Re: conditional prerequisite

[John Graham-Cumming]

On Fri, 2005-01-28 at 12:09, address@hidden wrote:
> I had asked this before and Boris Kolpackov was so kind to give some 
> hints. Unfortunately, they did not exactly address the problem and 
> I'm trying to explain once more, hopefully better, what I'm trying 
> to achieve:
> 
> I have the following two rules.
> 
> info.h:       info.txt
>       sed -f info.sed info.txt > info.h
> 
> info.txt:
>       wget -N http://blabla/info.txt

[snip]

I made a little example Makefile like this:

    info.h: info.txt
        touch info.h

    info.txt: info.marker
        touch info.txt

    info.marker:
        touch info.marker

    .INTERMEDIATE: info.txt

The first time you run this you'll get

touch info.marker
touch info.txt
touch info.h
rm info.txt

> info.h is newer than info.txt:
>       nothing happens

If you touch info.h then nothing happens.

make: `info.h' is up to date.

> info.h is older than info.txt:
>       sed is invoked

touch info.h ; sleep 1 ; touch info.txt then you'll get the output:

touch info.h

> info.h exists but info.txt does not exist:
>       nothing happens!

rm info.txt and then you'll get the output:

make: `info.h' is up to date.

> info.h does not exist but info.txt exists:
>       sed is invoked

rm info.h ; touch info.txt then you'll get

touch info.h

> info.h and info.txt both do not exist:
>       wget is invoked, then sed

rm info.h info.txt and then you'll get

touch info.txt
touch info.h
rm info.txt

Basically the info.marker is acting as the driver for this build.  If
you touch it/remove it you'll force everything to be remade.  info.txt
is marked intermediate so it only gets rebuilt if GNU Make needs to make
it to make info.h and it wont make info.txt just because it's missing,
that'll only happen if it builds info.marker or info.marker is newer
than info.h.  Naturally you would change my 'touch' commands for info.h
and info.txt into your sed/wget.

John.
-- 
John Graham-Cumming

Home: http://www.jgc.org/
Work: http://www.electric-cloud.com/
POPFile: http://getpopfile.org/