Makefile variables ($@, $< …)

Ever wondered what the funny-looking vars in the GNU Makefiles are? These two are the most common ones:  $@ and @< there are some other are special vars as well.

Here’s what these two mean, and you can look up the rest on the GNU Makefile “Automatic Variables” help page:

$@     -is the name of the target currently being processed.

$<     -is the name of the first dependency.

1. So if you have a makefile target that looks like this:

all: clean libs app
    @echo 'Building target: $@. First dependency is: $<
    ...


Then when you invoke  ’make all’  you will see the message:
 Building target: all. First dependency is: clean.'

2. It becomes more interesting when the first dependency is a list of several values, e.g.:

OBJS := lib1/src1.c lib1/src2.c lib2/src2.c lib2/src3.c
...
myapp: $(OBJS) $(USER_OBJS)

In this case the first dependency which is the variable OBJS is evaluated. It resolves to a list of values and the first value ‘lib1/src1.c’ will be assigned to the ‘$<’ variable.

So when making  ’myapp’  you will see the message:

 Building target: myapp. First dependency is: lib1/src1.c.'

One more thing to look for, particularly if you are using echo messages to debug your make process:
Your first dependency may often be another target in the make file! In the first example (all: clean libs app) the dependencies are ‘clean’, ‘libs’ and ‘app’ and by the looks of it these are most likely other targets. Now, what I wanted point out is that if one of these targets prints a message… Then the sequence of the messages you see may not be what you expected!  Or worse yet – if one of these pre-requisite targets breaks the build (!) then you may never see your message! Just keep these in mind when debugging your makefiles…