'\" t
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH sem_open 3 2024-05-02 "Linux man-pages 6.8"
.SH NAME
sem_open \- initialize and open a named semaphore
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.BR "#include <fcntl.h>" "           /* For O_* constants */"
.BR "#include <sys/stat.h>" "        /* For mode constants */"
.B #include <semaphore.h>
.P
.BI "sem_t *sem_open(const char *" name ", int " oflag );
.BI "sem_t *sem_open(const char *" name ", int " oflag ,
.BI "                mode_t " mode ", unsigned int " value );
.fi
.SH DESCRIPTION
.BR sem_open ()
creates a new POSIX semaphore or opens an existing semaphore.
The semaphore is identified by
.IR name .
For details of the construction of
.IR name ,
see
.BR sem_overview (7).
.P
The
.I oflag
argument specifies flags that control the operation of the call.
(Definitions of the flags values can be obtained by including
.IR <fcntl.h> .)
If
.B O_CREAT
is specified in
.IR oflag ,
then the semaphore is created if
it does not already exist.
The owner (user ID) of the semaphore is set to the effective
user ID of the calling process.
The group ownership (group ID) is set to the effective group ID
of the calling process.
.\" In reality the filesystem IDs are used on Linux.
If both
.B O_CREAT
and
.B O_EXCL
are specified in
.IR oflag ,
then an error is returned if a semaphore with the given
.I name
already exists.
.P
If
.B O_CREAT
is specified in
.IR oflag ,
then two additional arguments must be supplied.
The
.I mode
argument specifies the permissions to be placed on the new semaphore,
as for
.BR open (2).
(Symbolic definitions for the permissions bits can be obtained by including
.IR <sys/stat.h> .)
The permissions settings are masked against the process umask.
Both read and write permission should be granted to each class of
user that will access the semaphore.
The
.I value
argument specifies the initial value for the new semaphore.
If
.B O_CREAT
is specified, and a semaphore with the given
.I name
already exists, then
.I mode
and
.I value
are ignored.
.SH RETURN VALUE
On success,
.BR sem_open ()
returns the address of the new semaphore;
this address is used when calling other semaphore-related functions.
On error,
.BR sem_open ()
returns
.BR SEM_FAILED ,
with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EACCES
The semaphore exists, but the caller does not have permission to
open it.
.TP
.B EEXIST
Both
.B O_CREAT
and
.B O_EXCL
were specified in
.IR oflag ,
but a semaphore with this
.I name
already exists.
.TP
.B EINVAL
.I value
was greater than
.BR SEM_VALUE_MAX .
.TP
.B EINVAL
.I name
consists of just "/", followed by no other characters.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENAMETOOLONG
.I name
was too long.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ENOENT
The
.B O_CREAT
flag was not specified in
.I oflag
and no semaphore with this
.I name
exists;
or,
.\" this error can occur if we have a name of the (nonportable) form
.\" /dir/name, and the directory /dev/shm/dir does not exist.
.B O_CREAT
was specified, but
.I name
wasn't well formed.
.TP
.B ENOMEM
Insufficient memory.
.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_open ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH SEE ALSO
.BR sem_close (3),
.BR sem_getvalue (3),
.BR sem_post (3),
.BR sem_unlink (3),
.BR sem_wait (3),
.BR sem_overview (7)