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.TH SEM_INIT 3 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

sem_init \- initialize an unnamed semaphore

.SH SYNOPSIS

.nf

.B #include <semaphore.h>

.PP

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

.fi

.PP

Link with \fI\-pthread\fP.

.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.

.PP

The

.I pshared

argument indicates whether this semaphore is to be shared

between the threads of a process, or between processes.

.PP

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).

.PP

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.

.PP

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;

lb lb lb

l l l.

Interface	Attribute	Value

T{

.BR sem_init ()

T}	Thread safety	MT-Safe

.TE

.SH CONFORMING TO

POSIX.1-2001.

.SH NOTES

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 SEE ALSO

.BR sem_destroy (3),

.BR sem_post (3),

.BR sem_wait (3),

.BR sem_overview (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.