Dependency fix

From: Chris Dragan (cdragan_at_[hidden])
Date: 2005-11-17 16:56:44

Hi!

The project site suggested contributing patches here, so here I am. :-)

The proposed patch fixes header dependency scanning in Jam.

The Problem
-----------

Consider the following example:

+++ Jamfile
Main deptest : deptest.c ;

+++ deptest.c
#include "subdir/a.h"
int main() { return 0; }

+++ subdir/a.h
#include "b.h"

+++ subdir/b.h
#define SOMETHING

Now run jam. After you build the executable, touch file b.h and re-run 
jam. You
would expect it to rebuild the executable, but it fails:

# jam
...found 12 target(s)...
...updating 2 target(s)...
Cc deptest.o
Link deptest
Chmod1 deptest
...updated 2 target(s)...

# touch subdir/b.h

# jam
...found 12 target(s)...

The Solution
------------

/*
For simplicity:
- includer - the header that includes another header
- includee - the header being included by the includer.
*/

Apparently when searching for the includee Jam does not search relatively 
to the directory where the includer was found. This becomes apparent, when 
we look in HdrRule - we see there the following line:
SEARCH on $(s) = $(HDRSEARCH)

HDRSEARCH, among system and explicitly specified directories also contains 
".", which does not necessarily have to be the directory where the 
includer is located, especially when the includer itself was included by 
another file.

Fortunately, Boost Jam passes an undocumented, third argument to HdrRule - 
bound file name of the includer. From it, we can extract the path where 
the includer sits, and make Jam look there for the includee:
SEARCH on $(s) = $(3:D) $(HDRSEARCH)
Notice, that we should look there first, before we check other directories.

The Patch
---------

The patch in the attachment applies the above solution both to Jambase and 
to jambase.c. The patch was made against Boost Jam 3.1.9, but it is 
working as well with e.g. 3.1.6.

I hope that someone will find it useful.

Best Regards,
Chris Dragan
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