SEMGET(2)		   Linux Programmers Manual		    SEMGET(2)



NAME
       semget - get a semaphore set identifier

SYNOPSIS
       #include 
       #include 
       #include 

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

DESCRIPTION
       The  semget()  system call returns the semaphore set identifier associ
       ated 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 with key and IPC_CREAT is specified in semflg.

       If semflg specifies both IPC_CREAT and IPC_EXCL	and  a	semaphore  set
       already	exists	for key, then semget() fails with errno set to EEXIST.
       (This is analogous to the effect of the combination  O_CREAT  |	O_EXCL
       for open(2).)

       Upon  creation,	the  least  significant  9 bits of the argument semflg
       define the permissions (for owner, group and others) for the  semaphore
       set.   These  bits  have  the same format, and the same meaning, as the
       mode argument of open(2) (though the execute permissions are not  mean
       ingful  for  semaphores, and write permissions mean permission to alter
       semaphore values).

       The values of the semaphores in a newly created set are	indeterminate.
       (POSIX.1-2001  is  explicit  on this point.)  Although Linux, like many
       other  implementations,	initializes  the  semaphore  values  to  0,  a
       portable application cannot rely on this: it should explicitly initial
       ize the semaphores to the desired values.

       When creating a new semaphore set, semget() initializes the sets asso
       ciated data structure, semid_ds (see semctl(2)), 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  least  significant  9  bits of sem_perm.mode are set to the
	      least significant 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 dont 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 permissions are verified.

RETURN VALUE
       If successful, the return value will be the semaphore set identifier (a
       non-negative  integer), 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 permission to access the set, and does not have the
		  CAP_IPC_OWNER capability.

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

       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.

       ENOENT	  No semaphore set exists for key and semflg did  not  specify
		  IPC_CREAT.

       ENOMEM	  A  semaphore	set  has to be created but the system does not
		  have 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.

CONFORMING TO
       SVr4, POSIX.1-2001.

NOTES
       IPC_PRIVATE isnt a flag field but a key_t type.	If this special value
       is used for key, the system call ignores everything but the least  sig
       nificant 9 bits of semflg and creates a new semaphore set (on success).

       The following limits on semaphore set  resources  affect  the  semget()
       call:

       SEMMNI	  System  wide maximum number of semaphore sets: policy depen
		  dent (on Linux, this limit can be read and modified via  the
		  fourth field of /proc/sys/kernel/sem).

       SEMMSL	  Maximum  number  of  semaphores  per	semid:	implementation
		  dependent (on Linux, this limit can be read and modified via
		  the first field of /proc/sys/kernel/sem).

       SEMMNS	  System  wide	maximum number of semaphores: policy dependent
		  (on Linux, this limit can be read and modified via the  sec
		  ond  field  of  /proc/sys/kernel/sem).   Values greater than
		  SEMMSL * SEMMNI makes it irrelevant.

BUGS
       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its function.

       The  semaphores	in a set are not initialized by semget().  In order to
       initialize the semaphores, semctl(2) must be used to perform  a	SETVAL
       or  a  SETALL operation on the semaphore set.  (Where multiple peers do
       not know who will be the first to initialize the set,  checking	for  a
       non-zero sem_otime in the associated data structure retrieved by a sem
       ctl(2) IPC_STAT operation can be used to avoid races.)

SEE ALSO
       semctl(2),   semop(2),	ftok(3),   capabilities(7),   sem_overview(7),
       svipc(7)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  2004-05-27			     SEMGET(2)