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Linux 2.6 - man page for semget (linux section 3posix)

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

       semget - get set of XSI semaphores

       #include <sys/sem.h>

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

       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

	* 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

       Upon  successful  completion, semget() shall return a non-negative integer, namely a sema-
       phore identifier; otherwise, it shall return -1 and set errno to indicate the error.

       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) &&(sem-
	      flg &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  num-
	      ber 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.

   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);

       The  POSIX  Realtime  Extension defines alternative interfaces for interprocess communica-
       tion. 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.



       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>

       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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					SEMGET(P)

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