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SEMGET(3P)                 POSIX Programmer's Manual                SEMGET(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       semget - get set of XSI semaphores
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
              as specified by the low-order 9 bits  of  semflg  would  not  be
              granted; see XSI Interprocess Communication .
       EEXIST A  semaphore identifier exists for the argument key but ((semflg
              &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
       EINVAL The value of nsems is either less than or equal to 0 or  greater
              than  the system-imposed limit, or a semaphore identifier exists
              for the argument key, but the number of semaphores  in  the  set
              associated  with it is less than nsems and nsems is not equal to
              0.
       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);
              }

              /* Get semaphore ID associated with this key. */
              if ((semid = semget(semkey, 0, 0)) == -1) {

                  /* Semaphore does not exist - Create. */
                  if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
                      S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
                  {
                      /* Initialize the semaphore. */
                      sbuf.sem_num = 0;
                      sbuf.sem_op = 2;  /* This is the number of runs
                                           without queuing. */
                      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(3P)