Provided by: manpages-dev_5.10-1ubuntu1_all bug


       semget - get a System V semaphore set identifier


       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

       int semget(key_t key, int nsems, int semflg);


       The semget() system call returns the System V semaphore set identifier associated with the
       argument key.  It may be used either to obtain the  identifier  of  a  previously  created
       semaphore  set  (when  semflg  is zero and key does not have the value IPC_PRIVATE), or to
       create a new set.

       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

       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

       • 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.


       If  successful,  the  return  value  will  be  the semaphore set identifier (a nonnegative
       integer), otherwise, -1 is returned, with errno indicating the error.


       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 in the user
              namespace that governs its IPC namespace.

       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
              semaphore 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.


       SVr4, POSIX.1-2001.


       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.  On Linux systems before version
              3.19, the default value for this limit was 128.   Since  Linux  3.19,  the  default
              value  is  32,000.   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.  On  Linux  systems  before  version
              3.19,  the  default  value  for  this limit was 250.  Since Linux 3.19, the default
              value is 32,000.  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  the number of semaphores system-wide is also limited by the product of SEMMSL
              and SEMMNI.


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


       The  program shown below uses semget() to create a new semaphore set or retrieve the ID of
       an existing set.  It generates the key for semget() using ftok(3).  The first two command-
       line  arguments  are  used  as  the pathname and proj_id arguments for ftok(3).  The third
       command-line argument is an integer  that  specifies  the  nsems  argument  for  semget().
       Command-line options can be used to specify the IPC_CREAT (-c) and IPC_EXCL (-x) flags for
       the call to semget().  The usage of this program is demonstrated below.

       We first create two files that will be used to generate keys  using  ftok(3),  create  two
       semaphore sets using those files, and then list the sets using ipcs(1):

           $ touch mykey mykey2
           $ ./t_semget -c mykey p 1
           ID = 9
           $ ./t_semget -c mykey2 p 2
           ID = 10
           $ ipcs -s

           ------ Semaphore Arrays --------
           key        semid      owner      perms      nsems
           0x7004136d 9          mtk        600        1
           0x70041368 10         mtk        600        2

       Next,  we  demonstrate that when semctl(2) is given the same key (as generated by the same
       arguments to ftok(3)), it returns the ID of the already existing semaphore set:

           $ ./t_semget -c mykey p 1
           ID = 9

       Finally, we demonstrate the kind of  collision  that  can  occur  when  ftok(3)  is  given
       different pathname arguments that have the same inode number:

           $ ln mykey link
           $ ls -i1 link mykey
           2233197 link
           2233197 mykey
           $ ./t_semget link p 1       # Generates same key as 'mykey'
           ID = 9

   Program source

       /* t_semget.c

          Licensed under GNU General Public License v2 or later.
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>
       #include <sys/stat.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       static void
       usage(const char *pname)
           fprintf(stderr, "Usage: %s [-cx] pathname proj-id num-sems\n",
           fprintf(stderr, "    -c           Use IPC_CREAT flag\n");
           fprintf(stderr, "    -x           Use IPC_EXCL flag\n");

       main(int argc, char *argv[])
           int semid, nsems, flags, opt;
           key_t key;

           flags = 0;
           while ((opt = getopt(argc, argv, "cx")) != -1) {
               switch (opt) {
               case 'c': flags |= IPC_CREAT;   break;
               case 'x': flags |= IPC_EXCL;    break;
               default:  usage(argv[0]);

           if (argc != optind + 3)

           key = ftok(argv[optind], argv[optind + 1][0]);
           if (key == -1) {

           nsems = atoi(argv[optind + 2]);

           semid = semget(key, nsems, flags | 0600);
           if (semid == -1) {

           printf("ID = %d\n", semid);



       semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7), sysvipc(7)


       This  page  is  part of release 5.10 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