SEM_OPEN
Section: POSIX Programmer's Manual (3P)
Updated: 2017
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PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
sem_open
--- initialize and open a named semaphore
SYNOPSIS
#include <semaphore.h>
sem_t *sem_open(const char *name, int oflag, ...);
DESCRIPTION
The
sem_open()
function shall establish a connection between a named semaphore
and a process. Following a call to
sem_open()
with semaphore name
name,
the process may reference the semaphore associated with
name
using the address returned from the call. This semaphore may be used
in subsequent calls to
sem_wait(),
sem_timedwait(),
sem_trywait(),
sem_post(),
and
sem_close().
The semaphore remains usable by this process until the semaphore is
closed by a successful call to
sem_close(),
_exit(),
or one of the
exec
functions.
The
oflag
argument controls whether the semaphore is created or merely accessed
by the call to
sem_open().
The following flag bits may be set in
oflag:
- O_CREAT
-
This flag is used to create a semaphore if it does not already exist.
If O_CREAT is set and the semaphore already exists, then O_CREAT has no
effect, except as noted under O_EXCL. Otherwise,
sem_open()
creates a named semaphore. The O_CREAT flag requires a third and a
fourth argument:
mode,
which is of type
mode_t,
and
value,
which is of type
unsigned.
The semaphore is created with an initial value of
value.
Valid initial values for semaphores are less than or equal to
{SEM_VALUE_MAX}.
-
The user ID of the semaphore shall be set to the effective user ID of
the process. The group ID of the semaphore shall be set to the effective
group ID of the process; however, if the
name
argument is visible in the file system, the group ID may be set to
the group ID of the containing directory. The permission bits of the
semaphore are set to the value of the
mode
argument except those set in the file mode creation mask of the
process. When bits in
mode
other than the file permission bits are specified, the effect is
unspecified.
After the semaphore named
name
has been created by
sem_open()
with the O_CREAT flag, other processes can connect to the semaphore by
calling
sem_open()
with the same value of
name.
- O_EXCL
-
If O_EXCL and O_CREAT are set,
sem_open()
fails if the semaphore
name
exists. The check for the existence of the semaphore and the creation
of the semaphore if it does not exist are atomic with respect to other
processes executing
sem_open()
with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set,
the effect is undefined.
-
If flags other than O_CREAT and O_EXCL are specified in the
oflag
parameter, the effect is unspecified.
The
name
argument points to a string naming a semaphore object. It is
unspecified whether the name appears in the file system and is visible
to functions that take pathnames as arguments. The
name
argument conforms to the construction rules for a pathname, except
that the interpretation of
<slash>
characters other than the leading
<slash>
character in
name
is implementation-defined, and that the length limits for the
name
argument are implementation-defined and need not be the same as
the pathname limits
{PATH_MAX}
and
{NAME_MAX}.
If
name
begins with the
<slash>
character, then processes calling
sem_open()
with the same value of
name
shall refer to the same semaphore object, as long as that name has not
been removed. If
name
does not begin with the
<slash>
character, the effect is implementation-defined.
If a process makes multiple successful calls to
sem_open()
with the same value for
name,
the same semaphore address shall be returned for each such successful
call, provided that there have been no calls to
sem_unlink()
for this semaphore, and at least one previous successful
sem_open()
call for this semaphore has not been matched with a
sem_close()
call.
References to copies of the semaphore produce undefined results.
RETURN VALUE
Upon successful completion, the
sem_open()
function shall return the address of the semaphore. Otherwise, it
shall return a value of SEM_FAILED and set
errno
to indicate the error. The symbol SEM_FAILED is defined in the
<semaphore.h>
header. No successful return from
sem_open()
shall return the value SEM_FAILED.
ERRORS
If any of the following conditions occur, the
sem_open()
function shall return SEM_FAILED and set
errno
to the corresponding value:
- EACCES
-
The named semaphore exists and the permissions specified by
oflag
are denied, or the named semaphore does not exist and permission to
create the named semaphore is denied.
- EEXIST
-
O_CREAT and O_EXCL are set and the named semaphore already exists.
- EINTR
-
The
sem_open()
operation was interrupted by a signal.
- EINVAL
-
The
sem_open()
operation is not supported for the given name, or O_CREAT was specified
in
oflag
and
value
was greater than
{SEM_VALUE_MAX}.
- EMFILE
-
Too many semaphore descriptors or file descriptors are currently in use
by this process.
- ENFILE
-
Too many semaphores are currently open in the system.
- ENOENT
-
O_CREAT is not set and the named semaphore does not exist.
- ENOMEM
-
There is insufficient memory for the creation of the new named
semaphore.
- ENOSPC
-
There is insufficient space on a storage device for the creation of the
new named semaphore.
If any of the following conditions occur, the
sem_open()
function may return SEM_FAILED and set
errno
to the corresponding value:
- ENAMETOOLONG
-
The length of the
name
argument exceeds
{_POSIX_PATH_MAX}
on systems that do not support the XSI option
or exceeds
{_XOPEN_PATH_MAX}
on XSI systems,
or has a pathname component that is longer than
{_POSIX_NAME_MAX}
on systems that do not support the XSI option
or longer than
{_XOPEN_NAME_MAX}
on XSI systems.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
Early drafts required an error return value of -1 with the type
sem_t *
for the
sem_open()
function, which is not guaranteed to be portable across
implementations. The revised text provides the symbolic error code
SEM_FAILED to eliminate the type conflict.
FUTURE DIRECTIONS
A future version might require the
sem_open()
and
sem_unlink()
functions to have semantics similar to normal file system operations.
SEE ALSO
semctl(),
semget(),
semop(),
sem_close(),
sem_post(),
sem_timedwait(),
sem_trywait(),
sem_unlink()
The Base Definitions volume of POSIX.1-2017,
<semaphore.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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