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

       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

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.1‐2017, sys_ipc.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       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               2017                          FTOK(3P)

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