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

       semget - get a System V semaphore set identifier

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

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

       The  semget() system call returns the System V 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 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	porta‐
       ble  application	 cannot	 rely on this: it should explicitly initialize
       the semaphores to the desired values.

       When creating a new semaphore set, semget() initializes the set's 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 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 permissions are verified.

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

       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 corre‐
	      sponding	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

       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.

       SVr4, POSIX.1-2001.

       The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on	 Linux
       or by any version of POSIX.  However, some old implementations required
       the inclusion of these header files, and the SVID also documented their
       inclusion.   Applications  intended  to be portable to such old systems
       may need to include these header files.

       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 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()

       SEMMNI System  wide  maximum number of semaphore sets: policy dependent
	      (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 second	 field
	      of  /proc/sys/kernel/sem).   Values greater than SEMMSL * SEMMNI
	      makes it irrelevant.

       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
       nonzero	sem_otime in the associated data structure retrieved by a sem‐
       ctl(2) IPC_STAT operation can be used to avoid races.)

       semctl(2),   semop(2),	ftok(3),   capabilities(7),   sem_overview(7),

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2012-05-31			     SEMGET(2)

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