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
       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 |
       +--------------------------------------------+---------------+---------+
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 6.7               2023-10-31                           ftok(3)