The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> sem_trywait, sem_wait - lock a semaphore

#### SYNOPSIS

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

#### DESCRIPTION

> The sem_trywait() function shall lock the semaphore referenced by sem only if the semaphore is currently not locked; that is, if the semaphore value is currently positive. Otherwise, it shall not lock the semaphore.

> The sem_wait() function shall lock the semaphore referenced by sem by performing a semaphore lock operation on that semaphore. If the semaphore value is currently zero, then the calling thread shall not return from the call to sem_wait() until it either locks the semaphore or the call is interrupted by a signal.

> Upon successful return, the state of the semaphore shall be locked and shall remain locked until the sem_post() function is executed and returns successfully.

> The sem_wait() function is interruptible by the delivery of a signal.

#### RETURN VALUE

> The sem_trywait() and sem_wait() functions shall return zero if the calling process successfully performed the semaphore lock operation on the semaphore designated by sem. If the call was unsuccessful, the state of the semaphore shall be unchanged, and the function shall return a value of -1 and set errno to indicate the error.

#### ERRORS

> The sem_trywait() function shall fail if:

> [EAGAIN]
>   The semaphore was already locked, so it cannot be immediately locked by the sem_trywait() operation.

> The sem_trywait() and sem_wait() functions may fail if:

> [EDEADLK]
>   A deadlock condition was detected.
> [EINTR]
>   A signal interrupted this function.
> [EINVAL]
>   The sem argument does not refer to a valid semaphore.

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> Applications using these functions may be subject to priority inversion, as discussed in XBD Priority Inversion.

#### RATIONALE

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> semctl, semget, semop, sem_post, sem_timedwait

> XBD Priority Inversion, Memory Synchronization, <semaphore.h>

#### CHANGE HISTORY

> First released in Issue 5. Included for alignment with the POSIX Realtime Extension.

#### Issue 6

> The sem_trywait() and sem_wait() functions are marked as part of the Semaphores option.

> The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Semaphores option.

> The sem_timedwait() function is added to the SEE ALSO section for alignment with IEEE Std 1003.1d-1999.

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/121 is applied, updating the ERRORS section so that the [EINVAL] error becomes optional.

#### Issue 7

> SD5-XSH-ERN-54 is applied, removing the sem_wait() function from the ``shall fail'' error cases.

> The sem_trywait() and sem_wait() functions are moved from the Semaphores option to the Base.

> POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0530 [37] is applied.

End of informative text.

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