The function returns the semaphore set identifier associated to the value of the argument key. A new set of nsems semaphores is created if key has value IPC_PRIVATE or key isn't IPC_PRIVATE, no existing semaphore set is associated to key, and IPC_CREAT is asserted in semflg (i.e. semflg IPC_CREAT isn't zero). The presence in semflg of the fields IPC_CREAT and IPC_EXCL plays the same role, with respect to the existence of the semaphore set, as the presence of O_CREAT and O_EXCL in the mode argument of the open(2) system call: i.e. the semget function fails if semflg asserts both IPC_CREAT and IPC_EXCL__ and a semaphore set already exists for key''.
Upon creation, the lower 9 bits of the argument semflg define the access permissions (for owner, group and others) to the semaphore set in the same format, and with the same meaning, as for the access permissions parameter in the open(2) or creat(2) system calls (though the execute permissions are not used by the system, and write permissions, for a semaphore set, effectively means alter permissions).
Furthermore, while creating, the system call initializes the system semaphore set data structure semid_ds as follows:
sem_perm.cuid and sem_perm.uid are set to the effective user-ID of the calling process.
sem_perm.cgid and sem_perm.gid are set to the effective group-ID of the calling process.
The lowest order 9 bits of sem_perm.mode are set to the lowest order 9 bit of semflg.
sem_nsems is set to the value of nsems.
sem_otime is set to 0.
sem_ctime is set to the current time.
The argument nsems can be 0 (a don't care) when the system call isn't a create one. Otherwise nsems must be greater than 0 and less or equal to the maximum number of semaphores per semid, (SEMMSL).
For a failing return, errno will be set to one among the following values:
A semaphore set exists for key, but the calling process has no access permissions to the set.
A semaphore set exists for key and semflg was asserting both IPC_CREAT and IPC_EXCL.
The semaphore set is marked as to be deleted.
No semaphore set exists for key and semflg wasn't asserting IPC_CREAT.
A semaphore set has to be created but the system has not enough memory for the new data structure.
IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the lowest order 9 bits of semflg and creates a new semaphore set (on success).
The followings are limits on semaphore set resources affecting a semget call:
System wide maximum number of semaphore sets: policy dependent.
Maximum number of semaphores per semid: implementation dependent (500 currently).
Use of IPC_PRIVATE doesn't inhibit to other processes the access to the allocated semaphore set.
As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a semaphore set. Asserting both IPC_CREAT and IPC_EXCL in semflg only ensures (on success) that a new semaphore set will be created, it doesn't imply exclusive access to the semaphore set.