ftok(3p) — Linux manual page


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

PROLOG         top

       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         top

       ftok — generate an IPC key

SYNOPSIS         top

       #include <sys/ipc.h>

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

DESCRIPTION         top

       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         top

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

ERRORS         top

       The ftok() function shall fail if:

       EACCES Search permission is denied for a component of the path

       EIO    An error occurred while reading from the file system.

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

              The length of a component of a pathname is longer than

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

              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.

              The length of a pathname exceeds {PATH_MAX}, or pathname
              resolution of a symbolic link produced an intermediate result
              with a length that exceeds {PATH_MAX}.

       The following sections are informative.

EXAMPLES         top

   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);


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

       Applications should not assume that the resulting key value is

RATIONALE         top



       Future versions of this standard may add new interfaces to provide
       unique keys.

SEE ALSO         top

       msgget(3p), semget(3p), shmget(3p)

       The Base Definitions volume of POSIX.1‐2008, sys_ipc.h(0p)

COPYRIGHT         top

       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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                            FTOK(3P)

Pages that refer to this page: sys_ipc.h(0p)msgget(3p)semget(3p)shmget(3p)