sem_wait(3) [netbsd man page]

SEM_WAIT(3)						   BSD Library Functions Manual 					       SEM_WAIT(3)

NAME

     sem_wait, sem_trywait -- decrement (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 indi-
     cate 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

     sem_post(3)

STANDARDS

     sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (``POSIX.1'').

BSD
								 January 22, 2003							       BSD

sem_wait(3)						     Library Functions Manual						       sem_wait(3)

NAME

       sem_wait, sem_trywait - Performs (or conditionally performs) a semaphore lock (P1003.1b)

LIBRARY

       Realtime Library (librt.so, librt.a)

SYNOPSIS

       #include <semaphore.h>

       int sem_wait (
       sem_t *sem);

       int sem_trywait (
       sem_t *sem);

PARAMETERS

       sem	 Specifies a pointer to the semaphore to be locked.

DESCRIPTION

       The sem_wait function locks the semaphore referenced by sem by performing 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 semaphore cannot be accessed by other processes.  The semaphore remains
       locked until the process unlocks it with a call to the sem_post function.

       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 conditions:

       [EAGAIN]       The semaphore was already locked and cannot be locked by the sem_trywait operation (sem_trywait only).

       [EINTR]	      A signal interrupted this function.

       [EINVAL]       The sem argument does not refer to a valid semaphore.

RELATED INFORMATION

       Functions: sem_post(3)

       Guide to Realtime Programming delim off

																       sem_wait(3)