FTOK(3) Linux Programmer's Manual FTOK(3)NAMEftok - 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 non-zero) to gener‐
ate 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.
CONFORMING TO
POSIX.1-2001.
NOTES
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 behaviour 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 i-node 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 ALSOmsgget(2), semget(2), shmget(2), stat(2), svipc(7)Linux 2.4 2001-11-28 FTOK(3)
