semget(2) — Linux manual page


semget(2)                  System Calls Manual                 semget(2)

NAME         top

       semget - get a System V semaphore set identifier

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/sem.h>

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

DESCRIPTION         top

       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 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

RETURN VALUE         top

       On success, semget() returns the semaphore set identifier (a
       nonnegative integer).  On failure, -1 is returned, and errno is
       set to indicate the error.

ERRORS         top

       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

       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.

STANDARDS         top


HISTORY         top

       SVr4, POSIX.1-2001.

NOTES         top

       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

   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.  Before
              Linux 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.  Before
              Linux 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

       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.

BUGS         top

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

EXAMPLES         top

       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 <stdio.h>
       #include <stdlib.h>
       #include <sys/ipc.h>
       #include <sys/sem.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);


SEE ALSO         top

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

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-06-15                      semget(2)

Pages that refer to this page: ipcrm(1)ipcs(1)lsipc(1)pcp-ipcs(1)ipc(2)semctl(2)semop(2)syscalls(2)umask(2)ftok(3)sem_overview(7)sysvipc(7)