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
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
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.
Applications using these functions may be subject to priority inversion, as
discussed in the Base Definitions volume of
IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
The sem_trywait() and sem_wait() functions are part of the
Semaphores option and need not be provided on all implementations.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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