Make Depend

[John Mills]

Freebies -

On Sat, 27 Nov 2004, Conrad J. Sabatier wrote:

> On Sat, 27 Nov 2004 09:43:17 +0100, Gert Cuykens
> <gert.cuykens at gmail.com> wrote:
> 
> > "A Makefile rule that typically scans all C/C++ source files in a
> > directory, and generates rules that indicate that an object file
> > depends on certain header files, and must be recompiled if they are
> > recompiled."
> > 
> > i dont understand this. how can a object depend on something that is
> > not compiled yet? Would the freebsd world not be a happier place if
> > make did the dependancy thingies what ever they are automatically ?
 ... 
> Re: dependencies, it should be simple to understand if you give it a
> moment's thought.  Let's say you have a file "main.c" that calls
> functions in "foo.c".  In order for main.c to compile and link properly
> to create a complete, executable program, it's absolutely essential that
> foo.c be compiled and linked in as well.
 
> What Makefile dependencies are about is ensuring that, if a change is
> made to foo.c, it will be recompiled and relinked with main.c to
> guarantee that the final executable is up to date in all respects.

Certainly a sensible point, but not the way I understood 'makedepend' to 
work.

As Conrad said, 'make' can be directed to compare the currency of the 
files upon which a particular product file (compiled object, library, 
executable, or other type) depends, so that all product files for which 
the components have changed _are_ rebuilt, but a maximum number of product 
files (i.e., unchanged objects being linked into a library) are 
unnecessarily rebuilt. Many of these rules I put in manually.

'make' only knows some 'generic' rules (what is done to change a *.c into 
a *.o, for example), plus the explicit dependencies I have written into my 
Makefile. 'makedepend' is a way to automatically generate the 
file-specific rules that can be deduced from a [source] file's own 
contents: usually those secondary files that are brought into it by 
'#include' pragmas. These auxiliary rules are written onto the Makefile 
and become part of it. These files are not necessarily separately 
compiled; I find your definition a bit misleading on this.

'makedepend' is given a list of files to scan, and places to look for 
included files. My 'depend' rule looks like this:

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
depend:
	makedepend -- $(CFLAGS) -- $(SRCS) -- $(INCLUDES)

# DO NOT DELETE THIS LINE -- make  depend  depends  on it.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

That funky last line really tells 'makedepend' where it should write the 
new rules onto my Makefile. Before using a Makefile on a group of sources, 
or when source files are added to the build, I remove all the generate 
rules which have been added below the '# DO NOT DELETE ...' line and 
rebuild the 'depend' target - which is the Makefile itself:

 $ make depend

Typical rules automagically added by 'makedepend' are:

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
...
BufRing.o: ../Llcommon/SEBase.h StdAfx.h BufRing.h
Camera.o: ../Llcommon/SEBase.h StdAfx.h ../Llcommon/commonStruct.h
Camera.o: ../Llcommon/secureeye.h ../Llcommon/memCtrl.h
Camera.o: ../Llcommon/retCodes.h ../Llcommon/LiveShare.h Camera.h
Camera.o: ../Llcommon/Common.h Pump.h BufRing.h CamData.h Snap.h INet.h
Camera.o: Player.h
INet.o: ../Llcommon/SEBase.h StdAfx.h /usr/include/stdlib.h
...
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The effect of these added rules is that if I change [say] 'BufRing.h' then 
do 'make all', 'BufRing.c' and 'Camera.c' would be recompiled, but not 
necessarily 'INet.c'

'make' isn't very bright, but (like 'cpp') it can be _very_ handy.

 - John Mills
   john.m.mills at alum.mit.edu