SEMGET(2)						     Linux Programmer's Manual							 SEMGET(2)

NAME

       semget - get a semaphore set identifier

SYNOPSIS

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

       int semget(key_t key, int nsems, int semflg);

DESCRIPTION

       This  function  returns the semaphore set identifier associated with the argument key.  A new set of nsems semaphores is created if key has
       the value IPC_PRIVATE or if 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  low-order 9 bits of the argument semflg define the access permissions (for owner, group and others) for the semaphore
       set.  These bits have the same format, and the same meaning, as the mode argument in the open(2) or creat(2) system calls (though the  exe-
       cute permissions are not meaningful for semaphores, and write permissions mean permission to alter semaphore values).

       When creating a new semaphore set, semget initializes the semaphore set's associated 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 low-order 9 bits of sem_perm.mode are set to the low-order 9 bits 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 a semaphore set is not being created.  Otherwise nsems must be greater than 0 and less than
       or equal to the maximum number of semaphores per semaphore set (SEMMSL).

       If the semaphore set already exists, the access permissions are verified.

RETURN VALUE

       If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicat-
       ing the error.

ERRORS

       On failure errno will be set to one of the following:

       EACCES	  A semaphore set exists for key, but the calling process does not have permission to access the set.

       EEXIST	  A semaphore set exists for key and semflg was asserting both IPC_CREAT and IPC_EXCL.

       ENOENT	  No semaphore set exists for key and semflg wasn't asserting IPC_CREAT.

       EINVAL	  nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL), or a semaphore set corre-
		  sponding to key already exists, and nsems is larger than the number of semaphores in that set.

       ENOMEM	  A semaphore set has to be created but the system has not enough memory for the new data structure.

       ENOSPC	  A semaphore set has to be created but the system limit for the maximum number of semaphore sets (SEMMNI),  or  the  system  wide
		  maximum number of semaphores (SEMMNS), would be exceeded.

NOTES

       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 low-
       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:

       SEMMNI	  System wide maximum number of semaphore sets: policy dependent.

       SEMMSL	  Maximum number of semaphores per semid: implementation dependent (500 currently).

       SEMMNS	  System wide maximum number of semaphores: policy dependent.  Values greater than SEMMSL * SEMMNI makes it irrelevant.

BUGS

       Use of IPC_PRIVATE doesn't inhibit to other processes the access to the allocated semaphore set.

       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.

       The  data  structure  associated  with  each  semaphore in the set isn't initialized by the system call.  In order to initialize those data
       structures, one has to execute a subsequent call to semctl(2) to perform a SETVAL or a SETALL command on the semaphore set.

CONFORMING TO

       SVr4, SVID.  SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT.

SEE ALSO

       ftok(3), ipc(5), semctl(2), semop(2)

Linux 2.5							    2002-01-04								 SEMGET(2)