# $Id: README,v 2.1 2007/12/03 17:13:43 ksb Exp $

Flock applies or removes an advisory lock on the named file or file
descriptor.  Advisory locks allow cooperating processes to perform
consistent operations on files see flock(2).  This program is *very*
useful on a Sequent machine; almost as useful on fast uniprocessors.

Here is a case where flock(1) is very useful: on a Sequent Balance
series machine parallel make can be used to compile many files at the
same time.  This doesn't work for programs that use xstr(1) because
competing processes using xstrings fails.  In the Makefile for vi one
might use a flock on the Makefile to force (only) the xstr to be run
sequentially:

	.c.o:
		${CC} -E ${CFLAGS} $*.c | \
		flock Makefile sh -c "${XSTR} -c -; mv x.c $*.p.x"; \
		sed '/rcsid\[\]/d' < $*.p.x > $*.p.c; \
		${CC} ${CFLAGS} -c $*.p.c; \
		mv $*.p.o $*.o; \
		rm $*.p.x $*.p.c
	  
Which will start a few cpps in parallel and run only one xstr at a time,
as soon as it can it will run the sed/cc/mv/rm allowing another process
to begin using xstr.

(I flock the Makefile by convention, because I know I always have one.
 If more then one command set needs to work from the same Makefile I
 flock `complie.lock', and `yacc.lock' or some such, not using Makefile
 for either.  The clean target in the Makefile removes these files.)

As a wrapper, flock makes xapply(1) even more spiffy.  Also flock(1)
works well with xclate's file descriptor passing (-E, -I, -D) and escrow.

--
kayessbee (Kevin S Braunsdorf), Dec 2007
flock-spam@ksb.npcguild.org s,-span,,p