NAME

       semget - get a System V 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 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 meaningful 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 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  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

       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 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 3.54 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/.