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Re: Make does not build a missing prerequisite.

From: Reinier Post
Subject: Re: Make does not build a missing prerequisite.
Date: Tue, 7 Oct 2014 16:51:00 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Tue Oct  7 13:09:11 2014, address@hidden (Maxim Yegorushkin) wrote:
> My dependencies are:
> build/Linux-x86_64-64.g++-release/lib/libdata_access.so :
> build/Linux-x86_64-64.g++-release/obj-mt-pic/data_access/data_access.o
> build/Linux-x86_64-64.g++-release/obj-mt-pic/data_access/data_access.o :
> src/c++/data_access/data_access.cc
> src/c++/data_access/data_access.cc :
> src/c++/data_access/data_access.reflect.h
> src/c++/data_access/data_access.reflect.h :
> src/c++/data_access/data_access.h src/c++/reflect/generate.py

This is not a complete list.
> What happens is that make correctly identifies that data_access.cc depends
> on data_access.reflect.h, finds an implicit rule that generates
> data_access.reflect.h, rebuilds the dependencies of data_access.reflect.h
> but neglects to build data_access.reflect.h itself.

Really?  That's not what I'm seeing below.
> Why does it not build data_access.reflect.h please?
> I am using GNU make-4.0. Below is the relevant bit from `make -r -d` output:
>    Considering target file
> 'build/Linux-x86_64-64.g++-release/lib/libdata_access.so'.
>     File 'build/Linux-x86_64-64.g++-release/lib/libdata_access.so' does not 
> exist.
> 'src/c++/data_access/data_access.reflect.h'.
>         Pruning file 'src/c++/data_access/data_access.h'.
>         Considering target file 'src/c++/reflect/generate.py'.
>          Looking for an implicit rule for 'src/c++/reflect/generate.py'.
>          Trying pattern rule with stem 'generate'.
>          Trying rule prerequisite 'src/python/olivetree_c.py'.
>            Finished prerequisites of target file 'src/python/olivetree_c.py'.
>           Must remake target 'src/python/olivetree_c.py'.
> cd src/python && ln -fs olivetree_c.py.release olivetree_c.py
> Putting child 0x1e74d10 (src/python/olivetree_c.py) PID 4317 on the chain.
> Live child 0x1e74d10 (src/python/olivetree_c.py) PID 4317
> Reaping winning child 0x1e74d10 PID 4317
> Removing child 0x1e74d10 PID 4317 from chain.
>           Successfully remade target file 'src/python/olivetree_c.py'.
>          Considering target file 'src/python/olivetree_c.py'.
>          File 'src/python/olivetree_c.py' was considered already.
>          Finished prerequisites of target file 'src/c++/reflect/generate.py'.
>          Prerequisite 'src/python/olivetree_c.py' of target 
> 'src/c++/reflect/generate.py' does not exist.

This is weird.  You just 'created' it, except that you really didn't:
all you did was create a symbolic link to what is supposedly
a pre-existing file.  There are two risks wih this:

1) Does that file actually exist?  Make claims it doesn't.

2) Even if it exists, you didn't update its modification time;
   make will notice it's older than the target,
   so why should it remake the target?

>         Must remake target 'src/c++/reflect/generate.py'.
>         Successfully remade target file 'src/c++/reflect/generate.py'.
>         Pruning file 'etc/rules.mk'.
>       Finished prerequisites of target file 
> 'src/c++/data_access/data_access.cc'.
>       Prerequisite 'src/c++/data_access/data_access.reflect.h' of target
> 'src/c++/data_access/data_access.cc' does not exist.
>      No need to remake target 'src/c++/data_access/data_access.cc'.
>      Finished prerequisites of target file
> 'build/Linux-x86_64-64.g++-release/obj-mt-pic/data_access/data_access.o'
> Maxim

Reinier Post
TU Eindhoven

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