semget

SYNOPSIS
       #include <sys/sem.h>

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


DESCRIPTION
       The  semget() function operates on XSI semaphores (see the Base Defini-
       tions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore).  It  is
       unspecified  whether  this  function  interoperates  with  the realtime
       interprocess communication facilities defined in Realtime .

       The semget() function shall return the semaphore identifier  associated
       with key.

       A  semaphore identifier with its associated semid_ds data structure and
       its associated set of nsems semaphores (see <sys/sem.h>) is created for
       key if one of the following is true:

        * The argument key is equal to IPC_PRIVATE.

        * The  argument key does not already have a semaphore identifier asso-
          ciated with it and (semflg &IPC_CREAT) is non-zero.

       Upon creation, the semid_ds data structure associated with the new sem-
       aphore identifier is initialized as follows:

        * In  the operation permissions structure sem_perm.cuid, sem_perm.uid,
          sem_perm.cgid, and sem_perm.gid shall be set equal to the  effective
          user  ID  and  effective  group  ID,  respectively,  of  the calling
          process.

        * The low-order 9 bits of sem_perm.mode shall be set equal to the low-
          order 9 bits of semflg.

        * The variable sem_nsems shall be set equal to the value of nsems.

        * The  variable  sem_otime shall be set equal to 0 and sem_ctime shall
          be set equal to the current time.

        * The data structure associated with each semaphore in the  set  shall
          not be initialized. The semctl() function with the command SETVAL or
          SETALL can be used to initialize each semaphore.

RETURN VALUE
       Upon successful completion, semget() shall return a non-negative  inte-
       ger,  namely  a semaphore identifier; otherwise, it shall return -1 and
       set errno to indicate the error.

ERRORS
       The semget() function shall fail if:

       EACCES A semaphore identifier exists for key, but operation  permission
       ENOENT A  semaphore  identifier does not exist for the argument key and
              (semflg &IPC_CREAT) is equal to 0.

       ENOSPC A semaphore identifier is to be created but  the  system-imposed
              limit  on  the  maximum number of allowed semaphores system-wide
              would be exceeded.


       The following sections are informative.

EXAMPLES
   Creating a Semaphore Identifier
       The following example gets a unique  semaphore  key  using  the  ftok()
       function,  then  gets a semaphore ID associated with that key using the
       semget() function (the first call also tests to make sure the semaphore
       exists).  If  the  semaphore does not exist, the program creates it, as
       shown by the second call to semget(). In creating the semaphore for the
       queuing  process,  the  program  attempts  to create one semaphore with
       read/write permission for all. It also uses the  IPC_EXCL  flag,  which
       forces semget() to fail if the semaphore already exists.

       After  creating  the  semaphore,  the program uses a call to semop() to
       initialize it to the values in the sbuf array. The number of  processes
       that  can  execute  concurrently without queuing is initially set to 2.
       The final call to semget() creates a semaphore identifier that  can  be
       used later in the program.


              #include <sys/types.h>
              #include <stdio.h>
              #include <sys/ipc.h>
              #include <sys/sem.h>
              #include <sys/stat.h>
              #include <errno.h>
              #include <unistd.h>
              #include <stdlib.h>
              #include <pwd.h>
              #include <fcntl.h>
              #include <limits.h>
              ...
              key_t semkey;
              int semid, pfd, fv;
              struct sembuf sbuf;
              char *lgn;
              char filename[PATH_MAX+1];
              struct stat outstat;
              struct passwd *pw;
              ...
              /* Get unique key for semaphore. */
              if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
                  perror("IPC error: ftok"); exit(1);
              }


                      sbuf.sem_flg = 0;
                      if (semop(semid, &sbuf, 1) == -1) {
                          perror("IPC error: semop"); exit(1);
                      }
                  }
                  else if (errno == EEXIST) {
                      if ((semid = semget(semkey, 0, 0)) == -1) {
                          perror("IPC error 1: semget"); exit(1);
                      }
                  }
                  else {
                      perror("IPC error 2: semget"); exit(1);
                  }
              }
              ...

APPLICATION USAGE
       The  POSIX Realtime Extension defines alternative interfaces for inter-
       process communication. Application  developers  who  need  to  use  IPC
       should design their applications so that modules using the IPC routines
       described in XSI Interprocess Communication can be easily  modified  to
       use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       XSI  Interprocess  Communication  ,  Realtime  ,  semctl()  , semop() ,
       sem_close() , sem_destroy() , sem_getvalue() , sem_init() ,  sem_open()
       ,  sem_post() , sem_unlink() , sem_wait() , the Base Definitions volume
       of IEEE Std 1003.1-2001, <sys/sem.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .



IEEE/The Open Group                  2003                            SEMGET(P)