semget(2) — Linux manual page


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

NAME         top

       semget - get a System V semaphore set identifier

SYNOPSIS         top

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

       If the semaphore set already exists, the permissions are verified.

RETURN VALUE         top

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

ERRORS         top

       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

       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         top

       SVr4, POSIX.1-2001.

NOTES         top

       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

   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

       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()

       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.

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 <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);


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.07 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

Linux                            2020-04-11                        SEMGET(2)

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