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().

       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 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.

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

       ENAMETOOLONG
              The length of the path argument exceeds  {PATH_MAX}  or  a  pathname  component  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 is not a directory.

       The ftok() function may fail if:

       ELOOP  More  than  {SYMLOOP_MAX}  symbolic links were encountered during resolution of the
              path argument.

       ENAMETOOLONG
              Pathname resolution of a symbolic link produced an intermediate result whose length
              exceeds {PATH_MAX}.

       The following sections are informative.

EXAMPLES

   Getting an IPC Key
       The  following  example  gets a unique key that can be used by the IPC functions semget(),
       msgget(), and shmget(). The key returned by ftok() for this example is  based  on  the  ID
       value S and the pathname /tmp.

              #include <sys/ipc.h>
              ...
              key_t key;
              char *path = "/tmp";
              int id = 'S';

              key = ftok(path, id);

   Saving an IPC Key
       The  following example gets a unique 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.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       msgget()  ,  semget()  ,  shmget()  , the Base Definitions volume of IEEE Std 1003.1-2001,
       <sys/ipc.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2003  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  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 .