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.
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 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
The sem_wait() function is interruptible by the delivery of a signal.
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.
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:
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.
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
IEEE/The Open Group 2013 SEM_TRYWAIT(3P)