The sem_timedwait() function shall lock the semaphore referenced by
sem as in the sem_wait() function. However, if the semaphore
cannot be locked without waiting for another process or thread to unlock the
semaphore by performing a sem_post() function, this wait shall be
terminated when the specified timeout expires.
The timeout shall expire when the absolute time specified by abs_timeout
passes, as measured by the clock on which timeouts are based (that is, when
the value of that clock equals or exceeds abs_timeout), or if the
absolute time specified by abs_timeout has already been passed at the
time of the call.
If the Timers option is supported, the timeout shall be based on the
CLOCK_REALTIME clock. If the Timers option is not supported, the timeout shall
be based on the system clock as returned by the time() function. The
resolution of the timeout shall be the resolution of the clock on which it is
based. The timespec data type is defined as a structure in the
Under no circumstance shall the function fail with a timeout if the semaphore
can be locked immediately. The validity of the abs_timeout need not be
checked if the semaphore can be locked immediately.
The sem_timedwait() function 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_timedwait() function is part of the Semaphores and Timeouts
options 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