'\" t .\" Copyright (C) 2006 Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .TH sem_init 3 2024-05-02 "Linux man-pages 6.9.1" .SH NAME sem_init \- initialize an unnamed semaphore .SH LIBRARY POSIX threads library .RI ( libpthread ", " \-lpthread ) .SH SYNOPSIS .nf .B #include .P .BI "int sem_init(sem_t *" sem ", int " pshared ", unsigned int " value ); .fi .SH DESCRIPTION .BR sem_init () initializes the unnamed semaphore at the address pointed to by .IR sem . The .I value argument specifies the initial value for the semaphore. .P The .I pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes. .P If .I pshared has the value 0, then the semaphore is shared between the threads of a process, and should be located at some address that is visible to all threads (e.g., a global variable, or a variable allocated dynamically on the heap). .P If .I pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see .BR shm_open (3), .BR mmap (2), and .BR shmget (2)). (Since a child created by .BR fork (2) inherits its parent's memory mappings, it can also access the semaphore.) Any process that can access the shared memory region can operate on the semaphore using .BR sem_post (3), .BR sem_wait (3), and so on. .P Initializing a semaphore that has already been initialized results in undefined behavior. .SH RETURN VALUE .BR sem_init () returns 0 on success; on error, \-1 is returned, and .I errno is set to indicate the error. .SH ERRORS .TP .B EINVAL .I value exceeds .BR SEM_VALUE_MAX . .TP .B ENOSYS .I pshared is nonzero, but the system does not support process-shared semaphores (see .BR sem_overview (7)). .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR sem_init () T} Thread safety MT-Safe .TE .SH STANDARDS POSIX.1-2008. .SH HISTORY POSIX.1-2001. .P Bizarrely, POSIX.1-2001 does not specify the value that should be returned by a successful call to .BR sem_init (). POSIX.1-2008 rectifies this, specifying the zero return on success. .SH EXAMPLES See .BR shm_open (3) and .BR sem_wait (3). .SH SEE ALSO .BR sem_destroy (3), .BR sem_post (3), .BR sem_wait (3), .BR sem_overview (7)