sem_t *sem_open(const char *name, int oflag,
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_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:
The user ID of the semaphore is set to the effective user ID of the process; the group ID of the semaphore is set to a system default group ID or to the effective group ID of the process. 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.
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. 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. The interpretation of slash characters other than the leading slash character in name 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.
References to copies of the semaphore produce undefined results.
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.
If any of the following conditions occur, the sem_open() function shall return SEM_FAILED and set errno to the corresponding value:
The following sections are informative.
The sem_open() function is part of the Semaphores option and need not be available on all implementations.
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.
semctl() , semget() , semop() , sem_close() , sem_post() , sem_timedwait() , sem_trywait() , sem_unlink() , sem_wait() , the Base Definitions volume of IEEE Std 1003.1-2001, <semaphore.h>