ftok(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

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

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 call.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │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 happen, 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 5.10 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/.

GNU                            2020-04-11                        FTOK(3)

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