SEM_WAIT(3) Library Functions Manual SEM_WAIT(3)
NAME
sem_wait, sem_trywaitdecrement (lock) a semaphore
LIBRARY
POSIX Real-time Library (librt, -lrt)
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
sem_wait() and sem_trywait() will fail if:
[EINVAL]
sem points to an invalid semaphore.
 
Additionally, sem_trywait() will fail if:
[EAGAIN]
The semaphore value was zero, and thus could not be decremented.
SEE ALSO
STANDARDS
sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (“POSIX.1”).