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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

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