SEM_TRYWAIT(3)             POSIX Programmer's Manual            SEM_TRYWAIT(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 the Base Definitions volume of POSIX.1-2017,
       Section 3.291, Priority Inversion.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       semctl(), semget(), semop(), sem__post(), sem__timedwait()

       The Base Definitions volume of POSIX.1-2017, Section 3.291, Priority
       Inversion, Section 4.12, Memory Synchronization, <semaphore.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                       SEM_TRYWAIT(3)
