Identifiers in this page not found in current SDK headers: PSEMTEAMIDLEN
documents: sem_open
SEM_OPEN(2) System Calls Manual SEM_OPEN(2)

sem_open — initialize and open a named semaphore

#include <semaphore.h>

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

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

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 '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, () 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, () 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 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.

There are additional restrictions for non-macOS platforms only: if the calling process is signed with a Team ID entitlement, then name is ‘namespaced’, meaning calls to () or () will not observe any semaphores created by processes signed with different Team IDs. If a process lacks a Team ID entitlement, then sem_open() and sem_unlink() operate on a global namespace, which is always searched as a fallback.

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

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

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

The named semaphore is opened unless:

[]
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.
[]
and O_EXCL were specified and the semaphore exists.
[]
The sem_open() operation was interrupted by a signal.
[]
The shm_open() operation is not supported; or O_CREAT is specified and value exceeds SEM_VALUE_MAX.
[]
The process has already reached its limit for semaphores or file descriptors in use.
[]
name exceeded PSEMNAMLEN characters, or Team ID of calling binary exceeded PSEMTEAMIDLEN characters.
[]
Too many semaphores or file descriptors are open on the system.
[]
is not set and the named semaphore does not exist.
[]
is specified, the file does not exist, and there is insufficient space available to create the semaphore.

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

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

June 8, 2000 Darwin