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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 .



IEEE/The Open Group                  2017                          SEM_OPEN(3)
