| '\" t |
| .\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" |
| .\" SPDX-License-Identifier: Linux-man-pages-copyleft |
| .\" |
| .TH sem_init 3 (date) "Linux man-pages (unreleased)" |
| .SH NAME |
| sem_init \- initialize an unnamed semaphore |
| .SH LIBRARY |
| POSIX threads library |
| .RI ( libpthread ,\~ \-lpthread ) |
| .SH SYNOPSIS |
| .nf |
| .B #include <semaphore.h> |
| .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) |
