Re: Make question

[John Calcote]

Hi Timothy,

On 2/5/2010 8:59 AM, Canham, Timothy K (316C) wrote:
> Newbie on the list, so forgive me if this is asked frequently:
>
> We have code generation, and we have  a rule like this:
>
> fileout1.txt fileout2.txt fileout3.txt: filein.txt
>         filegen.sh filein.txt
>    

What you need to realize about a rule like this is that it's really just 
a shortcut notation for this:

fileout1.txt : filein.txt
        filegen.sh filein.txt

fileout2.txt : filein.txt
        filegen.sh filein.txt

fileout3.txt: filein.txt
        filegen.sh filein.txt


The advantages of using the single rule are clear - you only need to 
maintain one common command and rule, but the fact remains that make 
ultimately considers it to be three separate rules.

> We run a parallel make (make -j x), and what I've found is that the rule often 
> gets invoked three times, causes files to overwrite each other. We've had to 
> make that part serial to avoid this, and it slows down the build. Some have 
> suggested an approach like this:
>
> filemarker.txt: filein.txt
>         filegen.sh filein.txt
>         touch filemarker.txt
>    

This is a common pattern for managing commands with multiple output 
files. It's called a stamp rule, and it does have it's problems. The 
single file (filemarker.txt) acts as a representative of the three 
generated files in a way that make can understand and consume. This 
marker can also be used as a dependency in another rule. One problem is 
that if filemarker.txt is left in place, the outputs will not be 
generated, even if they're missing. That's because, as far as make is 
concerned, the file that represents those three is still there and up to 
date. The only way to get make to regenerate the outputs that you 
actually want is to touch filein.txt, or remove the stamp file.

> This will work, but it leaves a clutter of files all over the place.
>
> Is there a way to specify the rule so that the rule is only invoked once in a 
> parallel make, or is there a technique that accomplishes the same thing?
>    

By "a clutter of files all over the place" I presume you mean that the 
filemarker.txt file is left behind. Be sure to add this file to your 
clean rule so it get's deleted when you clean things up.

Regards,
John