ftok

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



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  gener-
       ate  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.

CONFORMING TO
       XPG4

NOTES
       Under libc4 and libc5 (and under SunOS 4.x) 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 behaviour 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 i-node 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.

SEE ALSO
       ipc(5), msgget(2), semget(2), shmget(2), stat(2)



Linux 2.4			  2001-11-28			      FTOK(3)