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SEM_OPEN(2)                 BSD System Calls Manual                SEM_OPEN(2)

NAME
     sem_open -- initialize and open a named semaphore

SYNOPSIS
     #include <semaphore.h>

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

     The parameters "mode_t mode" and "unsigned int value" are optional.

DESCRIPTION
     The named semaphore named name is initialized and opened as specified by
     the argument oflag and a semaphore descriptor is returned to the calling
     process.

     The value of oflag is formed by or'ing the following values:

           O_CREAT         create the semaphore if it does not exist
           O_EXCL          error if create and semaphore exists

     If O_CREAT is specified, sem_open() requires an additional two arguments.
     mode specifies the permissions for the semaphore as described in chmod(2)
     and modified by the process' umask value (see umask(2)).  The semaphore
     is created with an initial value, which must be less than or equal to
     SEM_VALUE_MAX.

     If O_EXCL is specified and the semaphore exists, sem_open() fails.  The
     check for the existence of the semaphore and the creation of the sema-phore semaphore
     phore are atomic with respect to all processes calling sem_open() with
     O_CREAT and O_EXCL set.

     When a new semaphore is created, it is given the user ID and group ID
     which correspond to the effective user and group IDs of the calling
     process. There is no visible entry in the file system for the created
     object in this implementation.

     The returned semaphore descriptor is available to the calling process
     until it is closed with sem_close(), or until the caller exits or execs.

     If a process makes repeated calls to sem_open(), with the same name argu-ment, argument,
     ment, the same descriptor is returned for each successful call, unless
     sem_unlink() has been called on the semaphore in the interim.

     If sem_open() fails for any reason, it will return a value of SEM_FAILED
     and sets errno.  On success, it returns a semaphore descriptor.

ERRORS
     The named semaphore is opened unless:

     [EACCES]           The required permissions (for reading and/or writing)
                        are denied for the given flags; or O_CREAT is speci-fied, specified,
                        fied, the object does not exist, and permission to
                        create the semaphore is denied.

     [EEXIST]           O_CREAT and O_EXCL were specified and the semaphore
                        exists.

     [EINTR]            The sem_open() operation was interrupted by a signal.

     [EINVAL]           The shm_open() operation is not supported; or O_CREAT
                        is specified and value exceeds SEM_VALUE_MAX.

     [EMFILE]           The process has already reached its limit for sema-phores semaphores
                        phores or file descriptors in use.

     [ENAMETOOLONG]     name exceeded SEM_NAME_LEN characters.

     [ENFILE]           Too many semaphores or file descriptors are open on
                        the system.

     [ENOENT]           O_CREAT is not set and the named semaphore does not
                        exist.

     [ENOSPC]           O_CREAT is specified, the file does not exist, and
                        there is insufficient space available to create the
                        semaphore.

SEE ALSO
     sem_close(2), sem_post(2), sem_trywait(2), sem_unlink(2), sem_wait(2),
     semctl(2), semget(2), semop(2), umask(2)

HISTORY
     sem_open() is specified in the POSIX Realtime Extension
     (1003.1b-1993/1003.1i-1995).

Darwin                           June 8, 2000                           Darwin