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       semget - get a System V semaphore set identifier


       #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.


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


       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.


       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)


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