Re: Does (normal) prerequisite order matter?

[Brendan Heading]

Matt England wrote:

> ...will the rule (if any) for prereq1 complete (and not just start) 
> before the rule for prereq2 starts?  This is an important behavior for 
> my Makefiles, and thus far it seems to be behaving this way (but I have  
> more testing to do); I'm using GNU make 3.80 (on cygwin, mingw, and 
> Redhat systems).

The behaviour you have described will continue to work with gmake on any 
platform as far as I know - provided you do not use parallel make (-jx). 
 But what you are suggesting is that there is a dependency you are not 
telling make about - a dependency between the prereqs.

When I am writing makefiles I always ensure that there are no 
dependencies between the prereqs; if there are then I explicitly define 
them in the makefile. It's like a programmer ensuring their C code is 
re-entrant - not only is it good practice but it allows you to easily 
use multiprocessor machines effectively.

I'd strongly recommend you consider reworking your makefiles such that 
the dependencies are explicitly defined.

> I don't yet see this explained clearly in any of the any of the manual 
> (at http://www.gnu.org/software/make/manual/html_chapter/make_toc.html 
> ), and I have read most of the manual, including the parts on rule-writing.

The manual definitely says that the prereqs are always processed 
left-to-right for non-parallel builds.

> As an aside, I tried understanding order-only prerequisites, but they 
> don't seem to address what I seek above:
> http://www.gnu.org/software/make/manual/html_chapter/make_4.html#SEC29
> http://kolpackov.net/pipermail/notes/2004-January/000001.html

I'd suggest avoiding using any of make's more advanced syntax, and 
instead look into your prerequisites and understand why they are 
interdependent. Then you can do something like

target: prereq1 prereq2

prereq2:prereq1
        <do prereq2>
prereq1:
        <do prereq1>

With the above syntax, we are explicitly telling make to order the 
prereq processing in a certain way even if there are parallel builds.