ftok(3)
NAME
ftok - convert a pathname and a project identifier to a System V IPC
key
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
key_t ftok(const char *pathname, int proj_id);
DESCRIPTION
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 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.
RETURN VALUE
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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
Interface Attribute Value
ftok() Thread safety MT-Safe
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
NOTES
On some ancient systems, 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.
SEE ALSO
msgget(2), semget(2), shmget(2), stat(2), svipc(7)
COLOPHON
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