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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  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
       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 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
       non-zero  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 3.01 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/.