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/.