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.TH SEM_INIT 3 2020-08-13 "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 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)
.SH COLOPHON
This page is part of release 5.10 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/.