SEM_OPEN(2) BSD System Calls Manual SEM_OPEN(2)
sem_open -- initialize and open a named semaphore
sem_open(const char *name, int flags);
sem_open(const char *name, int flags, mode_t mode, unsigned int value);
The named semaphore named name is initialized and opened as specified by
the argument flags and a semaphore descriptor is returned to the calling
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 argu-
ments. 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 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 argu-
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.
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, the object does not exist, and permission to
create the semaphore is denied.
[EEXIST] O_CREAT and O_EXCL were specified and the semaphore
[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 or file descriptors in use.
[ENAMETOOLONG] name exceeded SEM_NAME_LEN characters.
[ENFILE] Too many semaphores or file descriptors are open on
[ENOENT] O_CREAT is not set and the named semaphore does not
[ENOSPC] O_CREAT is specified, the file does not exist, and
there is insufficient space available to create the
semctl(2), semget(2), semop(2), sem_close(2), sem_post(2),
sem_trywait(2), sem_unlink(2), sem_wait(2), umask(2)
sem_open() is specified in the POSIX Realtime Extension
Darwin June 8, 2000 Darwin