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Man Pages
SEM_WAIT(3) FreeBSD Library Functions Manual SEM_WAIT(3)

sem_wait, sem_trywait
decrement (lock) a semaphore

Standard C Library (libc, -lc)

#include <semaphore.h>
int
sem_wait(sem_t *sem);
int
sem_trywait(sem_t *sem);

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.

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.

The sem_wait() and sem_trywait() functions will fail if:
[]
The sem argument points to an invalid semaphore.
Additionally, sem_wait() will fail if:
[]
A signal interrupted this function.
Additionally, sem_trywait() will fail if:
[]
The semaphore value was zero, and thus could not be decremented.

sem_getvalue(3), sem_post(3), sem_timedwait(3)

The sem_wait() and sem_trywait() functions conform to ISO/IEC 9945-1:1996 (“POSIX.1”).
April 16, 2013 FreeBSD 12.0-RELEASE

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