PROLOG

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

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 .