How can I make a target “private” in GNU make for internal use only? OR: how to best enforce target-specific variable-values?

Question

I have some ancillary targets in a makefile that I want to restrict for internal or \"private\" use (only) inside the makefile. That is, I want to be able to specify these targ

Answer 1

The problem you are trying to solve is legitimate but you are heading on the worse possible path to solve it.

Declaring private targets does not make any sense

When we write a Makefile, we are describing a compilation job in terms of targets, sources and recipes. The advancement of this job is described by the set of targets which are already built. Now you are accurately observing that the sequence

make clean
make foo.o
make debug

will produce objects whose format is inconsistent with foo.o thus leaving your build directory in an inconsistent state. But it is very wrong to deduce that the user should not be able to construct foo.o explicitly. Consider the following sequence:

make clean
# Wait for foo.o being compiles and
#  interrupt the build job with a signal
make debug

Since make sees that foo.o it will resume its task where it was at and left foo.o untouched while compiling subsequent units with different flags, leaving the build directory the same inconsistent state as in the first scenario.

Hence, if we could implement private targets in Makefiles, this would be ineffective and could convey a false sense of security, which is even worse than insecurity by itself. Also the solution you imagined annihilates one of the most important advantages of using Makefiles over shell scripts: Make makes it easy to continue an interrupted task where it was at.

I documented some other aspects of using Makefiles in relation to the set of targets already built in my answer to the question “What is the purpose of linking object files separately in a Makefile?”.

Another solution to your problem

To address the issue of compilation flags inconsistency, we can arrange to store built targets into a special directory, depending on the compilation flags used. Implementing this would fix the issue without forcing us to resign upon the ease of resuming an interrupted compilation job.

Here is an implementation roadmap:

1. Identify build profiles, here you have release and build.
2. Choose which compilation to use for each build profile.
3. Choose in which directory to store built targets for each build profile.
4. Write your Makefile so that built targets are stored in the directories you choosed. Please refer Gnu make - how to get object files in separate subdirectory.

Note. In my opinion, the BSD variant of make has a much nicer support for writing targets in a special directory, see my answer to the question “How to write a Makefile using different directories for targets and sources”. Generally I prefer the BSD variant of make because its documentation is short and to the point and it enjoys a lot of useful advanced examples, since operating system build and ports build in the BSD world are orchestrated by this program.