NAME
sem_wait, sem_trywait – decrement (lock) a semaphore
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <semaphore.h>
int
sem_wait(sem_t *sem);
int
sem_trywait(sem_t *sem);
DESCRIPTION
The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-zero and the value can be decremented.
The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero. Otherwise, the semaphore is not decremented and an error is returned.
RETURN VALUES
The sem_wait() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS
The sem_wait() and sem_trywait() functions will fail if:
- [EINVAL]
-
The sem argument points to an invalid semaphore.
Additionally, sem_wait() will fail if:
- [EINTR]
-
A signal interrupted this function.
Additionally, sem_trywait() will fail if:
- [EAGAIN]
-
The semaphore value was zero, and thus could not be decremented.
SEE ALSO
STANDARDS
The sem_wait() and sem_trywait() functions conform to ISO/IEC 9945-1:1996 (“POSIX.1”).