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.
sem_trywait, sem_wait — lock a semaphore
int sem_trywait(sem_t * sem);
int sem_wait(sem_t * sem);
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.
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:
The sem_trywait() and sem_wait() functions may fail if:
- The semaphore was already locked, so it cannot be immediately locked by the sem_trywait() operation.
- A deadlock condition was detected.
- A signal interrupted this function.
The following sections are informative.
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.
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>
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 .
- The sem argument does not refer to a valid semaphore.
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