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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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sem_open(3) Library Functions Manual sem_open(3)
sem_open - initialize and open a named semaphore
POSIX threads library (libpthread, -lpthread)
#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
#include <semaphore.h>
sem_t *sem_open(const char *name, int oflag, ...
/* mode_t mode, unsigned int value */ );
sem_open() creates a new POSIX semaphore or opens an existing
semaphore. The semaphore is identified by name. For details of
the construction of name, see sem_overview(7).
The oflag argument specifies flags that control the operation of
the call. (Definitions of the flags values can be obtained by
including <fcntl.h>.) If O_CREAT is specified in 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. If both O_CREAT and
O_EXCL are specified in oflag, then an error is returned if a
semaphore with the given name already exists.
If O_CREAT is specified in oflag, then two additional arguments
must be supplied. The mode argument specifies the permissions to
be placed on the new semaphore, as for open(2). (Symbolic
definitions for the permissions bits can be obtained by including
<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 value
argument specifies the initial value for the new semaphore. If
O_CREAT is specified, and a semaphore with the given name already
exists, then mode and value are ignored.
On success, sem_open() returns the address of the new semaphore;
this address is used when calling other semaphore-related
functions. On error, sem_open() returns SEM_FAILED, with errno
set to indicate the error.
EACCES The semaphore exists, but the caller does not have
permission to open it.
EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a
semaphore with this name already exists.
EINVAL value was greater than SEM_VALUE_MAX.
EINVAL name consists of just "/", followed by no other characters.
EMFILE The per-process limit on the number of open file
descriptors has been reached.
ENAMETOOLONG
name was too long.
ENFILE The system-wide limit on the total number of open files has
been reached.
ENOENT The O_CREAT flag was not specified in oflag and no
semaphore with this name exists; or, O_CREAT was specified,
but name wasn't well formed.
ENOMEM Insufficient memory.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ sem_open() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001.
sem_close(3), sem_getvalue(3), sem_post(3), sem_unlink(3),
sem_wait(3), sem_overview(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sem_open(3)
Pages that refer to this page: umask(2), sem_close(3), sem_unlink(3), sem_overview(7)
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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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