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 correspond-
       ing 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 Definitions 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 associated with it and (semflg &IPC_CREAT) is non-zero.

       Upon creation, the semid_ds data structure associated with the new semaphore 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 integer, 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 interprocess 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 Technol-
       ogy -- 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)