semget


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 associ-
       ated 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  mean-
       ingful  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  porta-
       ble  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 asso-
       ciated 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).

              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 corre-
              sponding  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 sig-
       nificant 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 sem-
       ctl(2) IPC_STAT operation can be used to avoid races.)

Linux                             2004-05-27                         SEMGET(2)
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