthread.h>
         int pthread_attr_getschedparam(pthread_attr_t *attr,
    				      struct sched_param *param);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_getschedparam() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.7 pthread_attr_setinheritsched

Function Prototype:

       #include <pthread.h>
        int pthread_attr_setinheritsched(pthread_attr_t *attr,
    					int inheritsched);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_setinheritsched() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.8 pthread_attr_getinheritsched

Function Prototype:

       #include <pthread.h>
         int pthread_attr_getinheritsched(const pthread_attr_t *attr,
    					int *inheritsched);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_getinheritsched() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.9 pthread_attr_setstacksize

Function Prototype:

       #include <pthread.h>
        int pthread_attr_setstacksize(pthread_attr_t *attr, long stacksize);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_setstacksize() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.10 pthread_attr_getstacksize

Function Prototype:

        #include <pthread.h>
       int pthread_attr_getstacksize(pthread_attr_t *attr, long *stackaddr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_attr_getstacksize() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.11 pthread_create

Function Prototype:

        #include <pthread.h>
        int pthread_create(pthread_t *thread, pthread_attr_t *attr,
    			  pthread_startroutine_t startRoutine,
    			  pthread_addr_t arg);

Description: To create a thread object and runnable thread, a routine must be specified as the new thread's start routine. An argument may be passed to this routine, as an untyped address; an untyped address may also be returned as the routine's value. An attributes object may be used to specify details about the kind of thread being created.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_create() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.12 pthread_detach

Function Prototype:

        #include <pthread.h>
        int pthread_detach(pthread_t thread);

Description: A thread object may be "detached" to specify that the return value and completion status will not be requested.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_detach() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.13 pthread_exit

Function Prototype:

        #include <pthread.h>
        void pthread_exit(pthread_addr_t pvValue);

Description: A thread may terminate it's own execution.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_exit() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.14 pthread_cancel

Function Prototype:

        #include <pthread.h>
        int pthread_cancel(pthread_t thread);

Description:

The pthread_cancel() function shall request that thread be canceled. The target thread's cancelability state determines when the cancellation takes effect. When the cancellation is acted on, thread shall be terminated.

When cancelability is disabled, all cancels are held pending in the target thread until the thread changes the cancelability. When cancelability is deferred, all cancels are held pending in the target thread until the thread changes the cancelability or calls pthread_testcancel().

Cancelability is asynchronous; all cancels are acted upon immediately (when enable), interrupting the thread with its processing.

Input Parameters:

- thread. Identifies the thread to be canceled.

Returned Values:

If successful, the ptnread_cancel() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- ESRCH. No thread could be found corresponding to that specified by the given thread ID.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name. Except:

- The thread-specific data destructor functions shall be called for thread. However, these destructors are not currently supported.
- Cancellation types are not supported. The thread will be canceled at the time that pthread_cancel() is called or, if cancelation is disabled, at the time when cancelation is re-enabled.
- `pthread_testcancel()` is not supported.
- Thread cancellation at cancellation points is not supported.

### 2.9.15 pthread_setcancelstate

Function Prototype:

        #include <pthread.h>
        int pthread_setcancelstate(int state, int *oldstate);

Description:

The pthread_setcancelstate() function atomically sets both the calling thread's cancelability state to the indicated state and returns the previous cancelability state at the location referenced by oldstate. Legal values for state are PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DISABLE.<.li>

Any pending thread cancelation may occur at the time that the cancelation state is set to PTHREAD_CANCEL_ENABLE.

Input Parameters:

- state New cancelation state. One of PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE.<.li>
- oldstate. Location to return the previous cancelation state.

Returned Values:

If successful, the pthread_setcancelstate() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- ESRCH. No thread could be found corresponding to that specified by the given thread ID.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.16 pthread_testcancelstate

Function Prototype:

        #include <pthread.h>
        int pthread_setcancelstate(void);

Description:

NOT SUPPORTED Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_setcancelstate() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.17 pthread_join

Function Prototype:

        #include <pthread.h>
        int pthread_join(pthread_t thread, pthread_addr_t *ppvValue);

Description: A thread can await termination of another thread and retrieve the return value of the thread.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_join() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.18 pthread_yield

Function Prototype:

        #include <pthread.h>
        void pthread_yield(void);

Description: A thread may tell the scheduler that its processor can be made available.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_yield() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.19 pthread_self

Function Prototype:

        #include <pthread.h>
        pthread_t pthread_self(void);

Description: A thread may obtain a copy of its own thread handle.

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_self() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.20 pthread_getschedparam

Function Prototype:

        #include <pthread.h>
        int pthread_getschedparam(pthread_t thread, int *policy,
                                  struct sched_param *param);

Description: The `pthread_getschedparam()` functions will get the scheduling policy and parameters of threads. For `SCHED_FIFO` and `SCHED_RR`, the only required member of the `sched_param` structure is the priority `sched_priority`.

The `pthread_getschedparam()` function will retrieve the scheduling policy and scheduling parameters for the thread whose thread ID is given by `thread` and will store those values in `policy` and `param`, respectively. The priority value returned from `pthread_getschedparam()` will be the value specified by the most recent `pthread_setschedparam()`, `pthread_setschedprio()`, or `pthread_create()` call affecting the target thread. It will not reflect any temporary adjustments to its priority (such as might result of any priority inheritance, for example).

The policy parameter may have the value `SCHED_FIFO` or `SCHED_RR` (`SCHED_OTHER` and `SCHED_SPORADIC`, in particular, are not supported). The `SCHED_FIFO` and `SCHED_RR`policies will have a single scheduling parameter, `sched_priority`.``

Input Parameters:

- `thread`. The ID of thread whose scheduling parameters will be queried.
- `policy`. The location to store the thread's scheduling policy.
- `param`. The location to store the thread's priority.

Returned Values: 0 (`OK`) if successful. Otherwise, the error code `ESRCH` if the value specified by `thread` does not refer to an existing thread.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.21 pthread_setschedparam

Function Prototype:

        #include <pthread.h>
        int pthread_setschedparam(pthread_t thread, int policy,
                                  const struct sched_param *param);

Description: The `pthread_setschedparam()` functions will set the scheduling policy and parameters of threads. For `SCHED_FIFO` and `SCHED_RR`, the only required member of the `sched_param` structure is the priority `sched_priority`.

The `pthread_setschedparam()` function will set the scheduling policy and associated scheduling parameters for the thread whose thread ID is given by `thread` to the policy and associated parameters provided in `policy` and `param`, respectively.

The policy parameter may have the value `SCHED_FIFO` or `SCHED_RR`. (`SCHED_OTHER` and `SCHED_SPORADIC`, in particular, are not supported). The `SCHED_FIFO` and `SCHED_RR` policies will have a single scheduling parameter, `sched_priority`.

If the `pthread_setschedparam()` function fails, the scheduling parameters will not be changed for the target thread.

Input Parameters:

- `thread`. The ID of thread whose scheduling parameters will be modified.
- `policy`. The new scheduling policy of the thread. Either `SCHED_FIFO` or `SCHED_RR`. `SCHED_OTHER`and `SCHED_SPORADIC`are not supported.``````
- `param`. The location to store the thread's priority.

Returned Values:

If successful, the pthread_setschedparam() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `EINVAL`. The value specified by `policy` or one of the scheduling parameters associated with the scheduling policy `policy` is invalid.
- `ENOTSUP`. An attempt was made to set the policy or scheduling parameters to an unsupported value (`SCHED_OTHER` and `SCHED_SPORADIC` in particular are not supported)
- `EPERM`. The caller does not have the appropriate permission to set either the scheduling parameters or the scheduling policy of the specified thread. Or, the implementation does not allow the application to modify one of the parameters to the value specified.
- `ESRCH`. The value specified by thread does not refer to a existing thread.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.22 pthread_key_create

Function Prototype:

        #include <pthread.h>
        int pthread_key_create( pthread_key_t *key, void (*destructor)(void*) )

Description:

This function creates a thread-specific data key visible to all threads in the system. Although the same key value may be used by different threads, the values bound to the key by pthread_setspecific() are maintained on a per-thread basis and persist for the life of the calling thread.

Upon key creation, the value NULL will be associated with the the new key in all active threads. Upon thread creation, the value NULL will be associated with all defined keys in the new thread.

Input Parameters:

- key is a pointer to the key to create.
- destructor is an optional destructor() function that may be associated with each key that is invoked when a thread exits. However, this argument is ignored in the current implementation.

Returned Values:

If successful, the pthread_key_create() function will store the newly created key value at *key and return zero (OK). Otherwise, an error number will be returned to indicate the error:

- EAGAIN. The system lacked sufficient resources to create another thread-specific data key, or the system-imposed limit on the total number of keys per task {PTHREAD_KEYS_MAX} has been exceeded
- ENONMEM Insufficient memory exists to create the key.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

- The present implementation ignores the destructor argument.

### 2.9.23 pthread_setspecific

Function Prototype:

        #include <pthread.h>
        int pthread_setspecific( pthread_key_t key, void *value )

Description:

The pthread_setspecific() function associates a thread- specific value with a key obtained via a previous call to pthread_key_create(). Different threads may bind different values to the same key. These values are typically pointers to blocks of dynamically allocated memory that have been reserved for use by the calling thread.

The effect of calling pthread_setspecific() with a key value not obtained from pthread_key_create() or after a key has been deleted with pthread_key_delete() is undefined.

Input Parameters:

- key. The data key to set the binding for.
- value. The value to bind to the key.

Returned Values:

If successful, pthread_setspecific() will return zero (OK). Otherwise, an error number will be returned:

- ENOMEM. Insufficient memory exists to associate the value with the key.
- EINVAL. The key value is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

- pthread_setspecific() may be called from a thread-specific data destructor function.

### 2.9.24 pthread_getspecific

Function Prototype:

        #include <pthread.h>
        void *pthread_getspecific( pthread_key_t key )

Description:

The pthread_getspecific() function returns the value currently bound to the specified key on behalf of the calling thread.

The effect of calling pthread_getspecific() with a key value not obtained from pthread_key_create() or after a key has been deleted with pthread_key_delete() is undefined.

Input Parameters:

- key. The data key to get the binding for.

Returned Values:

The function pthread_getspecific() returns the thread- specific data associated with the given key. If no thread specific data is associated with the key, then the value NULL is returned.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

- pthread_getspecific() may be called from a thread-specific data destructor function.

### 2.9.25 pthread_key_delete

Function Prototype:

        #include <pthread.h>
        int pthread_key_delete( pthread_key_t key )

Description:

This POSIX function should delete a thread-specific data key previously returned by pthread_key_create(). However, this function does nothing in the present implementation.

Input Parameters:

- key. The key to delete

Returned Values:

- Always returns EINVAL.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.26 pthread_mutexattr_init

Function Prototype:

        #include <pthread.h>
        int pthread_mutexattr_init(pthread_mutexattr_t *attr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_mutexattr_init() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.27 pthread_mutexattr_destroy

Function Protoype:

        #include <pthread.h>
        int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_mutexattr_destroy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.28 pthread_mutexattr_getpshared

Function Prototype:

        #include <pthread.h>
        int pthread_mutexattr_getpshared(pthread_mutexattr_t *attr,
    					int *pshared);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_mutexattr_getpshared() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.29 pthread_mutexattr_setpshared

Function Prototype:

        #include <pthread.h>
       int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr,
    					int pshared);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_mutexattr_setpshared() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.30 pthread_mutexattr_gettype

Function Prototype:

        #include <pthread.h>
    #ifdef CONFIG_MUTEX_TYPES
        int pthread_mutexattr_gettype(const pthread_mutexattr_t *attr, int *type);
    #endif

Description: Return the mutex type from the mutex attributes.

Input Parameters:

- `attr`. The mutex attributes to query
- `type`. Location to return the mutex type. See `pthread_mutexattr_setttyp()` for a description of possible mutex types that may be returned.

Returned Values:

If successful, the pthread_mutexattr_settype() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `EINVAL`. Parameters `attr` and/or `attr` are invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.31 pthread_mutexattr_settype

Function Prototype:

        #include <pthread.h>
    #ifdef CONFIG_MUTEX_TYPES
        int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type);
    #endif

Description: Set the mutex type in the mutex attributes.

Input Parameters:

- `attr`. The mutex attributes in which to set the mutex type.
- `type`. The mutex type value to set. The following values are supported:
  - `PTHREAD_MUTEX_NORMAL`. This type of mutex does not detect deadlock. A thread attempting to relock this mutex without first unlocking it will deadlock. Attempting to unlock a mutex locked by a different thread results in undefined behavior. Attempting to unlock an unlocked mutex results in undefined behavior.
  - `PTHREAD_MUTEX_ERRORCHECK`. This type of mutex provides error checking. A thread attempting to relock this mutex without first unlocking it will return with an error. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.
  - `PTHREAD_MUTEX_RECURSIVE`. A thread attempting to relock this mutex without first unlocking it will succeed in locking the mutex. The relocking deadlock which can occur with mutexes of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex require the same number of unlocks to release the mutex before another thread can acquire the mutex. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.
  - `PTHREAD_MUTEX_DEFAULT`. The default mutex type (PTHREAD_MUTEX_NORMAL).

  In NuttX, `PTHREAD_MUTEX_NORMAL` is not implemented. Rather, the behavior described for `PTHREAD_MUTEX_ERRORCHECK` is the normal behavior.

Returned Values:

If successful, the pthread_mutexattr_settype() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `EINVAL`. Parameters `attr` and/or `attr` are invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.32 pthread_mutex_init

Function Prototype:

        #include <pthread.h>
        int pthread_mutex_init(pthread_mutex_t *mutex,
    			      pthread_mutexattr_t *attr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_mutex_init() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.33 pthread_mutex_destroy

Function Prototype:

        #include <pthread.h>
        int pthread_mutex_destroy(pthread_mutex_t *mutex);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_mutex_destroy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.34 pthread_mutex_lock

Function Prototype:

        #include <pthread.h>
        int pthread_mutex_lock(pthread_mutex_t *mutex);

Description: The mutex object referenced by mutex is locked by calling `pthread_mutex_lock()`. If the mutex is already locked, the calling thread blocks until the mutex becomes available. This operation returns with the mutex object referenced by mutex in the locked state with the calling thread as its owner.

If the mutex type is `PTHREAD_MUTEX_NORMAL`, deadlock detection is not provided. Attempting to relock the mutex causes deadlock. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior results.

In NuttX, `PTHREAD_MUTEX_NORMAL` is not implemented. Rather, the behavior described for `PTHREAD_MUTEX_ERRORCHECK` is the normal behavior.

If the mutex type is `PTHREAD_MUTEX_ERRORCHECK`, then error checking is provided. If a thread attempts to relock a mutex that it has already locked, an error will be returned. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error will be returned.

If the mutex type is `PTHREAD_MUTEX_RECURSIVE`, then the mutex maintains the concept of a lock count. When a thread successfully acquires a mutex for the first time, the lock count is set to one. Every time a thread relocks this mutex, the lock count is incremented by one. Each time the thread unlocks the mutex, the lock count is decremented by one. When the lock count reaches zero, the mutex becomes available for other threads to acquire. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error will be returned.

If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread resumes waiting for the mutex as if it was not interrupted.

Input Parameters:

- `mutex`. A reference to the mutex to be locked.

Returned Values:

If successful, the pthread_mutex_lock() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Note that this function will never return the error EINTR.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.35 pthread_mutex_trylock

Function Prototype:

        #include <pthread.h>
        int pthread_mutex_trylock(pthread_mutex_t *mutex);

Description: The function pthread_mutex_trylock() is identical to `pthread_mutex_lock()` except that if the mutex object referenced by mutex is currently locked (by any thread, including the current thread), the call returns immediately with the errno `EBUSY`.

If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread resumes waiting for the mutex as if it was not interrupted.

Input Parameters:

- `mutex`. A reference to the mutex to be locked.

Returned Values:

If successful, the pthread_mutex_trylock() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Note that this function will never return the error EINTR.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.36 pthread_mutex_unlock

Function Prototype:

        #include <pthread.h>
        int pthread_mutex_unlock(pthread_mutex_t *mutex);

Description:

The `pthread_mutex_unlock()` function releases the mutex object referenced by mutex. The manner in which a mutex is released is dependent upon the mutex's type attribute. If there are threads blocked on the mutex object referenced by mutex when `pthread_mutex_unlock()` is called, resulting in the mutex becoming available, the scheduling policy is used to determine which thread shall acquire the mutex. (In the case of `PTHREAD_MUTEX_RECURSIVE` mutexes, the mutex becomes available when the count reaches zero and the calling thread no longer has any locks on this mutex).

If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread resumes waiting for the mutex as if it was not interrupted.

Input Parameters:

- `param`.``

Returned Values:

If successful, the pthread_mutex_unlock() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Note that this function will never return the error EINTR.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.37 pthread_condattr_init

Function Prototype:

        #include <pthread.h>
        int pthread_condattr_init(pthread_condattr_t *attr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_condattr_init() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.38 pthread_condattr_destroy

Function Prototype:

        #include <pthread.h>
        int pthread_condattr_destroy(pthread_condattr_t *attr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_condattr_destroy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.39 pthread_cond_init

Function Prototype:

        #include <pthread.h>
        int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *attr);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_cond_init() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.40 pthread_cond_destroy

Function Prototype:

        #include <pthread.h>
        int pthread_cond_destroy(pthread_cond_t *cond);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_cond_destroy() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.41 pthread_cond_broadcast

Function Prototype:

        #include <pthread.h>
        int pthread_cond_broadcast(pthread_cond_t *cond);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_cond_broadcast() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.42 pthread_cond_signal

Function Prototype:

        #include <pthread.h>
        int pthread_cond_signal(pthread_cond_t *dond);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_cond_signal() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.43 pthread_cond_wait

Function Prototype:

        #include <pthread.h>
        int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the pthread_cond_wait() function will return zero (OK). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.44 pthread_cond_timedwait

Function Prototype:

        #include <pthread.h>
        int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex,
    				  const struct timespec *abstime);

Description:

Input Parameters:

- `To be provided`.

Returned Values:

If successful, the `pthread_cond_timedwait()` function will return zero (`OK`). Otherwise, an error number will be returned to indicate the error:

- `To be provided`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.45 pthread_barrierattr_init

Function Prototype:

        #include <pthread.h>
        int pthread_barrierattr_init(FAR pthread_barrierattr_t *attr);

Description: The `pthread_barrierattr_init()` function will initialize a barrier attribute object `attr` with the default value for all of the attributes defined by the implementation.

Input Parameters:

- `attr`. Barrier attributes to be initialized.

Returned Values: 0 (`OK`) on success or `EINVAL` if `attr` is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.46 pthread_barrierattr_destroy

Function Prototype:

        #include <pthread.h>
        int pthread_barrierattr_destroy(FAR pthread_barrierattr_t *attr);

Description: The `pthread_barrierattr_destroy()` function will destroy a barrier attributes object. A destroyed attributes object can be reinitialized using `pthread_barrierattr_init()`; the results of otherwise referencing the object after it has been destroyed are undefined.

Input Parameters:

- `attr`. Barrier attributes to be destroyed.

Returned Values: 0 (OK) on success or EINVAL if attr is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.47 pthread_barrierattr_setpshared

Function Prototype:

        #include <pthread.h>
        int pthread_barrierattr_setpshared(FAR pthread_barrierattr_t *attr, int pshared);

Description: The process-shared attribute is set to `PTHREAD_PROCESS_SHARED` to permit a barrier to be operated upon by any thread that has access to the memory where the barrier is allocated. If the process-shared attribute is `PTHREAD_PROCESS_PRIVATE`, the barrier can only be operated upon by threads created within the same process as the thread that initialized the barrier. If threads of different processes attempt to operate on such a barrier, the behavior is undefined. The default value of the attribute is `PTHREAD_PROCESS_PRIVATE`.

Input Parameters:

- `attr`. Barrier attributes to be modified.
- `pshared`. The new value of the pshared attribute.

Returned Values: 0 (`OK`) on success or `EINVAL` if either `attr` is invalid or `pshared` is not one of `PTHREAD_PROCESS_SHARED` or `PTHREAD_PROCESS_PRIVATE`.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.48 pthread_barrierattr_getpshared

Function Prototype:

        #include <pthread.h>
        int pthread_barrierattr_getpshared(FAR const pthread_barrierattr_t *attr, FAR int *pshared);

Description: The `pthread_barrierattr_getpshared()` function will obtain the value of the process-shared attribute from the attributes object referenced by `attr`.

Input Parameters:

- `attr`. Barrier attributes to be queried.
- `pshared`. The location to stored the current value of the pshared attribute.

Returned Values: 0 (`OK`) on success or `EINVAL` if either `attr` or `pshared` is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.49 pthread_barrier_init

Function Prototype:

        #include <pthread.h>
        int pthread_barrier_init(FAR pthread_barrier_t *barrier,
                                 FAR const pthread_barrierattr_t *attr, unsigned int count);

Description: The `pthread_barrier_init()` function allocates any resources required to use the barrier referenced by `barrier` and initialized the barrier with the attributes referenced by `attr`. If `attr` is NULL, the default barrier attributes will be used. The results are undefined if `pthread_barrier_init()` is called when any thread is blocked on the barrier. The results are undefined if a barrier is used without first being initialized. The results are undefined if `pthread_barrier_init()` is called specifying an already initialized barrier.

Input Parameters:

- `barrier`. The barrier to be initialized.
- `attr`. Barrier attributes to be used in the initialization.
- `count`. The count to be associated with the barrier. The count argument specifies the number of threads that must call `pthread_barrier_wait()` before any of them successfully return from the call. The value specified by count must be greater than zero.

Returned Values:0 (OK) on success or on of the following error numbers:

- `EAGAIN`. The system lacks the necessary resources to initialize another barrier.
- `EINVAL`. The barrier reference is invalid, or the values specified by attr are invalid, or the value specified by count is equal to zero.
- `ENOMEM`. Insufficient memory exists to initialize the barrier.
- `EBUSY`. The implementation has detected an attempt to reinitialize a barrier while it is in use.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.50 pthread_barrier_destroy

Function Prototype:

        #include <pthread.h>
        int pthread_barrier_destroy(FAR pthread_barrier_t *barrier);

Description: The `pthread_barrier_destroy()` function destroys the barrier referenced by `barrie` and releases any resources used by the barrier. The effect of subsequent use of the barrier is undefined until the barrier is reinitialized by another call to `pthread_barrier_init()`. The results are undefined if `pthread_barrier_destroy()` is called when any thread is blocked on the barrier, or if this function is called with an uninitialized barrier.

Input Parameters:

- `barrier`. The barrier to be destroyed.

Returned Values: 0 (`OK`) on success or on of the following error numbers:

- `EBUSY`. The implementation has detected an attempt to destroy a barrier while it is in use.
- `EINVAL`. The value specified by barrier is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.51 pthread_barrier_wait

Function Prototype:

        #include <pthread.h>
        int pthread_barrier_wait(FAR pthread_barrier_t *barrier);

Description: The `pthread_barrier_wait()` function synchronizse participating threads at the barrier referenced by `barrier`. The calling thread is blocked until the required number of threads have called `pthread_barrier_wait()` specifying the same `barrier`. When the required number of threads have called `pthread_barrier_wait()` specifying the `barrier`, the constant `PTHREAD_BARRIER_SERIAL_THREAD` will be returned to one unspecified thread and zero will be returned to each of the remaining threads. At this point, the barrier will be reset to the state it had as a result of the most recent `pthread_barrier_init()` function that referenced it.

The constant `PTHREAD_BARRIER_SERIAL_THREAD` is defined in `pthread.h` and its value must be distinct from any other value returned by `pthread_barrier_wait()`.

The results are undefined if this function is called with an uninitialized barrier.

If a signal is delivered to a thread blocked on a barrier, upon return from the signal handler the thread will resume waiting at the barrier if the barrier wait has not completed. Otherwise, the thread will continue as normal from the completed barrier wait. Until the thread in the signal handler returns from it, it is unspecified whether other threads may proceed past the barrier once they have all reached it.

A thread that has blocked on a barrier will not prevent any unblocked thread that is eligible to use the same processing resources from eventually making forward progress in its execution. Eligibility for processing resources will be determined by the scheduling policy.

Input Parameters:

- `barrier`. The barrier on which to wait.

Returned Values: 0 (`OK`) on success or `EINVAL` if the barrier is not valid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.52 pthread_once

Function Prototype:

        #include <pthread.h>
        int pthread_once(FAR pthread_once_t *once_control, CODE void (*init_routine)(void));

Description: The first call to `pthread_once()` by any thread with a given `once_control`, will call the `init_routine()` with no arguments. Subsequent calls to `pthread_once()` with the same `once_control` will have no effect. On return from `pthread_once()`, `init_routine()` will have completed.

Input Parameters:

- `once_control`. Determines if `init_routine()` should be called. `once_control` should be declared and intialized as follows:

      pthread_once_t once_control = PTHREAD_ONCE_INIT;

  `PTHREAD_ONCE_INIT` is defined in `pthread.h`.
- `init_routine`. The initialization routine that will be called once.

Returned Values: 0 (OK) on success or EINVAL if either once_control or init_routine are invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.53 pthread_kill

Function Prototype:

        #include <signal.h>
        #include <pthread.h>
        int pthread_kill(pthread_t thread, int signo)

Description: The `pthread_kill()` system call can be used to send any signal to a thread. See `kill()` for further information as this is just a simple wrapper around the `kill()` function.

Input Parameters:

- `thread`. The id of the thread to receive the signal. Only positive, non-zero values of `tthread`t are supported.
- `signo`. The signal number to send. If `signo` is zero, no signal is sent, but all error checking is performed.

Returned Values:

On success, the signal was sent and zero is returned. On error one of the following error numbers is returned.

- `EINVAL`. An invalid signal was specified.
- `EPERM`. The thread does not have permission to send the signal to the target thread.
- `ESRCH`. No thread could be found corresponding to that specified by the given thread ID.
- `ENOSYS`. Do not support sending signals to process groups.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

### 2.9.54 pthread_sigmask

Function Prototype:

        #include <signal.h>
        #include <pthread.h>
        int pthread_sigmask(int how, FAR const sigset_t *set, FAR sigset_t *oset);

Description: This function is a simple wrapper around `sigprocmask()`. See the `sigprocmask()` function description for further information.

Input Parameters:

- `how`. How the signal mast will be changed:
  - `SIG_BLOCK`: The resulting set is the union of the current set and the signal set pointed to by `set`.
  - `SIG_UNBLOCK`: The resulting set is the intersection of the current set and the complement of the signal set pointed to by `set`.
  - `SIG_SETMASK`: The resulting set is the signal set pointed to by `set`.
- `set`. Location of the new signal mask.
- `oset`. Location to store the old signal mask.

Returned Values:

0 (OK) on succes or EINVAL if `how` is invalid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name.

## 2.10 Environment Variables

Overview. NuttX supports environment variables that can be used to control the behavior of programs. In the spirit of NuttX the environment variable behavior attempts to emulate the behavior of environment variables in the mulit-processing OS:

- Task environments. When a new task is created using task_create, the environment of the child task is an inherited, exact copy of the environment of the parent. However, after child task has been created, subsequent operations by the child task on its environment does not alter the environment of the parent. No do operations by the parent effect the child's environment. The environments start identical but are independent and may diverge.
- Thread environments. When a pthread is created using pthread_create, the child thread also inherits that envirnment of the parent. However, the child does not recieve a copy of the environment but, rather, shares the same environment. Changes to the environment are visiable to all threads with the same parentage.

Programming Interfaces. The following environment variable programming interfaces are provided by Nuttx and are described in detail in the following paragraphs.

- 2.10.1 `getenv`
- 2.10.2 `putenv`
- 2.10.3 `clearenv`
- 2.10.4 `setenv`
- 2.10.5 `unsetenv`

Disabling Environment Variable Support. All support for environment variables can be disabled by setting `CONFIG_DISABLE_ENVIRON` in the board configuration file.

### 2.10.1 `getenv`

Function Prototype:

      #include <stdlib.h>
      FAR char *getenv(const char *name);

Description: The `getenv()` function searches the environment list for a string that matches the string pointed to by `name`.

Input Parameters:

- `name`. The name of the variable to find.

Returned Values: The value of the valiable (read-only) or NULL on failure.

### 2.10.2 `putenv`

Function Prototype:

      #include <stdlib.h>
      int putenv(char *string);

Description: The `putenv()` function adds or changes the value of environment variables. The argument string is of the form name=value. If name does not already exist in the environment, then string is added to the environment. If name does exist, then the value of name in the environment is changed to value.

Input Parameters:

- `string` name=value string describing the environment setting to add/modify.

Returned Values: Zero on sucess.

### 2.10.3 `clearenv`

Function Prototype:

      #include <stdlib.h>
      int clearenv(void);

Description: The `clearenv()` function clears the environment of all name-value pairs and sets the value of the external variable environ to NULL.

Input Parameters: None

Returned Values: Zero on success.

### 2.10.4 `setenv`

Function Prototype:

      #include <stdlib.h>
      int setenv(const char *name, const char *value, int overwrite);

Description: The `setenv()` function adds the variable `name` to the environment with the specified `value` if the variable `name` does not exist. If the `name` does exist in the environment, then its value is changed to `value` if `overwrite` is non-zero; if `overwrite` is zero, then the value of `name` is unaltered.

Input Parameters:

- `name` The name of the variable to change.
- `value` The new value of the variable.
- `value` Replace any existing value if non-zero.

Returned Values: Zero on success.

### 2.10.5 `unsetenv`

Function Prototype:

      #include <stdlib.h>
      int unsetenv(const char *name);

Description: The `unsetenv()` function deletes the variable `name` from the environment.

Input Parameters:

- `name` The name of the variable to delete.

Returned Values: Zero on success.

## 2.11 File System Interfaces

- 2.11.1 NuttX File System Overview
- 2.11.2 Driver Operations
- 2.11.3 Directory Operations
- 2.11.4 Standard I/O
- 2.11.5 Pipes and FIFOs
- 2.11.6 FAT File System Support
- 2.11.7 `mmap()` and eXecute In Place (XIP)

### 2.11.1 NuttX File System Overview

Overview. NuttX includes an optional, scalable file system. This file-system may be omitted altogther; NuttX does not depend on the presence of any file system.

Pseudo Root File System. Or, a simple in-memory, psuedo file system can be enabled. This simple file system can be enabled setting the CONFIG_NFILE_DESCRIPTORS option to a non-zero value. This is an in-memory file system because it does not require any storage medium or block driver support. Rather, file system contents are generated on-the-fly as referenced via standard file system operations (open, close, read, write, etc.). In this sense, the file system is psuedo file system (in the same sense that the Linux `/proc` file system is also referred to as a psuedo file system).

Any user supplied data or logic can be accessed via the psuedo-file system. Built in support is provided for character and block drivers in the `/dev` psuedo file system directory.

Mounted File Systems The simple in-memory file system can be extended my mounting block devices that provide access to true file systems backed up via some mass storage device. NuttX supports the standard `mount()` command that allows a block driver to be bound to a mountpoint within the psuedo file system and to a a file system. At present, NuttX supports only the VFAT file system.

Comparison to Linux From a programming perspective, the NuttX file system appears very similar to a Linux file system. However, there is a fundamental difference: The NuttX root file system is a psuedo file system and true file systems may be mounted in the psuedo file system. In the typical Linux installation by comparison, the Linux root file system is a true file system and psuedo file systems may be mounted in the true, root file system. The approach selected by NuttX is intended to support greater scalability from the very tiny platform to the moderate platform.

File System Interfaces. The NuttX file system simply supports a set of standard, file system APIs (`open()`, `close()`, `read()`, `write`, etc.) and a registration mechanism that allows devices drivers to a associated with nodes in a file-system-like name space.

### 2.11.2 Driver Operations

- 2.11.2.1 `fcntl.h`
- 2.11.2.2 `unistd.h`
- 2.11.2.3 `sys/ioctl.h`
- 2.11.2.4 `poll.h`
- 2.11.2.5 `sys/select.h`

#### 2.11.2.1 fcntl.h

      #include <fcntl.h>
      int open(const char *path, int oflag, ...);

#### 2.11.2.2 unistd.h

      #include <unistd.h>
      int     close(int fd);
      int     dup(int fildes);
      int     dup2(int fildes1, int fildes2);
      off_t   lseek(int fd, off_t offset, int whence);
      ssize_t read(int fd, void *buf, size_t nbytes);
      int     unlink(const char *path);
      ssize_t write(int fd, const void *buf, size_t nbytes);

#### 2.11.2.3 sys/ioctl.h

      #include <sys/ioctl.h>
      int     ioctl(int fd, int req, unsigned long arg);

#### 2.11.2.4 poll.h

##### 2.11.2.4.1 poll

Function Prototype:

      #include <poll.h>
      int     poll(struct pollfd *fds, nfds_t nfds, int timeout);

Description: `poll()` waits for one of a set of file descriptors to become ready to perform I/O. If none of the events requested (and no error) has occurred for any of the file descriptors, then `poll()` blocks until one of the events occurs.

Configuration Settings. In order to use the `poll()` API, the following must be defined in your NuttX configuration file:

- `CONFIG_NFILE_DESCRIPTORS` Defined to be greater than 0
- `CONFIG_DISABLE_POLL` NOT defined

In order to use the select with TCP/IP sockets test, you must also have the following additional things selected in your NuttX configuration file:

- `CONFIG_NET` Defined for general network support
- `CONFIG_NET_TCP` Defined for TCP/IP support
- `CONFIG_NSOCKET_DESCRIPTORS` Defined to be greater than 0
- `CONFIG_NET_NTCP_READAHEAD_BUFFERS` Defined to be greater than zero

In order to for select to work with incoming connections, you must also select:

- `CONFIG_NET_TCPBACKLOG` Incoming connections pend in a backlog until `accept() is called. The size of the backlog is selected when `listen()` is called.`

Input Parameters:

- `fds`. List of structures describing file descriptors to be monitored.
- `nfds`. The number of entries in the list.
- `timeout`. Specifies an upper limit on the time for which `poll()` will block in milliseconds. A negative value of `timeout` means an infinite timeout.

Returned Values:

On success, the number of structures that have nonzero revents fields. A value of 0 indicates that the call timed out and no file descriptors were ready. On error, -1 is returned, and `errno` is set appropriately:

- `EBADF`. An invalid file descriptor was given in one of the sets.
- `EFAULT`. The fds address is invalid
- `EINTR`. A signal occurred before any requested event.
- `EINVAL`. The nfds value exceeds a system limit.
- `ENOMEM`. There was no space to allocate internal data structures.
- `ENOSYS`. One or more of the drivers supporting the file descriptor does not support the poll method.

#### 2.11.2.5 sys/select.h

##### 2.11.2.5.1 select

Function Prototype:

      #include <sys/select.h>
      int     select(int nfds, FAR fd_set *readfds, FAR fd_set *writefds,
                     FAR fd_set *exceptfds, FAR struct timeval *timeout);

Description: `select()` allows a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without blocking.

NOTE: `poll()` is the fundamental API for performing such monitoring operation under NuttX. `select()` is provided for compatibility and is simply a layer of added logic on top of `poll()`. As such, `select()` is more wasteful of resources and `poll()` is the recommended API to be used.

Input Parameters:

- `nfds`. the maximum file descriptor number (+1) of any descriptor in any of the three sets.
- `readfds`. the set of descriptions to monitor for read-ready events
- `writefds`. the set of descriptions to monitor for write-ready events
- `exceptfds`. the set of descriptions to monitor for error events
- `timeout`. Return at this time if none of these events of interest occur.

Returned Values:

- `0:` Timer expired
- `>0:` The number of bits set in the three sets of descriptors
- `-1:` An error occurred (`errno` will be set appropriately, see `poll()`).

### 2.11.3 Directory Operations

      #include <dirent.h>
      int        closedir(DIR *dirp);
      FAR DIR   *opendir(const char *path);
      FAR struct dirent *readdir(FAR DIR *dirp);
      int        readdir_r(FAR DIR *dirp, FAR struct dirent *entry, FAR struct dirent **result);
      void       rewinddir(FAR DIR *dirp);
      void       seekdir(FAR DIR *dirp, int loc);
      int        telldir(FAR DIR *dirp);

      #include <unistd.h>
      int       chdir(FAR const char *path);
      FAR char *getcwd(FAR char *buf, size_t size);

### 2.11.4 Standard I/O

      #include <stdio.h>
      int    fclose(FILE *stream);
      int    fflush(FILE *stream);
      int    feof(FILE *stream);                               /* Prototyped but not implemented */
      int    ferror(FILE *stream);                             /* Prototyped but not implemented */
      int    fgetc(FILE *stream);
      int    fgetpos(FILE *stream, fpos_t *pos);
      char  *fgets(char *s, int n, FILE *stream);
      FILE  *fopen(const char *path, const char *type);
      int    fprintf(FILE *stream, const char *format, ...);
      int    fputc(int c, FILE *stream);
      int    fputs(const char *s, FILE *stream);
      size_t fread(void *ptr, size_t size, size_t n_items, FILE *stream);
      int    fseek(FILE *stream, long int offset, int whence);
      int    fsetpos(FILE *stream, fpos_t *pos);
      long   ftell(FILE *stream);
      size_t fwrite(const void *ptr, size_t size, size_t n_items, FILE *stream);
      char  *gets(char *s);

      int    printf(const char *format, ...);
      int    puts(const char *s);
      int    rename(const char *source, const char *target);
      int    sprintf(char *dest, const char *format, ...);
      int    ungetc(int c, FILE *stream);
      int    vprintf(const char *s, va_list ap);
      int    vfprintf(FILE *stream, const char *s, va_list ap);
      int    vsprintf(char *buf, const char *s, va_list ap);

      FILE  *fdopen(int fd, const char *type);
      int    fstat(int fd, FAR struct stat *buf);              /* Prototyped but not implemented */
      int    mkdir(const char *path, mode_t mode);
      int    rmdir(const char *path);
      int    stat(const char *path, FAR struct stat *buf);
      int    statfs(const char *path, FAR struct statfs *buf); /* Prototyped but not implemented */

### 2.11.5 Pipes and FIFOs

### 2.11.5.1 `pipe`

Function Prototype:

      #include <unistd.h>
      int pipe(int filedes[2]);

Description:

`pipe()` creates a pair of file descriptors, pointing to a pipe inode, and places them in the array pointed to by `filedes`. `filedes[0]` is for reading, `filedes[1]` is for writing.

Input Parameters:

- `filedes[2]`. The user provided array in which to catch the pipe file descriptors.

Returned Values:

0 is returned on success; otherwise, -1 is returned with `errno` set appropriately.

### 2.11.5.2 `mkfifo`

Function Prototype:

      #include <sys/stat.h>
      int mkfifo(FAR const char *pathname, mode_t mode);

Description:

`mkfifo()` makes a FIFO device driver file with name `pathname`. Unlike Linux, a NuttX FIFO is not a special file type but simply a device driver instance. `mode` specifies the FIFO's permissions (but is ignored in the current implementation).

Once the FIFO has been created by `mkfifo()`, any thread can open it for reading or writing, in the same way as an ordinary file. However, it must have been opened from both reading and writing before input or output can be performed. This FIFO implementation will block all attempts to open a FIFO read-only until at least one thread has opened the FIFO for writing.

If all threads that write to the FIFO have closed, subsequent calls to `read()` on the FIFO will return 0 (end-of-file).

Input Parameters:

- `pathname`. The full path to the FIFO instance to attach to or to create (if not already created).
- `mode`. Ignored for now``

Returned Values:

0 is returned on success; otherwise, -1 is returned with `errno` set appropriately.

### 2.11.6 FAT File System Support

### 2.11.6.1 `mkfatfs`

Function Prototype:

    #include <nuttx/mkfatfs.h>
    int mkfatfs(FAR const char *pathname, FAR struct fat_format_s *fmt);

Description:

The `mkfats()` formats a FAT file system image on the block device specified by `pathname`

Assumptions: The caller must assure that the block driver is not mounted and not in use when this function is called. The result of formatting a mounted device is indeterminate (but likely not good).

Input Parameters:

- `pathname` The full path to the registered block driver in the file system.
- `fmt` A reference to an instance of a structure that provides caller-selectable attributes of the created FAT file system.

      struct fat_format_s
      {
         ubyte   ff_nfats;           /* Number of FATs */
         ubyte   ff_fattype;         /* FAT size: 0 (autoselect), 12, 16, or 32 */
         ubyte   ff_clustshift;      /* Log2 of sectors per cluster: 0-5, 0xff (autoselect) */
         ubyte   ff_volumelabel[11]; /* Volume label */
         uint16  ff_backupboot;      /* Sector number of the backup boot sector (0=use default)*/
         uint16  ff_rootdirentries;  /* Number of root directory entries */
         uint16  ff_rsvdseccount;    /* Reserved sectors */
         uint32  ff_hidsec;          /* Count of hidden sectors preceding fat */
         uint32  ff_volumeid;        /* FAT volume id */
         uint32  ff_nsectors;        /* Number of sectors from device to use: 0: Use all */
      };

Returned Values:

Zero (`OK`) on success; -1 (`ERROR`) on failure with `errno` set appropriately:

- `EINVAL` - NULL block driver string, bad number of FATS in `fmt`, bad FAT size in `fmt`, bad cluster size in `fmt`
- `ENOENT` - `pathname` does not refer to anything in the filesystem.
- `ENOTBLK` - `pathname` does not refer to a block driver
- `EACCESS` - block driver does not support write or geometry methods

### 2.11.7 `mmap()` and eXecute In Place (XIP)

NuttX operates in a flat open address space. Therefore, it generally does not require `mmap()` functionality. There is one one exception: `mmap()` is the API that is used to support direct access to random access media under the following very restrictive conditions:

1. The filesystem supports the `FIOC_MMAP` ioctl command. Any file system that maps files contiguously on the media should support this `ioctl` command. By comparison, most file system scatter files over the media in non-contiguous sectors. As of this writing, ROMFS is the only file system that meets this requirement.
2. The underly block driver supports the `BIOC_XIPBASE` `ioctl` command that maps the underlying media to a randomly accessible address. At present, only the RAM/ROM disk driver does this.

### 2.11.7.1 `mmap`

Function Prototype:

    #include <sys/mman.h>
    int mkfatfs(FAR const char *pathname, FAR struct fat_format_s *fmt);
    FAR void *mmap(FAR void *start, size_t length, int prot, int flags, int fd, off_t offset)

Description:

Provides minimal `mmap()` as needed to support eXecute In Place (XIP) operation (as described above).

Input Parameters:

- `start` A hint at where to map the memory -- ignored. The address of the underlying media is fixed and cannot be re-mapped without MMU support.
- `length` The length of the mapping -- ignored. The entire underlying media is always accessible.
- `prot` See the `PROT_*` definitions in `sys/mman.h`.
  - `PROT_NONE` - Will cause an error.
  - `PROT_READ` - `PROT_WRITE` and `PROT_EXEC` also assumed.
  - `PROT_WRITE` - `PROT_READ` and `PROT_EXEC` also assumed.
  - `PROT_EXEC` - `PROT_READ` and `PROT_WRITE` also assumed.
- `flags` See the `MAP_*` definitions in `sys/mman.h`.
  - `MAP_SHARED` - Required
  - `MAP_PRIVATE` - Will cause an error
  - `MAP_FIXED` - Will cause an error
  - `MAP_FILE` - Ignored
  - `MAP_ANONYMOUS` - Will cause an error
  - `MAP_ANON` - Will cause an error
  - `MAP_GROWSDOWN` - Ignored
  - `MAP_DENYWRITE` - Will cause an error
  - `MAP_EXECUTABLE` - Ignored
  - `MAP_LOCKED` - Ignored
  - `MAP_NORESERVE` - Ignored
  - `MAP_POPULATE` - Ignored
  - `AP_NONBLOCK` - Ignored
- `fd` file descriptor of the backing file -- required.
- `offset` The offset into the file to map.

Returned Values:

On success, `mmap()` returns a pointer to the mapped area. On error, the value `MAP_FAILED` is returned, and `errno` is set appropriately.

- `ENOSYS` - Returned if any of the unsupported `mmap()` features are attempted.
- `EBADF` - `fd` is not a valid file descriptor.
- `EINVAL` - Length is 0. flags contained neither `MAP_PRIVATE` or `MAP_SHARED`, or contained both of these values.
- `ENODEV` - The underlying filesystem of the specified file does not support memory mapping.

## 2.12 Network Interfaces

NuttX includes a simple interface layer based on uIP (see http://www.sics.se). NuttX supports subset of a standard socket interface to uIP. These network feature can be enabled by settings in the architecture configuration file. Those socket APIs are discussed in the following paragraphs.

- 2.12.1 socket
- 2.12.2 bind
- 2.12.3 connect
- 2.12.4 listen
- 2.12.5 accept
- 2.12.6 send
- 2.12.7 sendto
- 2.12.8 recv
- 2.12.9 recvfrom
- 2.12.10 setsockopt
- 2.12.11 getsockopt

### 2.12.1 `socket`

Function Prototype:

      #include <sys/socket.h>
      int socket(int domain, int type, int protocol);

Description: socket() creates an endpoint for communication and returns a descriptor.

Input Parameters:

- `domain`: (see sys/socket.h)
- `type`: (see sys/socket.h)
- `protocol`: (see sys/socket.h)

Returned Values: 0 on success; -1 on error with `errno` set appropriately:

- `EACCES`. Permission to create a socket of the specified type and/or protocol is denied.
- `EAFNOSUPPORT`. The implementation does not support the specified address family.
- `EINVAL`. Unknown protocol, or protocol family not available.
- `EMFILE`. Process file table overflow.
- `ENFILE` The system limit on the total number of open files has been reached.
- `ENOBUFS` or `ENOMEM`. Insufficient memory is available. The socket cannot be created until sufficient resources are freed.
- `EPROTONOSUPPORT`. The protocol type or the specified protocol is not supported within this domain.

### 2.12.2 `bind`

Function Prototype:

      #include <sys/socket.h>
      int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

Description: `bind()` gives the socket sockfd the local address `addr`. `addr` is `addrlen` bytes long. Traditionally, this is called "assigning a name to a socket." When a socket is created with `socket()`, it exists in a name space (address family) but has no name assigned.

Input Parameters:

- `sockfd`: Socket descriptor from socket.
- `addr`: Socket local address.
- `addrlen`: Length of `addr`.

Returned Values: 0 on success; -1 on error with `errno` set appropriately:

- `EACCES` The address is protected, and the user is not the superuser.
- `EADDRINUSE` The given address is already in use.
- `EBADF` `sockfd` is not a valid descriptor.
- `EINVAL` The socket is already bound to an address.
- `ENOTSOCK` `sockfd` is a descriptor for a file, not a socket.

### 2.12.3 `connect`

Function Prototype:

      #include <sys/socket.h>
      int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

Description: `connect()` connects the socket referred to by the file descriptor `sockfd` to the address specified by `addr`. The `addrlen` argument specifies the size of `addr`. The format of the address in `addr` is determined by the address space of the socket sockfd. If the socket sockfd is of type SOCK_DGRAM then `addr` is the address to which datagrams are sent by default, and the only address from which datagrams are received. If the socket is of type SOCK_STREAM or SOCK_SEQPACKET, this call attempts to make a connection to the socket that is bound to the address specified by `addr`. Generally, connection-based protocol sockets may successfully `connect()` only once; connectionless protocol sockets may use `connect()` multiple times to change their association. Connectionless sockets may dissolve the association by connecting to an address with the sa_family member of sockaddr set to AF_UNSPEC.

Input Parameters:

- `sockfd`: Socket descriptor returned by `socket()`
- `addr`: Server address (form depends on type of socket)
- `addrlen`: Length of actual `addr`

Returned Values: 0 on success; -1 on error with `errno` set appropriately:

- `EACCES` or EPERM: The user tried to connect to a broadcast address without having the socket broadcast flag enabled or the connection request failed because of a local firewall rule.
- `EADDRINUSE` Local address is already in use.
- `EAFNOSUPPORT` The passed address didn't have the correct address family in its sa_family field.
- `EAGAIN` No more free local ports or insufficient entries in the routing cache. For PF_INET.
- `EALREADY` The socket is non-blocking and a previous connection attempt has not yet been completed.
- `EBADF` The file descriptor is not a valid index in the descriptor table.
- `ECONNREFUSED` No one listening on the remote address.
- `EFAULT` The socket structure address is outside the user's address space.
- `EINPROGRESS` The socket is non-blocking and the connection cannot be completed immediately.
- `EINTR` The system call was interrupted by a signal that was caught.
- `EISCONN` The socket is already connected.
- `ENETUNREACH` Network is unreachable.
- `ENOTSOCK` The file descriptor is not associated with a socket.
- `ETIMEDOUT` Timeout while attempting connection. The server may be too busy to accept new connections.

### 2.12.4 listen

Function Prototype:

      #include <sys/socket.h>
      int listen(int sockfd, int backlog);

Description: To accept connections, a socket is first created with `socket()`, a willingness to accept incoming connections and a queue limit for incoming connections are specified with `listen()`, and then the connections are accepted with `accept()`. The `listen() call applies only to sockets of type `SOCK_STREAM` or `SOCK_SEQPACKET`.`

Input Parameters:

- `sockfd`: Socket descriptor of the bound socket.
- `backlog`: The maximum length the queue of pending connections may grow. If a connection request arrives with the queue full, the client may receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that retries succeed.

Returned Values: On success, zero is returned. On error, -1 is returned, and `errno` is set appropriately.

- `EADDRINUSE`: Another socket is already listening on the same port.
- `EBADF`: The argument `sockfd` is not a valid descriptor.
- `ENOTSOCK`: The argument `sockfd` is not a socket.
- `EOPNOTSUPP`: The socket is not of a type that supports the listen operation.

### 2.12.5 accept

Function Prototype:

      #include <sys/socket.h>
      int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

Description: The `accept()` function is used with connection-based socket types (`SOCK_STREAM`, `SOCK_SEQPACKET` and `SOCK_RDM`). It extracts the first connection request on the queue of pending connections, creates a new connected socket with most of the same properties as `sockfd`, and allocates a new socket descriptor for the socket, which is returned. The newly created socket is no longer in the listening state. The original socket `sockfd` is unaffected by this call. Per file descriptor flags are not inherited across an accept.

The `sockfd` argument is a socket descriptor that has been created with `socket()`, bound to a local address with `bind()`, and is listening for connections after a call to `listen()`.

On return, the `addr` structure is filled in with the address of the connecting entity. The `addrlen` argument initially contains the size of the structure pointed to by `addr`; on return it will contain the actual length of the address returned.

If no pending connections are present on the queue, and the socket is not marked as non-blocking, accept blocks the caller until a connection is present. If the socket is marked non-blocking and no pending connections are present on the queue, accept returns `EAGAIN`.

Input Parameters:

- `sockfd`: Socket descriptor of the listening socket.
- `addr`: Receives the address of the connecting client.
- `addrlen`: Input: allocated size of `addr`, Return: returned size of `addr`.

Returned Values: Returns -1 on error. If it succeeds, it returns a non-negative integer that is a descriptor for the accepted socket.

- `EAGAIN` or `EWOULDBLOCK`: The socket is marked non-blocking and no connections are present to be accepted.
- `EBADF`: The descriptor is invalid.
- `ENOTSOCK`: The descriptor references a file, not a socket.
- `EOPNOTSUPP`: The referenced socket is not of type `SOCK_STREAM`.
- `EINTR`: The system call was interrupted by a signal that was caught before a valid connection arrived.
- `ECONNABORTED`: A connection has been aborted.
- `EINVAL`: Socket is not listening for connections.
- `EMFILE`: The per-process limit of open file descriptors has been reached.
- `ENFILE`: The system maximum for file descriptors has been reached.
- `EFAULT`: The addr parameter is not in a writable part of the user address space.
- `ENOBUFS` or `ENOMEM`: Not enough free memory.
- `EPROTO`: Protocol error.
- `EPERM`: Firewall rules forbid connection.

### 2.12.6 `send`

Function Prototype:

      #include <sys/socket.h>
      ssize_t send(int sockfd, const void *buf, size_t len, int flags);

Description: The `send()` call may be used only when the socket is in a connected state (so that the intended recipient is known). The only difference between `send()` and `write()` is the presence of `flags`. With `zero` flags parameter, `send()` is equivalent to `write()`. Also, `send(s,buf,len,flags)` is equivalent to `sendto(s,buf,len,flags,NULL,0)`.

Input Parameters:

- `sockfd`: Socket descriptor of socket
- `buf`: Data to send
- `len`: Length of data to send
- `flags`: Send flags

Returned Values: See `sendto()`.

### 2.12.7 `sendto`

Function Prototype:

      #include <sys/socket.h>
      ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
                     const struct sockaddr *to, socklen_t tolen);

Description: If `sendto()` is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the parameters to and tolen are ignored (and the error EISCONN may be returned when they are not NULL and 0), and the error ENOTCONN is returned when the socket was not actually connected.

Input Parameters:

- `sockfd`: Socket descriptor of socket
- `buf`: Data to send
- `len`: Length of data to send
- `flags`: Send flags
- `to`: Address of recipient
- `tolen`: The length of the address structure

Returned Values: On success, returns the number of characters sent. On error, -1 is returned, and `errno` is set appropriately:

- `EAGAIN` or `EWOULDBLOCK`. The socket is marked non-blocking and the requested operation would block.
- `EBADF`. An invalid descriptor was specified.
- `ECONNRESET`. Connection reset by peer.
- `EDESTADDRREQ`. The socket is not connection-mode, and no peer address is set.
- `EFAULT`. An invalid user space address was specified for a parameter.
- `EINTR`. A signal occurred before any data was transmitted.
- `EINVAL`. Invalid argument passed.
- `EISCONN`. The connection-mode socket was connected already but a recipient was specified. (Now either this error is returned, or the recipient specification is ignored.)
- `EMSGSIZE`. The socket type requires that message be sent atomically, and the size of the message to be sent made this impossible.
- `ENOBUFS`. The output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion.
- `ENOMEM`. No memory available.
- `ENOTCONN`. The socket is not connected, and no target has been given.
- `ENOTSOCK`. The argument s is not a socket.
- `EOPNOTSUPP`. Some bit in the flags argument is inappropriate for the socket type.
- `EPIPE`. The local end has been shut down on a connection oriented socket. In this case the process will also receive a SIGPIPE unless MSG_NOSIGNAL is set.

### 2.12.8 `recv`

Function Prototype:

      #include <sys/socket.h>
      ssize_t recv(int sockfd, void *buf, size_t len, int flags);

Description: The `recv()` call is identical to `recvfrom()` with a NULL `from` parameter.

Input Parameters:

- sockfd: Socket descriptor of socket
- buf: Buffer to receive data
- len: Length of buffer
- flags: Receive flags

Returned Values: see `recvfrom()`. Zero on success.

### 2.12.9 `recvfrom`

Function Prototype:

      #include <sys/socket.h>
      ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                       struct sockaddr *from, socklen_t *fromlen);

Description: `recvfrom()` receives messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.

If `from` is not NULL, and the underlying protocol provides the source address, this source address is filled in. The argument `fromlen` initialized to the size of the buffer associated with `from`, and modified on return to indicate the actual size of the address stored there.

Input Parameters:

- `sockfd`: Socket descriptor of socket.
- `buf`: Buffer to receive data.
- `len`: Length of buffer.
- `flags`: Receive flags.
- `from`: Address of source.
- `fromlen`: The length of the address structure.

Returned Values: On success, returns the number of characters sent. On error, -1 is returned, and `errno` is set appropriately:

- `EAGAIN`. The socket is marked non-blocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received.
- `EBADF`. The argument `sockfd` is an invalid descriptor.
- `ECONNREFUSED`. A remote host refused to allow the network connection (typically because it is not running the requested service).
- `EFAULT`. The receive buffer pointer(s) point outside the process's address space.
- `EINTR`. The receive was interrupted by delivery of a signal before any data were available.
- `EINVAL`. Invalid argument passed.
- `ENOMEM`. Could not allocate memory.
- `ENOTCONN`. The socket is associated with a connection-oriented protocol and has not been connected.
- `ENOTSOCK`. The argument `sockfd` does not refer to a socket.

### 2.12.10 `setsockopt`

Function Prototype:

      #include <sys/socket.h>
      int setsockopt(int sockfd, int level, int option,
                     const void *value, socklen_t value_len);

Description: `setsockopt()` sets the option specified by the `option` argument, at the protocol level specified by the `level` argument, to the value pointed to by the `value` argument for the socket associated with the file descriptor specified by the `sockfd` argument.

The `level` argument specifies the protocol level of the option. To set options at the socket level, specify the level argument as SOL_SOCKET.

See `sys/socket.h` for a complete list of values for the `option` argument.

Input Parameters:

- `sockfd`: Socket descriptor of socket
- `level`: Protocol level to set the option
- `option`: identifies the option to set
- `value`: Points to the argument value
- `value_len`: The length of the argument value

Returned Values: On success, returns the number of characters sent. On error, -1 is returned, and `errno` is set appropriately:

- `BADF`. The `sockfd` argument is not a valid socket descriptor.
- `DOM`. The send and receive timeout values are too big to fit into the timeout fields in the socket structure.
- `INVAL`. The specified option is invalid at the specified socket `level` or the socket has been shut down.
- `ISCONN`. The socket is already connected, and a specified option cannot be set while the socket is connected.
- `NOPROTOOPT`. The `option` is not supported by the protocol.
- `NOTSOCK`. The `sockfd` argument does not refer to a socket.
- `NOMEM`. There was insufficient memory available for the operation to complete.
- `NOBUFS`. Insufficient resources are available in the system to complete the call.

### 2.12.11 `getsockopt`

Function Prototype:

      #include <sys/socket.h>
      int getsockopt(int sockfd, int level, int option,
                     void *value, socklen_t *value_len);

Description: `getsockopt()` retrieve thse value for the option specified by the `option` argument for the socket specified by the `sockfd` argument. If the size of the option value is greater than `value_len`, the value stored in the object pointed to by the `value` argument will be silently truncated. Otherwise, the length pointed to by the `value_len` argument will be modified to indicate the actual length of the`value`.

The `level` argument specifies the protocol level of the option. To retrieve options at the socket level, specify the level argument as SOL_SOCKET.

See `sys/socket.h`for a complete list of values for the `option` argument.

Input Parameters:

- `sockfd Socket descriptor of socket`
- `level Protocol level to set the option`
- `option identifies the option to get`
- `value Points to the argument value`
- `value_len The length of the argument value`

Returned Values: On success, returns the number of characters sent. On error, -1 is returned, and `errno` is set appropriately:

- `BADF`. The `sockfd` argument is not a valid socket descriptor.
- `INVAL`. The specified option is invalid at the specified socket `level` or the socket has been shutdown.
- `NOPROTOOPT`. The `option` is not supported by the protocol.
- `NOTSOCK`. The `sockfd` argument does not refer to a socket.
- `NOBUFS Insufficient resources are available in the system to complete the call.`

# 3.0 OS Data Structures

## 3.1 Scalar Types

Many of the types used to communicate with NuttX are simple scalar types. These types are used to provide architecture independence of the OS from the application. The scalar types used at the NuttX interface include:

- pid_t
- size_t
- sigset_t
- STATUS
- time_t

## 3.2 Hidden Interface Structures

Several of the types used to interface with NuttX are structures that are intended to be hidden from the application. From the standpoint of the application, these structures (and structure pointers) should be treated as simple handles to reference OS resources. These hidden structures include:

- _TCB
- mqd_t
- sem_t
- WDOG_ID
- pthread_key_t

In order to maintain portability, applications should not reference specific elements within these hidden structures. These hidden structures will not be described further in this user's manual.

## 3.3 Access to the `errno` Variable

A pointer to the thread-specific `errno` value is available through a function call:

Function Prototype:

        #include <errno.h>
        #define errno *get_errno_ptr()
        int *get_errno_ptr( void )

Description: `get_errno_ptr()` returns a pointer to the thread-specific `errno` value. Note that the symbol `errno` is defined to be `get_errno_ptr()` so that the usual access by referencing the symbol `errno` will work as expected.

There is a unique, private `errno` value for each NuttX task. However, the implementation of `errno` differs somewhat from the use of `errno` in most multi-threaded process environments: In NuttX, each pthread will also have its own private copy of `errno` and the `errno` will not be shared between pthreads. This is, perhaps, non-standard but promotes better thread independence.

Input Parameters: None

Returned Values:

- A pointer to the thread-specific `errno` value.

## 3.4 User Interface Structures

### 3.4.1 main_t

main_t defines the type of a task entry point. main_t is declared in sys/types.h as:

        typedef int (*main_t)(int argc, char *argv[]);

### 3.4.2 struct sched_param

This structure is used to pass scheduling priorities to and from NuttX;

        struct sched_param
        {
          int sched_priority;
        };

### 3.4.3 struct timespec

This structure is used to pass timing information between the NuttX and a user application:

        struct timespec
        {
          time_t tv_sec;  /* Seconds */
          long   tv_nsec; /* Nanoseconds */
        };

### 3.4.4 struct mq_attr

This structure is used to communicate message queue attributes between NuttX and a MoBY application:

        struct mq_attr {
          size_t       mq_maxmsg;   /* Max number of messages in queue */
          size_t       mq_msgsize;  /* Max message size */
          unsigned     mq_flags;    /* Queue flags */
          size_t       mq_curmsgs;  /* Number of messages currently in queue */
        };

### 3.4.5 struct sigaction

The following structure defines the action to take for given signal:

        struct sigaction
        {
          union
          {
            void (*_sa_handler)(int);
            void (*_sa_sigaction)(int, siginfo_t *, void *);
          } sa_u;
          sigset_t           sa_mask;
          int                sa_flags;
        };
        #define sa_handler   sa_u._sa_handler
        #define sa_sigaction sa_u._sa_sigaction

### 3.4.6 struct siginfo/siginfo_t

The following types is used to pass parameters to/from signal handlers:

        typedef struct siginfo
        {
          int          si_signo;
          int          si_code;
          union sigval si_value;
       } siginfo_t;

### 3.4.7 union sigval

This defines the type of the struct siginfo si_value field and is used to pass parameters with signals.

        union sigval
        {
          int   sival_int;
          void *sival_ptr;
        };

### 3.4.8 struct sigevent

The following is used to attach a signal to a message queue to notify a task when a message is available on a queue.

        struct sigevent
        {
          int          sigev_signo;
          union sigval sigev_value;
          int          sigev_notify;
        };

### 3.4.9 Watchdog Data Types

When a watchdog expires, the callback function with this type is called:

        typedef void (*wdentry_t)(int argc, ...);

Where argc is the number of uint32 type arguments that follow.

The arguments are passed as uint32 values. For systems where the sizeof(pointer) < sizeof(uint32), the following union defines the alignment of the pointer within the uint32. For example, the SDCC MCS51 general pointer is 24-bits, but uint32 is 32-bits (of course).

        union wdparm_u
        {
          void   *pvarg;
          uint32 *dwarg;
        };
        typedef union wdparm_u wdparm_t;

For most 32-bit systems, pointers and uint32 are the same size For systems where sizeof(pointer) > sizeof(uint32), we will have to do some redesign.

# Index

accept bind BIOC_XIPBASE chdir clock_getres clock_gettime Clocks clock_settime close closedir connect Data structures Directory operations dirent.h Driver operations dup dup2 eXecute In Place (XIP) exit FAT File System Support fclose fcntl.h fdopen feof ferror File system, interfaces File system, overview fflush fgetc fgetpos fgets FIOC_MMAP fopen fprintf fputc fputs fread fseek fsetpos fstat ftell fwrite getcwd getpid gets getsockopt gmtime_r Introduction ioctl kill listen localtime_r lseek Named Message Queue Interfaces mkdir mkfatfs mkfifo mktime mq_close mq_getattr mq_notify mq_open mq_receive mq_send mq_setattr mq_timedreceive mq_timedsend mq_unlink mmap Network Interfaces open opendir OS Interfaces pipe | poll poll.h printf Pthread Interfaces pthread_attr_destroy pthread_attr_getinheritsched pthread_attr_getschedparam pthread_attr_getschedpolicy pthread_attr_getstacksize pthread_attr_init pthread_attr_setinheritsched pthread_attr_setschedparam pthread_attr_setschedpolicy pthread_attr_setstacksize pthread_barrierattr_init pthread_barrierattr_destroy pthread_barrierattr_getpshared pthread_barrierattr_setpshared pthread_barrier_destroy pthread_barrier_init pthread_barrier_wait pthread_cancel pthread_condattr_init pthread_cond_broadcast pthread_cond_destroy pthread_cond_init pthread_cond_signal pthread_cond_timedwait pthread_cond_wait pthread_create pthread_detach pthread_exit pthread_getschedparam pthread_getspecific pthreads share some resources. pthread_join pthread_key_create pthread_key_delete pthread_kill pthread_mutexattr_destroy pthread_mutexattr_getpshared pthread_mutexattr_gettype pthread_mutexattr_init pthread_mutexattr_setpshared pthread_mutexattr_settype pthread_mutex_destroy pthread_mutex_init pthread_mutex_lock pthread_mutex_trylock pthread_mutex_unlock pthread_condattr_destroy pthread_once pthread_self pthread_setcancelstate pthread_setschedparam pthread_setspecific pthread_sigmask pthread_testcancelstate pthread_yield puts RAM disk driver read readdir readdir_r recv recvfrom rename rmdir rewinddir ROM disk driver ROMFS sched_getparam sched_get_priority_max sched_get_priority_min | sched_get_rr_interval sched_lockcount sched_lock sched_setparam sched_setscheduler sched_unlock sched_yield select Counting Semaphore Interfaces sem_close sem_destroy sem_getvalue sem_init sem_open sem_post sem_trywait sem_unlink sem_wait sched_getscheduler seekdir send sendto setsockopt sigaction sigaddset sigdelset sigemptyset sigfillset sigismember Signal Interfaces sigpending sigprocmask sigqueue sigsuspend sigtimedwait sigwaitinfo socket sprintf Standard I/O stat statfs stdio.h sys/select.h sys/ioctl.h task_activate Task Control Interfaces task_create task_delete task_init task_restart Task Scheduling Interfaces Task Switching Interfaces telldir timer_create timer_delete timer_getoverrun timer_gettime Timers timer_settime ungetc unistd.h, unistd.h unlink vfprintf vprintf vsprintf Watchdog Timer Interfaces wd_cancel wd_create wd_delete wd_gettime wd_start write XIP