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 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 .