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RedHat 9 (Linux i386) - man page for semget (redhat section 2)

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
       execute permissions are not meaningful for semaphores, and write permissions mean  permis-
       sion 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  inte-
       ger), otherwise -1 is returned, with errno indicating 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 permis-
		  sion 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 corresponding 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  cur-
		  rently).

       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  sema-
       phore set.

       There  is  currently  no intrinsic way for a process to ensure exclusive access to a sema-
       phore 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 sys-
       tem  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)


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