SEMGET(2)                           Linux Programmer's Manual                           SEMGET(2)



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 sem‐
       flg.

       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 val‐
       ues).

       When creating a new semaphore set, semget() initializes the set's associated  data  struc‐
       ture, 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  inte‐
       ger), 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 IPC_CREAT and IPC_EXCL were specified in semflg, but a semaphore set already exists
              for key.

       EINVAL nsems is less than 0 or greater than the limit on the number of semaphores per sem‐
              aphore set (SEMMSL).

       EINVAL A semaphore set corresponding to key already exists, but 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 all but the least significant 9 bits of semflg and creates  a  new
       semaphore set (on success).

   Semaphore initialization
       The  values of the semaphores in a newly created set are indeterminate.  (POSIX.1-2001 and
       POSIX.1-2008 are explicit on this point, although POSIX.1-2008 notes that a future version
       of  the  standard  may  require  an  implementation  to  initialize  the semaphores to 0.)
       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.

       Initialization can be done using semctl(2) SETVAL or  SETALL  operation.   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.

   Semaphore limits
       The following limits on semaphore set resources affect the semget() call:

       SEMMNI System-wide limit on the 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 semaphore ID: implementation dependent (on  Linux,
              this limit can be read and modified via the first field of /proc/sys/kernel/sem).

       SEMMNS System-wide  limit  on  the  number of semaphores: policy dependent (on Linux, this
              limit can be read and modified via the second field of /proc/sys/kernel/sem).  Note
              that  number of semaphores system-wide is also limited by the product of SEMMSL and
              SEMMNI.

BUGS
       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly  show  its
       function.

SEE ALSO
       semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7), svipc(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-05-21                                  SEMGET(2)