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

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

       ftok - generate an IPC key

       #include <sys/ipc.h>

       key_t ftok(const char *path, int id);

       The  ftok()  function shall return a key based on path and id that is usable in subsequent
       calls to msgget(), semget(), and shmget(). The application  shall  ensure  that	the  path
       argument is the pathname of an existing file that the process is able to stat().

       The ftok() function shall return the same key value for all paths that name the same file,
       when called with the same id value, and return different key values when called with  dif-
       ferent  id values or with paths that name different files existing on the same file system
       at the same time. It is unspecified whether ftok() shall return the same  key  value  when
       called again after the file named by path is removed and recreated with the same name.

       Only  the low-order 8-bits of id are significant. The behavior of ftok() is unspecified if
       these bits are 0.

       Upon successful completion, ftok() shall return a  key.	Otherwise,  ftok()  shall  return
       (key_t)-1 and set errno to indicate the error.

       The ftok() function shall fail if:

       EACCES Search permission is denied for a component of the path prefix.

       ELOOP  A loop exists in symbolic links encountered during resolution of the path argument.

	      The  length  of  the  path  argument  exceeds {PATH_MAX} or a pathname component is
	      longer than {NAME_MAX}.

       ENOENT A component of path does not name an existing file or path is an empty string.

	      A component of the path prefix is not a directory.

       The ftok() function may fail if:

       ELOOP  More than {SYMLOOP_MAX} symbolic links were encountered during  resolution  of  the
	      path argument.

	      Pathname resolution of a symbolic link produced an intermediate result whose length
	      exceeds {PATH_MAX}.

       The following sections are informative.

   Getting an IPC Key
       The following example gets a unique key that can be used by the	IPC  functions	semget(),
       msgget(),  and  shmget().  The  key returned by ftok() for this example is based on the ID
       value S and the pathname /tmp.

	      #include <sys/ipc.h>
	      key_t key;
	      char *path = "/tmp";
	      int id = 'S';

	      key = ftok(path, id);

   Saving an IPC Key
       The following example gets a unique key based on the pathname /tmp and the ID value a.  It
       also  assigns  the  value  of  the resulting key to the semkey variable so that it will be
       available to a later call to semget(), msgget(), or shmget().

	      #include <sys/ipc.h>
	      key_t semkey;

	      if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
		  perror("IPC error: ftok"); exit(1);

       For maximum portability, id should be a single-byte character.



       msgget() , semget() , shmget() , the  Base  Definitions	volume	of  IEEE Std 1003.1-2001,

       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					  FTOK(P)

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