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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  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

       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

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       pages/.