A semaphore identifier and associated data structure and set containing nsems semaphores (see intro(2)) are created for key if one of the following are true:

key is equal to IPC_PRIVATE.

key does not already have a semaphore identifier associated with it, and (semflg & IPC_CREAT) is ``true''.

Upon creation, the data structure associated with the new semaphore identifier is initialized as follows:

sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.

The low-order 9 bits of sem_perm.mode are set equal to the low-order 9 bits of semflg.

sem_nsems is set equal to the value of nsems.

sem_otime is set equal to 0 and sem_ctime is set equal to the current time.

A semaphore identifier (semid) is a unique positive integer created by a semget(2) system call. Each semid has a set of semaphores and a data structure associated with it. The data structure is referred to as semid_ds and contains the following members:

struct ipc_perm sem_perm; /* operation permission struct */ ushort sem_nsems; /* number of sems in set */ time_t sem_otime; /* last operation time */ time_t sem_ctime; /* last change time */ /* Times measured in secs since */ /* 00:00:00 GMT+1, Jan. 1, 1970 */

sem_perm is an ipc_perm structure that specifies the semaphore operation permission (see below). This structure includes the following members:

ushort cuid; /* creator user id */ ushort cgid; /* creator group id */ ushort uid; /* user id */ ushort gid; /* group id */ ushort mode; /* r/a permission */

The value of sem_nsems is equal to the number of semaphores in the set. Each semaphore in the set is referenced by a positive integer referred to as a sem_num. sem_num values run sequentially from 0 to the value of sem_nsems minus 1. sem_otime is the time of the last semop(2) operation, and sem_ctime is the time of the last semctl(2) operation that changed a member of the above structure.

A semaphore is a data structure that contains the following members:

ushort semval; /* semaphore value */ short sempid; /* pid of last operation */ ushort semncnt; /* # awaiting semval > cval */ ushort semzcnt; /* # awaiting semval = 0 */

semval is a non-negative integer. sempid is equal to the process ID of the last process that performed a semaphore operation on this semaphore. semncnt is a count of the number of processes that are currently suspended awaiting this semaphore's semval to become greater than its current value. semzcnt is a count of the number of processes that are currently suspended awaiting this semaphore's semval to become zero.

EACCES

A semaphore identifier exists for key, but operation permission (see intro(2)) as specified by the low-order 9 bits of semflg would not be granted.

EEXIST

A semaphore identifier exists for key but ( (semflg & IPC_CREAT) and (semflg & IPC_EXCL)) is ``true''.

EINVAL

nsems is either less than or equal to zero or greater than the system-imposed limit. A semaphore identifier exists for key, but the number of semaphores in the set associated with it is less than nsems and nsems is not equal to zero.

ENOENT

A semaphore identifier does not exist for key and (semflg & IPC_CREAT) is ``false''.

ENOSPC

A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphore identifiers system wide would be exceeded. A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system wide would be exceeded.