DragonFly On-Line Manual Pages


SEM_WAIT(3)	      DragonFly Library Functions Manual	   SEM_WAIT(3)

NAME

sem_wait, sem_trywait -- decrement (lock) a semaphore

LIBRARY

POSIX Threads Library (libpthread, -lpthread)

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

Upon successful completion, the value 0 is returned; 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

sem_post(3)

STANDARDS

sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (``POSIX.1''). DragonFly 3.9 April 15, 2009 DragonFly 3.9