NAME

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

SYNOPSIS

       #include <sys/types.h>
       #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 │
       └───────────┴───────────────┴─────────┘

CONFORMING TO

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

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)

COLOPHON

       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)