ftok(3) — Linux manual page


FTOK(3)                   Linux Programmer's Manual                  FTOK(3)

NAME         top

       ftok  - convert a pathname and a project identifier to a System V IPC

SYNOPSIS         top

       #include <sys/types.h>
       #include <sys/ipc.h>

       key_t ftok(const char *pathname, int proj_id);

DESCRIPTION         top

       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         top

       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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value   │
       │ftok()    │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       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 hap‐
       pen, for example between files on /dev/hda1 and files on /dev/sda1.

EXAMPLES         top

       See semget(2).

SEE ALSO         top

       msgget(2), semget(2), shmget(2), stat(2), sysvipc(7)

COLOPHON         top

       This page is part of release 5.08 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

GNU                              2020-04-11                          FTOK(3)

Pages that refer to this page: ipcrm(1)msgget(2)semget(2)shmget(2)svipc(7)sysvipc(7)migratepages(8)numactl(8)