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$FreeBSD: head/lib/libc/gen/sem_wait.3 57260 2000-02-16 19:31:53Z jasone $
.Dd February 15, 2000 .Dt SEM_WAIT 3 .Os .Sh NAME .Nm sem_wait , .Nm sem_trywait .Nd decrement (lock) a semaphore .Sh SYNOPSIS .Fd #include <semaphore.h> .Ft int .Fn sem_wait "sem_t *sem" .Ft int .Fn sem_trywait "sem_t *sem" .Sh DESCRIPTION The .Fn sem_wait function decrements (locks) the semaphore pointed to by .Fa sem , but blocks if the value of .Fa sem is zero, until the value is non-zero and the value can be decremented.
p The .Fn sem_trywait function decrements (locks) the semaphore pointed to by .Fa sem only if the value is non-zero. Otherwise, the semaphore is not decremented and an error is returned. .Sh RETURN VALUES If successful, .Fn sem_wait and .Fn sem_trywait return 0. Otherwise, -1 is returned and .Va errno is set to indicate the type of error. .Sh ERRORS .Fn sem_wait and .Fn sem_trywait will fail if: l -tag -width Er t Bq Er EINVAL .Fa sem points to an invalid semaphore. .El
p Additionally, .Fn sem_trywait will fail if: l -tag -width Er t Bq Er EAGAIN The semaphore value was zero, and thus could not be decremented. .El .Sh SEE ALSO .Xr sem_post 3 .Sh STANDARDS .Fn sem_wait and .Fn sem_trywait conform to ISO/IEC 9945-1 ANSI/IEEE
q Dq Tn POSIX Std 1003.1 1996 Edition.