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Re: outputting which dependencies triggered a target action

From: Shawn Halpenny
Subject: Re: outputting which dependencies triggered a target action
Date: Fri, 19 Aug 2005 16:22:59 -0400

On 8/19/05, Noel Yap <address@hidden> wrote:
> Is there an option less verbose than --debug that outputs which dependencies 
> triggered a target action?

You just gave me a great idea to add to my build system.  Here's
roughly how I went about it.

For my build system, all compilation+linkage rules are generated
dynamically based on variables' values in some set of makefiles; I
have one function that generates a compile rule and one function that
generates a link rule, for any combination of linker/compiler options,
source files, etc.  I mention this because it's easy for me to add
this to any project I currently build; if you have a bunch of rules
you want prereq information for, you'll need to adapt this solution.

The key is to use the $? (which prerequisites caused this target to
run) and $| (order-only prerequisites) variables.  I changed the rules
for my compile+link rule generation functions to do basically this:

ifneq ($(PREREQS),)
@echo $@: $? \| $|
@<generic link/compile rule cmds>

This works because the if*/endif stuff is parsed before function
definitions are parsed, so if I set PREREQS=1 on the cmd line, the
only thing the rules can do is output the prereqs lists.  Without
defining PREREQs everything builds normally.

This was really cool.  Thanks for the idea!

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