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 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.
See semget(2).
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
ftok() | Thread safety | MT-Safe |
POSIX.1-2001, POSIX.1-2008.
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
.
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.