NAME
sem_trywait, sem_wait - lock a semaphore (REALTIME)
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() and sem_wait() functions shall fail if:
EAGAIN The semaphore was already locked, so it cannot be immediately
locked by the sem_trywait() operation ( sem_trywait() only).
EINVAL The sem argument does not refer to a valid semaphore.
The sem_trywait() and sem_wait() functions may fail if:
EDEADLK
A deadlock condition was detected.
EINTR A signal interrupted this function.
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
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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
semctl() , semget() , semop() , sem_post() , sem_timedwait() , the Base
Definitions volume of IEEE Std 1003.1-2001, <semaphore.h>
COPYRIGHT
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 .