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FreeBSD 11.0 - man page for sem_trywait (freebsd section 3)

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

NAME
sem_wait, sem_trywait -- decrement (lock) a semaphore
LIBRARY
Standard C Library (libc, -lc)
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
The sem_wait() and sem_trywait() functions will fail if: [EINVAL] The sem argument points to an invalid semaphore. Additionally, sem_wait() will fail if: [EINTR] A signal interrupted this function. Additionally, sem_trywait() will fail if: [EAGAIN] The semaphore value was zero, and thus could not be decremented.
SEE ALSO
sem_getvalue(3), sem_post(3), sem_timedwait(3)
STANDARDS
The sem_wait() and sem_trywait() functions conform to ISO/IEC 9945-1:1996 (``POSIX.1'').
BSD
April 16, 2013 BSD

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