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

                 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 {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:

              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         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

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.1‐2017, semaphore.h(0p)

COPYRIGHT         top

       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 .

       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               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)