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advanced auto-dependencies

From: Boris Kolpackov
Subject: advanced auto-dependencies
Date: Fri, 6 Feb 2004 11:09:36 -0600
User-agent: Mutt/1.5.4i

Good day,

I think I found a flaw in a so-called "advanced auto-dependencies" 
generation technique. To start, consider this small excerpt from the 
article ( 

"Since we know some prerequisite of the target changed, we don't really 
 need the updated prerequisite list in this build. We already know that 
 we're going to rebuild the target, and having a more up-to-date list 
 won't affect that decision. What we really need is to ensure that the 
 prerequisite list is up-to-date for the next invocation of make, when 
 we need to decide whether to update it again."

This reasoning ignores the case when some of the prerequisites may not
exist and should in turn be built. In this case we do need up-to-date 
dependency list in this build. 

The following example illustrates this situation. Suppose we have 
'hello.idl' which can be compiled to 'hello_stub.hpp' and 'hello_stub.cpp'.
Plus we have 'client.cpp' which includes 'hello_stub.hpp' and gets compiled
into client.o. Finally, 'client.o' and 'hello_stub.o' are used to produce 

client : client.o hello_stub.o
        g++ -o $@ $^

-include client.o.d      # dependencies for client.o
-include hello_stub.o.d  # dependencies for hello_stub.o

hello_stub.hpp hello_stub.cpp : hello.idl
        idlc hello.idl

In the makefile fragment above I assume that there is an implicit rule
to compile .cpp to .o and generate dependencies at the same time.

Now let's see what happens when we try to build this project for the
first time. Make will ignore cleint.o.d and hello_stub.o.d since they
don't exist and there is no rule to build them. Then make proceeds to
build 'client' which depends on 'client.o' and  'hello_stub.o'. Since 
there is an implicit rule to build .o from .cpp make readily executes
the command to compile 'client.cpp'. Do you see the problem? 
'hello_stub.hpp' does not exist and it is included by 'client.cpp'.
But make has no idea about this dependency (since we decided it doesn't
need to know ;-).

Also note that this problem arise not only in the first build. Every time
you add a dependency (for example include a header) that does not merely
exist but should be built you will face this problem.


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