semget

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

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

DESCRIPTION
       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).

       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.

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


       EINVAL A  semaphore  set corresponding to key already exists, but 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
       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 all but the least significant
       9 bits of semflg and creates a new semaphore set (on success).

   Semaphore initialization
       The values of the semaphores in a newly created set are  indeterminate.
       (POSIX.1-2001  and  POSIX.1-2008  are  explicit on this point, although
       POSIX.1-2008 notes that a future version of the standard may require an
       implementation  to  initialize  the  semaphores to 0.)  Although Linux,
       like many other implementations, initializes the semaphore values to 0,
       a  portable  application cannot rely on this: it should explicitly ini-
       tialize the semaphores to the desired values.

       Initialization can be done using semctl(2) SETVAL or SETALL  operation.
       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 struc-
       ture  retrieved  by a semctl(2) IPC_STAT operation can be used to avoid
       races.

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

       SEMMNI System-wide  limit  on  the  number of semaphore sets.  On Linux
              systems before version 3.19, the default value  for  this  limit
              was  128.   Since  Linux  3.19, the default value is 32,000.  On
              Linux, this limit can be read and modified via the fourth  field
              of /proc/sys/kernel/sem.

       SEMMSL Maximum number of semaphores per semaphore ID.  On Linux systems
       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its function.

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

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-02-21                         SEMGET(2)
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