Section (3) ftok
ftok — convert a pathname and a project identifier to a System V IPC key
#include <sys/types.h> #include <sys/ipc.h>
||const char *pathname,|
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
(which must be nonzero) 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.
For an explanation of the terms used in this section, see attributes(7).
On some ancient systems, the prototype was:
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
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Copyright 1993 Giorgio Ciucci (giorgiocrcc.it)
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Modified 2001-11-28, by Michael Kerrisk, <mtk.manpagesgmail.com>
Changed data type of proj_id; minor fixes
aeb: further fixes; added notes.