Locks a semaphore.
Standard C Library (libc.a)
#include <semaphore.h>
int sem_trywait (sem)
sem_t *sem;
int sem_wait (sem)
sem_t *sem;The sem_trywait subroutine locks the semaphore referenced by the sem parameter only if the semaphore is currently not locked; that is, if the semaphore value is currently positive. Otherwise, it does not lock the semaphore.
The sem_wait subroutine locks the semaphore referenced by the sem parameter by performing a semaphore lock operation on that semaphore. If the semaphore value is currently zero, the calling thread does not return from the call to the sem_wait subroutine until it either locks the semaphore or the call is interrupted by a signal.
Upon successful return, the state of the semaphore will be locked and will remain locked until the sem_post subroutine is executed and returns successfully.
The sem_wait subroutine is interruptible by the delivery of a signal.
| Item | Description |
|---|---|
| sem | Specifies the semaphore to be locked. |
The sem_trywait and sem_wait subroutines return zero if the calling process successfully performed the semaphore lock operation. If the call was unsuccessful, the state of the semaphore is unchanged, and the subroutine returns -1 and sets errno to indicate the error.
| Item | Description |
|---|---|
| EACCES | Permission is denied to access the unnamed semaphore. |
| EAGAIN | The semaphore was already locked, so it cannot be immediately locked by the sem_trywait subroutine. |
| EFAULT | Invalid user address. |
| EIDRM | Semaphore was removed during the required operation. |
| EINTR | A signal interrupted the subroutine. |
| EINVAL | The sem parameter does not refer to a valid semaphore. |
| ENOMEM | Insufficient memory for the required operation. |
| ENOTSUP | This function is not supported with processes that have been checkpoint-restart'ed. |