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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 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
   Multithreading (see pthreads(7))
       The ftok() function is thread-safe.

CONFORMING TO
       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 us-
       age 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.

SEE ALSO
       msgget(2), semget(2), shmget(2),	stat(2), svipc(7)

COLOPHON
       This  page  is  part of release 3.74 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
       http://www.kernel.org/doc/man-pages/.

GNU				  2014-08-19			       FTOK(3)

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

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