make question

[Roman Divacky]

On Thu, Apr 28, 2011 at 05:52:58PM +0200, Hartmut Brandt wrote:
> Hi Roman,
> 
> On Wed, 27 Apr 2011, Roman Divacky wrote:
> 
> RD>You seem to have messed with bsd make so I have a question for you  :)
> 
> Yeah, that was some time ago ...
> 
> RD>When a job is about to be executed in JobStart() a pipe is created with
> RD>its ends connected to job->inPipe/job->outPipe. When the job is actually
> RD>created in JobExec() the ps.out is set to job->outPipe so that in
> RD>JobDoOutput() we can read from that pipe and basically just parse the output
> RD>for shell->noPrint and leaving it out from the output. This is meant (I think)
> RD>for supressing the "filter" thing. Ie. that if we do some @command the
> RD>restoration of setting of quiet mode is filtered out.
> RD>
> RD>
> RD>In -B mode we do it differently, as we invoke one shell per command we don't
> RD>have to insert quiet/verbose commands and thus avoid all the piping/parsing
> RD>dance.
> RD>
> RD>So my question is - why don't we invoke one shell per command by default
> RD>and avoid the piping/parsing? Is this because of performance? I think that
> RD>the piping/parsing of the output can have worse impact than invoking a shell
> RD>for every command. Especially given that most targets consists of just one
> RD>command.
> 
> The answer is in /usr/share/doc/psd/12.make. This is so one can write 
> something like
> 
> debug:
> 	DEBUG_FLAGS=-g	
> 	for i in $(SUBDIR); do
> 		$(MAKE) -C $$i all
> 	done
> 
> instead of:
> 
> debug:
> 	DEBUG_FLAGS=-g \
> 	for i in $(SUBDIR); do \
> 		$(MAKE) -C $$i all ; \
> 	done
> 
> -B means 'backward compatible' and does what the original v7 make did: one 
> shell per command. This means you don't have to write the backslashes and 
> the shell variable will be seen in the sub-makes and programs.
> 
> I think we can change this, because it would break makefiles that assume 
> that the entire script is given to the shell in one piece.

I think you answered the question why we parse the target. But I asked why
we parse the output from it.

Anyway, so you think it would be ok to change it to one shell per command and
avoid the shell output parsing or not?

I am interested in this so that "make -j*" lets the command know that the 
output is a TTY, eg. clang can emit coloured warnings.

Thank you, roman