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