sem_wait(3)sem_wait(3)NAME
sem_wait, sem_trywait - Performs (or conditionally performs) a sema‐
phore lock (P1003.1b)
SYNOPSIS
#include <semaphore.h>
int sem_wait ( sem_t *sem);
int sem_trywait ( sem_t *sem)
LIBRARY
Realtime Library (librt.so, librt.a)
PARAMETERS
Specifies a pointer to the semaphore to be locked.
DESCRIPTION
The sem_wait function locks the semaphore referenced by sem by perform‐
ing a semaphore lock operation on it. If the semaphore value is zero,
the sem_wait function blocks until it either locks the semaphore or is
interrupted by a signal.
The sem_trywait function locks a semaphore only if the semaphore is
currently not locked. If the semaphore value is zero, the sem_trywait
function returns without locking the semaphore.
These functions help ensure that the resource associated with the sema‐
phore cannot be accessed by other processes. The semaphore remains
locked until the process unlocks it with a call to the sem_post func‐
tion.
Use the sem_wait function instead of the sem_trywait function if the
process should wait for access to the semaphore.
RETURN VALUES
If the sem_wait or sem_trywait function returns successfully, a value
of 0 (zero) is returned and the function executes the semaphore lock
operation.
On an unsuccessful call, a value of -1 is returned and errno is set to
indicate that an error occurred. The state of the semaphore remains
unchanged.
ERRORS
The sem_wait and sem_trywait functions fail under the following condi‐
tions: The semaphore was already locked and cannot be locked by the
sem_trywait operation (sem_trywait only). A signal interrupted this
function. The sem argument does not refer to a valid semaphore.
SEE ALSO
Functions: sem_post(3)
Guide to Realtime Programming
sem_wait(3)