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

       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

       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 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.

       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

       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

       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

       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 function.

       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),

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Linux                              2020-04-11                          SEMGET(2)