NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
FTOK(3) Linux Programmer's Manual FTOK(3)
ftok - convert a pathname and a project identifier to a System V IPC key
#include <sys/types.h>
#include <sys/ipc.h>
key_t ftok(const char *pathname, int proj_id);
The ftok() function uses the identity of the file named by the given pathname
(which must refer to an existing, accessible file) and the least significant 8
bits of proj_id (which must be non-zero) to generate a key_t type System V IPC
key, suitable for use with msgget(2), semget(2), or shmget(2).
The resulting value is the same for all pathnames that name the same file,
when the same value of proj_id is used. The value returned should be
different when the (simultaneously existing) files or the project IDs differ.
On success the generated key_t value is returned. On failure -1 is returned,
with errno indicating the error as for the stat(2) system call.
POSIX.1-2001.
Under libc4 and libc5 (and under SunOS 4.x) the prototype was:
key_t ftok(char *pathname, char proj_id);
Today proj_id is an int, but still only 8 bits are used. Typical usage has an
ASCII character proj_id, that is why the behavior is said to be undefined when
proj_id is zero.
Of course no guarantee can be given that the resulting key_t is unique.
Typically, a best effort attempt combines the given proj_id byte, the lower 16
bits of the inode number, and the lower 8 bits of the device number into a
32-bit result. Collisions may easily happen, for example between files on
/dev/hda1 and files on /dev/sda1.
msgget(2), semget(2), shmget(2), stat(2), svipc(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2001-11-28 FTOK(3)