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.1‐2008, Section 3.287, Priority Inversion.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       semctl(), semget(), semop(), sem_post(), sem_timedwait()

       The Base Definitions volume of POSIX.1‐2008, Section 3.287,  Priority  Inversion,  Section  4.11,  Memory
       Synchronization, <semaphore.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications  Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .