Task Control Interfaces

Warning

This section name is duplicate with the first, how should it be named?

  • Scheduler locking interfaces. These non-standard interfaces are used to enable and disable pre-emption and to test is pre-emption is currently enabled.

  • Task synchronization interfaces are used to wait for termination of child tasks.

  • Task Exit Hooks may be used to register callback functions that are executed when a task group terminates. A task group is the functional analog of a process: It is a group that consists of the main task thread and of all of the pthreads created by the main task thread or any of the other pthreads within the task group. Members of a task group share certain resources such as environment variables, file descriptors, FILE streams, sockets, pthread keys and open message queues.

    Note

    Behavior of features related to task group’s depend of NuttX configuration settings. See the discussion of “Parent and Child Tasks,” below. See also theNuttX Taskingpage and theTasks vs. Threads FAQfor additional information on tasks and threads in NuttX.

    A task group terminates when the last thread within the group exits.

Parent and Child Tasks

The task synchronization interfaces historically depend upon parent and child relationships between tasks. But default, NuttX does not use any parent/child knowledge. However, there are three important configuration options that can change that.

  • CONFIG_SCHED_HAVE_PARENT: If this setting is defined, then it instructs NuttX to remember the task ID of the parent task when each new child task is created. This support enables some additional features (such as SIGCHLD) and modifies the behavior of other interfaces. For example, it makes waitpid() more standards complete by restricting the waited-for tasks to the children of the caller.

  • CONFIG_SCHED_CHILD_STATUS: If this option is selected, then the exit status of the child task will be retained after the child task exits. This option should be selected if you require knowledge of a child process’s exit status. Without this setting, wait(), waitpid() or waitid() may fail. For example, if you do:

    1. Start child task

    2. Wait for exit status (using wait(), waitpid() or waitid()).

    This may fail because the child task may run to completion before the wait begins. There is a non-standard work-around in this case: The above sequence will work if you disable pre-emption using sched_lock() prior to starting the child task, then re-enable pre-emption with sched_unlock() after the wait completes. This works because the child task is not permitted to run until the wait is in place.

    The standard solution would be to enable CONFIG_SCHED_CHILD_STATUS. In this case the exit status of the child task is retained after the child exits and the wait will successful obtain the child task’s exit status whether it is called before the child task exits or not.

  • CONFIG_PREALLOC_CHILDSTATUS. To prevent runaway child status allocations and to improve allocation performance, child task exit status structures are pre-allocated when the system boots. This setting determines the number of child status structures that will be pre-allocated. If this setting is not defined or if it is defined to be zero then a value of 2*MAX_TASKS is used.

    Note that there cannot be more that CONFIG_MAX_TASKS tasks in total. However, the number of child status structures may need to be significantly larger because this number includes the maximum number of tasks that are running PLUS the number of tasks that have exit’ed without having their exit status reaped (via wait(), waitpid() or waitid()).

    Obviously, if tasks spawn children indefinitely and never have the exit status reaped, then you may have a memory leak! (See Warning below)

Warning

If you enable the CONFIG_SCHED_CHILD_STATUS feature, then your application must either (1) take responsibility for reaping the child status with wait(), waitpid() or waitid(), or (2) suppress retention of child status. If you do not reap the child status, then you have a memory leak and your system will eventually fail.

Retention of child status can be suppressed on the parent using logic like:

struct sigaction sa;

sa.sa_handler = SIG_IGN;
sa.sa_flags = SA_NOCLDWAIT;
int ret = sigaction(SIGCHLD, &sa, NULL);

Functions

int sched_lock(void)

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.

Returns:

OK or ERROR.

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

STATUS taskLock(void);
int sched_unlock(void)

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.

Returns:

OK or ERROR.

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

STATUS taskUnlock(void);
int32_t sched_lockcount(void)

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.

Returns:

The current value of the lockCount.

POSIX Compatibility: None.

ipid_t waitpid(pid_t pid, int *stat_loc, int options)

Note

The following discussion is a general description of the waitpid() interface. However, as of this writing, the implementation of waitpid() is incomplete (but usable). If CONFIG_SCHED_HAVE_PARENT is defined, waitpid() will be a little more compliant to specifications. Without CONFIG_SCHED_HAVE_PARENT, waitpid() simply supports waiting for any task to complete execution. With CONFIG_SCHED_HAVE_PARENT, waitpid() will use SIGCHLD and can, therefore, wait for any child of the parent to complete. The implementation is incomplete in either case, however: NuttX does not support any concept of process groups. Nor does NuttX retain the status of exited tasks so if waitpid() is called after a task has exited, then no status will be available. The options argument is currently ignored.

The waitpid() functions will obtain status information pertaining to one of the caller’s child processes. The waitpid() function will suspend execution of the calling thread until status information for one of the terminated child processes of the calling process is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If more than one thread is suspended in waitpid() awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination. If status information is available prior to the call to waitpid(), return will be immediate.

NOTE: Because waitpid() is not fully POSIX compliant, it must be specifically enabled by setting CONFIG_SCHED_WAITPID in the NuttX configuration file.

Parameters:

  • pid – The task ID of the thread to wait for

  • stat_loc – The location to return the exit status

  • options – ignored

The pid argument specifies a set of child processes for which status is requested. The waitpid() function will only return the status of a child process from this set:

  • If pid is equal to (pid_t)-1), status is requested for any child process. In this respect, waitpid() is then equivalent to wait().

  • If pid is greater than 0, it specifies the process ID of a single child process for which status is requested.

  • If pid is 0, status is requested for any child process whose process group ID is equal to that of the calling process.

  • If pid is less than (pid_t)-1), status is requested for any child process whose process group ID is equal to the absolute value of pid.

The options argument is constructed from the bitwise-inclusive OR of zero or more of the following flags, defined in the <sys/wait.h> header:

  • WCONTINUED. The waitpid() function will report the status of any continued child process specified by pid whose status has not been reported since it continued from a job control stop.

  • WNOHANG. The waitpid() function will not suspend execution of the calling thread if status is not immediately available for one of the child processes specified by pid.

  • WUNTRACED. The status of any child processes specified by pid that are stopped, and whose status has not yet been reported since they stopped, will also be reported to the requesting process.

If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN, and the process has no unwaited-for children that were transformed into zombie processes, the calling thread will block until all of the children of the process containing the calling thread terminate, and waitpid() will fail and set errno to ECHILD.

If waitpid() returns because the status of a child process is available, these functions will return a value equal to the process ID of the child process. In this case, if the value of the argument stat_loc is not a null pointer, information will be stored in the location pointed to by stat_loc. The value stored at the location pointed to by stat_loc will be 0 if and only if the status returned is from a terminated child process that terminated by one of the following means:

  1. The process returned 0 from main().

  2. The process called _exit() or exit() with a status argument of 0.

  3. The process was terminated because the last thread in the process terminated.

Regardless of its value, this information may be interpreted using the following macros, which are defined in <sys/wait.h> and evaluate to integral expressions; the stat_val argument is the integer value pointed to by stat_loc.

  • WIFEXITED(stat_val). Evaluates to a non-zero value if status was returned for a child process that terminated normally.

  • WEXITSTATUS(stat_val). If the value of WIFEXITED(stat_val) is non-zero, this macro evaluates to the low-order 8 bits of the status argument that the child process passed to _exit() or exit(), or the value the child process returned from main().

  • WIFSIGNALED(stat_val). Evaluates to a non-zero value if status was returned for a child process that terminated due to the receipt of a signal that was not caught (see >signal.h<).

  • WTERMSIG(stat_val). If the value of WIFSIGNALED(stat_val) is non-zero, this macro evaluates to the number of the signal that caused the termination of the child process.

  • WIFSTOPPED(stat_val). Evaluates to a non-zero value if status was returned for a child process that is currently stopped.

  • WSTOPSIG(stat_val). If the value of WIFSTOPPED(stat_val) is non-zero, this macro evaluates to the number of the signal that caused the child process to stop.

  • WIFCONTINUED(stat_val). Evaluates to a non-zero value if status was returned for a child process that has continued from a job control stop.

Returns:

If waitpid() returns because the status of a child process is available, it will return a value equal to the process ID of the child process for which status is reported.

If waitpid() returns due to the delivery of a signal to the calling process, -1 will be returned and errno set to EINTR.

If waitpid() was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, 0 is returned.

Otherwise, (pid_t)-1errno set to indicate the error:

  • ECHILD. The process specified by pid does not exist or is not a child of the calling process, or the process group specified by pid does not exist or does not have any member process that is a child of the calling process.

  • EINTR. The function was interrupted by a signal. The value of the location pointed to by stat_loc is undefined.

  • EINVAL. The options argument is not valid.

Assumptions/Limitations:

POSIX Compatibility: Comparable to the POSIX interface of the same name, but the implementation is incomplete (as detailed above).

int waitid(idtype_t idtype, id_t id, FAR siginfo_t *info, int options)

Note

The following discussion is a general description of the waitid() interface. However, as of this writing, the implementation of waitid() is incomplete (but usable). If CONFIG_SCHED_HAVE_PARENT is defined, waitid() will be a little more compliant to specifications. waitpid() simply supports waiting a specific child task (P_PID or for any child task P_ALL to complete execution. SIGCHLD is used. The implementation is incomplete in either case, however: NuttX does not support any concept of process groups. Nor does NuttX retain the status of exited tasks so if waitpid() is called after a task has exited, then no status will be available. The options argument is currently ignored.

The waitid() function suspends the calling thread until one child of the process containing the calling thread changes state. It records the current state of a child in the structure pointed to by info. If a child process changed state prior to the call to waitid(), waitid() returns immediately. If more than one thread is suspended in wait() or waitpid() waiting termination of the same process, exactly one thread will return the process status at the time of the target process termination

The idtype and id arguments are used to specify which children waitid() will wait for.

  • If idtype is P_PID, waitid() will wait for the child with a process ID equal to (pid_t)``id``.

  • If idtype is P_PGID, waitid() will wait for any child with a process group ID equal to (pid_t)``id``.

  • If idtype is P_ALL, waitid() will wait for any children and id is ignored.

The options argument is used to specify which state changes waitid() will will wait for. It is formed by OR-ing together one or more of the following flags:

  • WEXITED: Wait for processes that have exited.

  • WSTOPPED: Status will be returned for any child that has stopped upon receipt of a signal.

  • WCONTINUES: Status will be returned for any child that was stopped and has been continued.

  • WNOHANG: Return immediately if there are no children to wait for.

  • WNOWAIT: Keep the process whose status is returned in info in a waitable state. This will not affect the state of the process; the process may be waited for again after this call completes.

The info argument must point to a siginfo_t structure. If waitid() returns because a child process was found that satisfied the conditions indicated by the arguments idtype and options, then the structure pointed to by info will be filled in by the system with the status of the process. The si_signo member will always be equal to SIGCHLD.

Returns:

If waitid() returns due to the change of state of one of its children, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The waitid() function will fail if:

  • ECHILD:

  • EINTR:

  • EINVAL: An invalid value was specified for options, or idtype and id specify an invalid set of processes.

POSIX Compatibility: Comparable to the POSIX interface of the same name, but the implementation is incomplete (as detailed in the description above).

pid_t wait(FAR int *stat_loc)

Note

The following discussion is a general description of the wait() interface. However, as of this writing, the implementation of wait() is incomplete (but usable). wait() is based on waitpid() (see description for further information).

The wait() function will suspend execution of the calling thread until status information for one of its terminated child processes is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If more than one thread is suspended in wait() awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination. If status information is available prior to the call towait(), return will be immediate.

The waitpid() function will behave identically to wait(), if its pid argument is (pid_t)-1 and the options argument is 0. Otherwise, its behavior will be modified by the values of the pid and options arguments.

Parameters:

  • stat_loc – The location to return the exit status

Returns:

See the values returned by waitpid()

POSIX Compatibility: Comparable to the POSIX interface of the same name, but the implementation is incomplete (as detailed in the description `waitpaid() <#waitpid>`__).

int atexit(void (*func)(void))

Registers a function to be called at program exit. The atexit() function registers the given function to be called at normal process termination, whether via exit() or via return from the program’s main().

Note

CONFIG_SCHED_ATEXIT must be defined to enable this function.

Parameters:

  • func – A pointer to the function to be called when the task exits.

Returns:

On success, atexit() returns OK (0). On error, ERROR (-1) is returned, and `errno <#ErrnoAccess>`__ is set to indicate the cause of the failure.

POSIX Compatibility: Comparable to the ISO C interface of the same name. Limitations in the current implementation:

  1. Only a single atexit function can be registered unless CONFIG_SCHED_ATEXIT_MAX defines a larger number.

  2. atexit() functions are not inherited when a new task is created.

int on_exit(CODE void (*func)(int, FAR void*), FAR void *arg)

Registers a function to be called at program exit. The on_exit() function registers the given function to be called at normal process termination, whether via exit() or via return from the program’s main(). The function is passed the status argument given to the last call to exit() and the arg argument from on_exit().

Parameters:

  • func – A pointer to the function to be called when the task exits.

  • arg – An argument that will be provided to the on_exit() function when the task exits.

Returns:

On success, on_exit() returns OK (0). On error, ERROR (-1) is returned, and `errno <#ErrnoAccess>`__ is set to indicate the cause of the failure.

POSIX Compatibility: This function comes from SunOS 4, but is also present in libc4, libc5 and glibc. It no longer occurs in Solaris (SunOS 5). Avoid this function, and use the standard atexit() instead.

  1. Only a single on_exit function can be registered unless CONFIG_SCHED_ONEXIT_MAX defines a larger number.

  2. on_exit() functions are not inherited when a new task is created.