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<HTML>

<HEAD>
<TITLE>Nuttx Users Manual</TITLE>
<META NAME="AUTHOR" CONTENT="Gregory Nutt">
</HEAD>

<BODY>
<CENTER><BIG><B>
Nuttx Operating System
<P>
User's Manual
</B></BIG>
<P>
<SMALL>by</SMALL>
<P>
Gregory Nutt
<P>
<SMALL>Last Update: January 28, 2007</SMALL>
</CENTER>

<H1>1.0 <A NAME="Introduction">Introduction</A></H1>

<P>
This user's manual is divided into five sections:
<UL>
<LI><B>Section 1.0, <A HREF="#Introduction">Introduction</A></B>:
This section provides an overview of the Nuttx user's manual.
<LI><B>Section 2.0, <A HREF="#OS_Interfaces">OS Interfaces</A></B>:
This section details the  interfaces provided by Nuttx from the
perspective of the firmware developer.  This section is divided
into several paragraphs that describe different groups of OS interfaces:
<UL>
<LI>Paragraph 2.1 <A HREF="#Task_Control">Task Control Interfaces</A>
<LI>Paragraph 2.2 <A HREF="#Task_Schedule">Task Scheduling Interfaces</A>
<LI>Paragraph 2.3 <A HREF="#Task_Switch">Task Switching Interfaces</A>
<LI>Paragraph 2.4 <A HREF="#Message_Queue">Named Message Queue Interfaces</A>
<LI>Paragraph 2.5 <A HREF="#Semaphores">Counting Semaphore Interfaces</A>
<LI>Paragraph 2.6 <A HREF="#Watchdogs">Watchdog Timer Interfaces</A>
<LI>Paragraph 2.7 <A HREF="#Signals">Signal Interfaces</A>
<LI>Paragraph 2.8 <A HREF="#Pthread">Pthread Interfaces</A>
</UL>
<LI><B>Section 3.0, <A HREF="#Data_Structures">OS Data Structures</A></B>:
This section documents the data structures that are used at the Nuttx
interface.
<LI><B>Section 4.0, <A HREF="#Problems">Known Problems</A></B>.  This section
lists known problems in the latest release of Nuttx.
</UL>
<HR>

<H1>2.0 <A NAME="OS_Interfaces">OS Interfaces</A></H1>

<P>
This section describes each C-callable interface to the Nuttx
Operating System. The description of each interface is presented
in the following format:
<P>
<B>Function Prototype:</B> The C prototype of the interface function
is provided.
<P>
<B>Description:</B> The operation performed by the interface function
is discussed.
<P>
<B>Input Parameters:</B> All input parameters are listed along
with brief descriptions of each input parameter.
<P>
<B>Returned Values:</B> 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.
<P>
<B>Assumptions/Limitations:</B> Any unusual assumptions made by
the interface function or any non-obvious limitations to the use
of the interface function will be indicated here.
<P>
<B>POSIX Compatibility:</B> Any significant differences between the
Nuttx interface and its corresponding POSIX interface will be noted
here.
<P>
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.
<HR>

<H2>2.1 <A NAME="Task_Control">Task Control Interfaces</A></H2>

<p>
<b>Tasks</b>.
Nuttx is a flat address OS.  As such it does not support &quot;processes&quot;
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 &quot;tasks&quot;
and pthreads:
<ul>
<li><i>tasks</i> are threads which have a degree of independence
<li><a href="#Pthread"><i>pthreads</i></a> share some resources.
</ul>
<p>
<b>File Descriptors and Streams</b>.
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.
<p>
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.

<H3>2.1.1 task_create</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
   #include &lt;sched.h&gt;
    int task_create(
        char *name,
        int priority,
        int stack_size,
        main_t entry,
        char *arg1, char *arg2, char *arg3, char *arg4);
</PRE>

<P>
<B>Description:</B> 
   This function creates and activates a new task with a
   specified priority and returns its system-assigned ID.
</p>

<P>The entry address entry is the address of the &quot;main&quot;
   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.
</P>
<P>
   Note that four (and only four) arguments must be passed for the
   spawned functions.
</P>
<p>
   The newly created task does not inherity 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.
</p>
<P>
<B>Input Parameters:</B> 
  <UL>
    <LI><I>name</I>. Name of the new task</LI>
    <LI><I>priority</I>. Priority of the new task</LI>
    <LI><I>stack_size</I>. size (in bytes) of the stack needed</LI>
    <LI><I>entry</I>. Entry point of a new task</LI>
    <LI><I>arg1, arg2, arg3, and arg4</I>. Four required task arguments to pass to the task</LI>
  </UL>
<P>
  <B>Returned Values:</B> 
</P>
<UL>
<LI>
  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).
</LI>
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>POSIX Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<PRE>
   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);
</PRE>

<P>
The Nuttx task_create() differs from VxWorks' taskSpawn() in the
following ways:
<UL>
<LI>Function name
<LI>Various differences in types or arguments
<LI>There is no options arguement.
<LI>One four parameters can be passed to a task (VxWorks allows
ten).
</UL>

<H3>2.1.2 task_init</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
   #include &lt;sched.h&gt;
   STATUS task_init(
      _TCB *tcb,
      char *name,
      int priority,
      uint32 *stack,
      uint32 stack_size,
      maint_t entry,
      int arg1, int arg2, int arg3, int arg4);
</PRE>

<P>
<B>Description:</B>
<P>
  This function initializes a Task Control Block (TCB)
   in preparation for starting a new thread.
</P>
<P>
  Unlike task_create(), task_init() does not activate the task.
  This must be done by calling task_activate().
</P>
<P>
<B>Input Parameters:</B> 
  <UL>
    <LI><I>tcb</I>. Address of the new task's TCB
    <LI><I>name</I>. Name of the new task (not used)
    <LI><I>priority</I>. Priority of the new task
    <LI><I>stack</I>. Start of the pre-allocated stack
    <LI><I>stack_size</I>. size (in bytes) of the pre-allocated stack
    <LI><I>entry</I>. Entry point of a new task
    <LI><I>arg1, arg2, arg3, and arg4</I>. Four required task arguments to pass to the task
  </UL>
</p>
<P>
<B>Returned Values:</B> 
</p>
<UL>
  <LI><P>OK, or ERROR if the task cannot be initialized.</P>
  <P>This function can only failure is it is unable to assign
    a new, unique task ID to the TCB (errno is not set).</P>
</UL>
<P>
<B>Assumptions/Limitations:</B>
<UL>
<LI>task_init() is provided to support internal OS functionality.  It is
<B>not recommended</B> for normal usage.  task_create() is the preferred
mechanism to initialize and start a new task.
</UL>
<P>
<B>POSIX Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<PRE>
   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);
</PRE>

<P>
The Nuttx task_init() differs from VxWorks' taskInit() in the
following ways:
<UL>
<LI>Function name
<LI>Various differences in types or arguments
<LI>There is no options argument.
<LI>One four parameters can be passed to a task (VxWorks allows ten).
</UL>

<H3>2.1.3 task_activate</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    STATUS task_activate( _TCB *tcb );
</PRE>

<P>
<B>Description:</B> This function activates tasks created by task_init().
Without activation, a task is ineligible for execution by the
scheduler.
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>tcb</I>. The TCB for the task for the task (same as the
task_init argument).
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>OK, or ERROR if the task cannot be activated (errno is not set).
</UL>

<P>
<B>Assumptions/Limitations:</B>
<UL>
<LI>task_activate() is provided to support internal OS functionality.  It is
<B>not recommended</B> for normal usage.  task_create() is the preferred
mechanism to initialize and start a new task.
</UL>
<P>
<B>POSIX Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<PRE>
    STATUS taskActivate( int tid );
</PRE>

<P>
The Nuttx task_activate() differs from VxWorks' taskActivate() in the
following ways:
<UL>
<LI>Function name
<LI>With VxWork's taskActivate, the pid argument is supposed to be
the pointer to the WIND_TCB cast to an integer.
</UL>

<H3>2.1.4 task_delete</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    STATUS task_delete( pid_t pid );
</PRE>

<P>
<B>Description:</B> 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().
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>pid</I>. The task ID of the task to delete. An ID of
zero signifies the calling task.
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>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)
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
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.
<P>
<B>POSIX Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<PRE>
    STATUS taskDelete( int tid );
</PRE>

<P>
The Nuttx task_delete() differs from VxWorks' taskDelete() in
the following ways:
<UL>
<LI>No support is provided for calling the tasks deletion routines
(because taskDeleteHookAdd() is not supported).
<LI>Deletion of self is not supported.  Use _exit();
</UL>

<H3>2.1.5 exit</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    void exit( int code );

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

<P>
<B>Description:</B> 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().
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>code</I>. (ignored)
</UL>

<P>
<B>Returned Values:</B>  None.

<P>
<B>Assumptions/Limitations:</B> 

<P>
<B>POSIX Compatibility:</B> This is equivalent to the ANSI interface:
<PRE>
    void exit( int code );
</PRE>
And the unix interface:
<PRE>
    void _exit( int code );
</PRE>

<P>
The Nuttx exit() differs from ANSI exit() in
the following ways:
<UL>
<LI>The <I>code</I> parameter is ignored.
</UL>

<H3>2.1.6 task_restart</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    STATUS task_restart( pid_t pid );
</PRE>

<P>
<B>Description:</B> This function &quot;restarts&quot; 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.
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>pid</I>. The task ID of the task to delete. An ID of
zero signifies the calling task.
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>
  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.
</LI>
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>POSIX Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the following similar interface:
<PRE>
    STATUS taskRestart (int tid);
</PRE>

<P>
The Nuttx task_restart() differs from VxWorks' taskRestart() in
the following ways:
<UL>
<LI>Restart of the currently running task is not supported.
<LI>The VxWorks description says that the ID, priority, etc. take
the value that they had when the task was <I>terminated</I>.
</UL>

<H3>2.1.7 getpid</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;unistd.h&gt;
    pid_t getpid( void );
</PRE>

<P>
<B>Description:</B> This function returns the task ID of the
calling task. The task ID will be invalid if called at the interrupt
level.
<P>
<B>Input Parameters:</B> None.
<P>
<B>Returned Values:</B> 
<UL>
<LI>The task ID of the calling task.
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>POSIX Compatibility:</B>
Compatible with the POSIX interface of the same name.

<H2>2.2 <A NAME="Task_Schedule">Task Scheduling Interfaces</A></H2>

<P>
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.
<P>
The OS interfaces described in the following paragraphs provide
a POSIX- compliant interface to the Nuttx scheduler. However, these
POSIX interfaces assume a greater range of scheduling options
than those provided by the Nuttx scheduler. As a result, many of
these POSIX interfaces are little more than stubs.

<H3>2.2.1 sched_setparam</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_setparam( pid_t pid, const struct sched_param *param );
</PRE>

<P>
<B>Description:</B> This function sets the priority of the task
specified by pid input parameter.
<P>
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.
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>pid</I>. The task ID of the task. If pid is zero, the
priority of the calling task is set.
<LI><I>param</I>. 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.
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>
  0 (OK) if successful, otherwise -1 (ERROR).
  This function can fail for the following reasons:
  (1) parm is NULL or parm->sched_priority is out of range.
  (2) pid does not correspond to any task.
</LI>
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.
Differences from the full POSIX implementation include:
<UL>
<LI>The range of priority values for the POSIX call is 0 to 255
</UL>

<H3>2.2.2 sched_getparam</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_getparam (pid_t pid, struct sched_param *param);
</PRE>

<P>
<B>Description:</B> This function gets the scheduling priority
of the task specified by pid.
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>pid</I>. The task ID of the task. If pid is zero, the
priority of the calling task is returned.
<LI><I>param</I>. A structure whose member sched_priority is the
integer priority. The task's priority is copied to the sched_priority
element of this structure.
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>0 (OK) if successful, otherwise -1 (ERROR).
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.

<H3>2.2.3 sched_setscheduler</H3>
<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param);
</PRE>
<P>
  <B>Description:</B>
  <i>sched_setscheduler()</i>  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.
</p>
<P>
<B>Input Parameters:</B> 
<UL>
  <LI><I>pid</I>. The task ID of the task. If pid is zero, the
    priority of the calling task is set.
  <LI><I>policy</I>. Scheduling policy requested (either SCHED_FIFO
    or SCHED_RR).
  <LI><I>param</I>. 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.
</UL>
<P>
  <B>Returned Values:</B> 
  On success, <i>sched_setscheduler()</i> returns OK (zero).  On
  error, ERROR (-1) is returned, and errno is set appropriately:
</p>
<ul>
  <li>EINVAL The scheduling policy is not one of the
    recognized policies.</li>
  <li>ESRCH  The task whose ID is pid could not be found.</li>
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.

<H3>2.2.4 sched_getscheduler</H3>
<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_getscheduler (pid_t pid);
</PRE>
<P>
  <B>Description:</B>
  <i>sched_getscheduler()</i> returns the scheduling policy
  currently applied to the process 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.
<P>
<B>Input Parameters:</B> 
<UL>
  <LI><I>pid</I>.
    The task ID of the task to query.
    If pid is zero, the calling task is queried.
  </LI>
</UL>
<P>
<B>Returned Values:</B> 
<UL>
  <LI>
    On success, <i>sched_getscheduler()</i> returns the policy for
    the task (either SCHED_FIFO or SCHED_RR).
    On error,  ERROR (-1) is returned, and errno is set appropriately:
    <ul>
      <li>ESRCH  The task whose ID is pid could not be found.</li>
    </ul>
  </li>
</UL>
<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.
Differences from the full POSIX implementation include:
<UL>
<LI>Does not report errors via <I>errno</I>.
</UL>

<H3>2.2.5 sched_yield</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_yield( void );
</PRE>

<P>
<B>Description:</B> This function forces the calling task to give
up the CPU (only to other tasks at the same priority).
<P>
<B>Input Parameters:</B> None.
<P>
<B>Returned Values:</B> 
<UL>
<LI>0 (OK) or -1 (ERROR)
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.

<H3>2.2.6 sched_get_priority_max</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_get_priority_max (int policy)
</PRE>

<P>
<B>Description:</B> This function returns the value of the highest
possible task priority for a specified scheduling policy.
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>policy</I>. Scheduling policy requested.
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>The maximum priority value or -1 (ERROR).
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.

<H3>2.2.7 sched_get_priority_min</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_get_priority_min (int policy);
</PRE>

<P>
<B>Description:</B> This function returns the value of the lowest
possible task priority for a specified scheduling policy.
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>policy</I>. Scheduling policy requested.
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>The minimum priority value or -1 (ERROR)
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX
interface of the same name.

<H3>2.2.8 sched_get_rr_interval</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    int sched_get_rr_interval (pid_t pid, struct timespec *interval);
</PRE>

<P>
  <B>Description:</B> 
  <i>sched_rr_get_interval()</i>  writes  the timeslice interval
  for task identified by <i>pid</i> into  the timespec structure
  pointed to by <i>interval</i>.  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.'
</p>
<P>
  <B>Input Parameters:</B> 
</p>
<UL>
<LI><I>pid</I>. The task ID of the task. If pid is zero, the
priority of the calling task is returned.
<LI><I>interval</I>. A structure used to return the time slice.
</UL>

<P>
  <B>Returned Values:</B> 
  On success, sched_rr_get_interval() returns OK (0).  On
  error, ERROR (-1) is returned, and errno is set to:
</p>
<UL>
  <LI>EFAULT Cannot copy to interval</LI>
  <LI>EINVAL Invalid pid.</LI>
  <LI>ENOSYS The system call is not yet implemented.</LI>
  <LI>ESRCH  The process whose ID is pid could not be found.</LI>
</UL>

<P>
  <B>Assumptions/Limitations:</B> 
<P>
  <B>POSIX  Compatibility:</B> Comparable to the POSIX
  interface of the same name.
</P>

<H2>2.3 <A NAME="Task_Switch">Task Switching Interfaces</A></H2>

<H3>2.3.1 sched_lock</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    STATUS sched_lock( void );
</PRE>

<P>
<B>Description:</B> 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.
<P>
<B>Input Parameters:</B> None.
<P>
<B>Returned Values:</B> 
<UL>
<LI>OK or ERROR.
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the comparable interface:
<PRE>
    STATUS taskLock( void );
</PRE>

<H3>2.3.2 sched_unlock</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    STATUS sched_unlock( void );
</PRE>

<P>
<B>Description:</B> 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.
<P>
<B>Input Parameters:</B> None.
<P>
<B>Returned Values:</B> 
<UL>
<LI>OK or ERROR.
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> This is a NON-POSIX interface.
VxWorks provides the comparable interface:
<PRE>
    STATUS taskUnlock( void );
</PRE>

<H3>2.3.3 sched_lockcount</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;sched.h&gt;
    sint32 sched_lockcount( void )
</PRE>

<P>
<B>Description:</B> 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.
<P>
<B>Input Parameters:</B> None.
<P>
<B>Returned Values:</B> 
<UL>
<LI>The current value of the lockCount.
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> None.
<HR>

<H2>2.4 <A NAME="Message_Queue">Named Message Queue Interfaces</A></H2>

<P>
The 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.

<H3>2.4.1 mq_open</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;mqueue.h&gt;
    mqd_t mq_open( const char *mqName, int oflags, ... );
</PRE>

<P>
<B>Description:</B> 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().
<P>
<B>Input Parameters:</B> 
<UL>
<LI><I>mqName</I>. Name of the queue to open
<LI><I>oflags</I>. Open flags. These may be any combination of:
<UL>
<LI><I>O_RDONLY</I>. Open for read access.
<LI><I>O_WRONLY</I>. Open for write access.
<LI><I>O_RDWR</I>. Open for both read &amp; write access.
<LI><I>O_CREAT</I>. Create message queue if it does not already
exist.
<LI><I>O_EXCL</I>. Name must not exist when opened.
<LI><I>O_NONBLOCK</I>. Don't wait for data.
</UL>

<LI><I>... Optional parameters</I>.
When the O_CREAT flag is specified, POSIX requires that a third
and fourth parameter be supplied:
<UL>
<LI><I>mode</I>.  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.
<LI><I>attr</I>.  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).
</UL>
</UL>

<P>
<B>Returned Values:</B> 
<UL>
<LI>A message queue descriptor or -1 (ERROR)
</UL>

<P>
<B>Assumptions/Limitations:</B> 
<P>
<B>  POSIX  Compatibility:</B> Comparable to the POSIX interface
of the same name.
Differences from the full POSIX implementation include:
<UL>
<LI>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.
</UL>

<H3>2.4.2 mq_close</H3>

<P>
<B>Function Prototype:</B> 
<PRE>
    #include &lt;mqueue.h&gt;
    int mq_close( mqd_t mqdes );
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