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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SEM_OPEN(3P) POSIX Programmer's Manual SEM_OPEN(3P)
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.
sem_open — initialize and open a named semaphore
#include <semaphore.h>
sem_t *sem_open(const char *name, int oflag, ...);
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.
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.
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.
None.
None.
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.
A future version might require the sem_open() and sem_unlink()
functions to have semantics similar to normal file system
operations.
semctl(3p), semget(3p), semop(3p), sem_close(3p), sem_post(3p),
sem_timedwait(3p), sem_trywait(3p), sem_unlink(3p)
The Base Definitions volume of POSIX.1‐2017, semaphore.h(0p)
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 .
IEEE/The Open Group 2017 SEM_OPEN(3P)
Pages that refer to this page: semaphore.h(0p), sem_close(3p), semctl(3p), sem_destroy(3p), semget(3p), semop(3p), sem_unlink(3p), sigaction(3p), umask(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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