sem_init Subroutine

Purpose

Initializes an unnamed semaphore.

Library

Standard C Library (libc.a)

Syntax

#include <semaphore.h>

int sem_init (sem, pshared, value)
sem_t *sem;
int pshared;
unsigned value;

Description

The sem_init subroutine initializes the unnamed semaphore referred to by the sem parameter. The value of the initialized semaphore is contained in the value parameter. Following a successful call to the sem_init subroutine, the semaphore might be used in subsequent calls to the sem_wait, sem_trywait, sem_post, and sem_destroy subroutines. This semaphore remains usable until it is destroyed.

If the pshared parameter has a nonzero value, the semaphore is shared between processes. In this case, any process that can access the sem parameter can use it for performing sem_wait, sem_trywait, sem_post, and sem_destroy operations.

Only the sem parameter itself may be used for performing synchronization.

If the pshared parameter is zero, the semaphore is shared between threads of the process. Any thread in this process can use the sem parameter for performing sem_wait, sem_trywait, sem_post, and sem_destroy operations. The use of the semaphore by threads other than those created in the same process returns an error.

Attempting to initialize a semaphore that has been already initialized results in the loss of access to the previous semaphore.

Parameters

Return Values

Upon successful completion, the sem_init subroutine initializes the semaphore in the sem parameter. Otherwise, it returns -1 and sets errno to indicate the error.

Error Codes