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
       The semget() system call returns the semaphore set identifier associated with the argument key.  A new set of nsems sema-
       phores 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 meaningful for semaphores, and write permissions mean permission to alter semaphore  val-
       ues).

       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 initialize the semaphores to the desired values.

       When creating a new semaphore set, semget() initializes the set's associated 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.

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 sema-
              phore 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  isn't  a flag field but a key_t type.  If this special value is used for key, the system call ignores every-
       thing but the least significant 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 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.

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  nonzero  sem_otime in the associated data structure retrieved by a semctl(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 3.25 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)