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FTOK(3P)                   POSIX Programmer's Manual                  FTOK(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
       ftok -- generate an IPC key
SYNOPSIS
       #include <sys/ipc.h>
       key_t ftok(const char *path, int id);
DESCRIPTION
       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(), with the exception
       that if stat() would fail with [EOVERFLOW] due  to  file  size,  ftok()
       shall still succeed.
       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  should
       return  different  key  values  when called with different 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.
RETURN VALUE
       Upon successful completion,  ftok()  shall  return  a  key.  Otherwise,
       ftok() shall return (key_t)-1 and set errno to indicate the error.
ERRORS
       The ftok() function shall fail if:
       EACCES Search permission is denied for a component of the path prefix.
       EIO    An error occurred while reading from the file system.
       ELOOP  A loop exists in symbolic links encountered during resolution of
              the path argument.
       ENAMETOOLONG
              The  length  of  a  component  of  a  pathname  is  longer  than
              {NAME_MAX}.
       ENOENT A component of path does not name an existing file or path is an
              empty string.
       ENOTDIR
              A component of the path prefix names an existing  file  that  is
              neither  a  directory nor a symbolic link to a directory, or the
              path argument contains at least one  non-<slash>  character  and
              ends  with  one or more trailing <slash> characters and the last
              pathname component names an existing  file  that  is  neither  a
              directory nor a symbolic link to a directory.
       The ftok() function may fail if:
       ELOOP  More  than  {SYMLOOP_MAX} symbolic links were encountered during
              resolution of the path argument.
       ENAMETOOLONG
              The length of a pathname exceeds {PATH_MAX}, or pathname resolu-
              tion  of  a symbolic link produced an intermediate result with a
              length that exceeds {PATH_MAX}.
       The following sections are informative.
EXAMPLES
   Getting an IPC Key
       The following example gets a 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);
           }
APPLICATION USAGE
       For maximum portability, id should be a single-byte character.
       Applications should not assume that the resulting key value is unique.
RATIONALE
       None.
FUTURE DIRECTIONS
       Future  versions  of  this  standard  may add new interfaces to provide
       unique keys.
SEE ALSO
       msgget(), semget(), shmget()
       The Base Definitions volume of POSIX.1-2008, <sys_ipc.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                             FTOK(3P)