sem_open(3p) — Linux manual page


SEM_OPEN(3P)              POSIX Programmer's Manual             SEM_OPEN(3P)

PROLOG         top

       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         top

       sem_open — initialize and open a named semaphore

SYNOPSIS         top

       #include <semaphore.h>

       sem_t *sem_open(const char *name, int oflag, ...);

DESCRIPTION         top

       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

                 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         top

       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         top

       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

       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

       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

       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:

              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

       The following sections are informative.

EXAMPLES         top




RATIONALE         top

       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.

SEE ALSO         top

       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.1‐2008, semaphore.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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 .

       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 .

IEEE/The Open Group                 2013                        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)