ftok(3) Library Functions Manual ftok(3)
NAME
ftok - convert a pathname and a project identifier to a System V IPC key
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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 re-
turned, 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 │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001.
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.
EXAMPLES
See semget(2).
SEE ALSO
msgget(2), semget(2), shmget(2), stat(2), sysvipc(7)
Linux man-pages 6.9.1 2024-05-02 ftok(3)
Generated by dwww version 1.16 on Tue Dec 16 04:05:23 CET 2025.