SEMGET(2)                 (2020-04-11)                  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 associ-
          ated with key and IPC_CREAT is specified in semflg.

          If semflg specifies both IPC_CREAT and IPC_EXCL and a sema-
          phore 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:

          +o sem_perm.cuid and sem_perm.uid are set to the effective
            user ID of the calling process.

          +o sem_perm.cgid and sem_perm.gid are set to the effective
            group ID of the calling process.

          +o The least significant 9 bits of sem_perm.mode are set to
            the least significant 9 bits of semflg.

          +o sem_nsems is set to the value of nsems.

          +o sem_otime is set to 0.

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          +o 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 sema-
          phores per semaphore set (SEMMSL).

          If the semaphore set already exists, the permissions are

          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:

               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.

               IPC_CREAT and IPC_EXCL were specified in semflg, but a
               semaphore set already exists for key.

               nsems is less than 0 or greater than the limit on the
               number of semaphores per semaphore set (SEMMSL).

               A semaphore set corresponding to key already exists,
               but nsems is larger than the number of semaphores in
               that set.

               No semaphore set exists for key and semflg did not
               specify IPC_CREAT.

               A semaphore set has to be created but the system does
               not have enough memory for the new data structure.

               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.


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          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.  Appli-
          cations 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:

               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.

               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.

               System-wide limit on the number of semaphores: policy
               dependent (on Linux, this limit can be read and

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               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 sema-
          phore set or retrieve the ID of an existing set.  It gener-
          ates 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 aqmykeyaq
              ID = 9

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        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 aqcaq: flags |= IPC_CREAT;   break;
                  case aqxaq: 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) {

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              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

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