John Graham-Cumming

The other day I wrote a blog post about finding the value of -j in GNU make and it received a lot of commentary. Daniel Frey wrote in with a different solution that goes like this. It's worth studying if you are interested in GNU make wrangling.

.PHONY: default FORCE token recursive
default: foo.out
    @$(eval JOB_COUNT := $(shell cat $<))
    @echo Job count: $(JOB_COUNT)

foo.out: FORCE
    @timeout 1 $(MAKE) --no-print-directory token recursive 2>/dev/null | wc -l >[email protected]

FORCE:

token:
    @echo X && sleep 2 && false

recursive:
    @$(MAKE) --no-print-directory token recursive

Sometimes when you are running a make you'd like to see what commands get executed (even if they've been muted by prefixing them with @) and where in the makefile those commands reside. But you can't. There's no way to override @ and make -n doesn't actually run the make.

But here's 'one weird trick' that makes it possible to see inside a make as its running. Just add this to the makefile:

_SHELL := $(SHELL)
SHELL = $(warning [[email protected]])$(_SHELL) -x

Before digging into how that gives you makefile X-ray vision, let's take a look at it in action. Here's an example makefile that builds an executable from foo.o and bar.o (which are built from corresponding foo.c and bar.c files).

.PHONY: all
all: runme

runme: foo.o bar.o ; @$(LINK.o) $^ -o [email protected]

foo.o: foo.c
bar.o: bar.c

%.o: %.c ; @$(COMPILE.C) -o [email protected] $<

Running simply make on this performs the compiles and link but produces no output because output has been suppressed by the @ signs on the compile and link comm…

The BBC Weather App sends the location of the user via HTTP to the BBC in order to determine the best location for weather information. It does this with roughly 100m accuracy, in the parameters of an unencrypted GET and even though the same API endpoint is available using HTTPS.

I discovered this accidentally using Charles Proxy to snoop on traffic from my iPhone at home. Here's the Charles Proxy view of that app interacting with the BBC's APIs:


It's hitting the endpoint http://open.live.bbc.co.uk/locator/locations with parameters la and lo containing the three decimal digit latitude and longitude (which give roughly 100m precision) over HTTP as a GET request.

The API then returns a JSON object containing nearby locations that the app can get weather information about.

Sadly, this API could have been accessed over HTTPS. Just switching the protocol from HTTP to HTTPS works fine. A legitimate wildcard certificate for *.bbc.co.uk is used.


So, the app could have used HTTPS.

…

If you're using GNU make and you need help debugging a makefile then there's a single line your should add. And it's so useful that you should add it to every makefile you create.

It's:

    print-%: ; @echo $*=$($*)

It allows you to quickly get the value of any makefile variable. For example, suppose you want to know the value of a variable called SOURCE_FILES. You'd just type:

    make print-SOURCE_FILES

If you are using GNU make 3.82 or above it's not even necessary to modify the makefile itself. Just do

    make --eval="print-%: ; @echo $*=$($*)" print-SOURCE_FILES

to get the value of SOURCE_FILES. It 'adds' the line above to the makefile by evaluating it. The --eval parameter is a handy way of adding to an existing makefile without modifying it.
How that works The line

    print-%: ; @echo $*=$($*)

defines a pattern-rule that matches any target in the form print-% (the % is the wildcard character). So when you run make print-SOURCE_FILES that…

So, it's finally here. My second book, entitled The GNU Make Book and published by No Starch Press later this month (electronically it's available now). If you are a long time reader of my blog then you'll know that I've had a very long interest in GNU make and written quite a bit about it. For a while I had self-published all my articles on GNU make as a print on demand book.

The folks at No Starch Press kindly took my self-published effort and did the serious work of actually editing, organizing, indexing and generally sprucing it up. I added some articles that weren't in the old version and updated for GNU make 4.0.

And now it's here.



The book is not intended for people learning GNU make from scratch. If you need to do that go get the FSF's GNU make manual and read it. It covers all that you need to get started with GNU make.

Once you've done that you'll be ready to read mine. It's based on years of working with real makefiles, on having wri…