SEM_OPEN(2)

NAME
sem_open - initialize and open a named semaphore

SYNOPSIS
#include <semaphore.h>
sem_t *
sem_open(const char *name, int flags);
sem_t *
sem_open(const char *name, int flags, mode_t mode, unsigned int value);

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

The flags specified are 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_CREATE if 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 semaphore 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 coorespond 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 argument, 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 specified, 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 semaphores 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
semctl(2), semget(2), semop(2), sem_close(2), sem_post(2), sem_trywait(2), sem_unlink(2), sem_wait(2), umask(2)

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