## NAME

sem_wait, sem_trywait, sem_timedwait - acquire or wait for a semaphore

## SYNOPSIS

#include <semaphore.h>

int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
typedef struct {
  ...
} sem_t; /*opaque POSIX semaphore*/

## DESCRIPTION

sem_wait() and sem_trywait() are the functions by which a calling thread waits or proceeds depending upon the state of a semaphore. A synchronizing process can proceed only if the value of the semaphore it accesses is currently greater than 0.

If at the time of a call to either sem_wait() or sem_trywait(), the value of sem is positive, these functions decrement the value of the semaphore, return immediately, and allow the calling process to continue.

If the semaphore's value is 0: sem_wait() blocks, awaiting the semaphore to be released by another thread.

sem_timedwait() locks the semaphore referenced by sem as in the sem_wait() function. However, if the semaphore cannot be locked without waiting for another process or thread to unlock the semaphore by performing a sem_post() function, this wait will be terminated when the specified timeout expires.

The timeout expires when the absolute time specified by abs_timeout passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified by abs_timeout has already been passed at the time of the call.

## RETURN VALUES

All functions return zero if the calling process successfully performed the semaphore lock operation on the semaphore designated by sem. If the call was unsuccessful, the state of the semaphore is unchanged, the calling function returns -1, and sets errno to indicate the error condition.

## ERRORS

sem_wait() function fails if:

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

sem_trywait() function fails if:

EINVAL
  The sem argument does not refer to a valid semaphore.
EAGAIN
  The semaphore was already locked, so it cannot be immediately locked.

sem_timedwait() function fails if:

EINVAL
  The sem argument does not refer to a valid semaphore.
  OR
  The process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.
ETIMEDOUT
  The semaphore could not be locked before the specified timeout expired.

## SEE ALSO

sem_post()

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