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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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
              resolution 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.1-2017, <sys_ipc.h>

COPYRIGHT
       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(3)
