Make and the ?= operator

The GNU crew improved Make with lots of handy stuff. One such gem that I found today is the ?= operator, which only defines a macro if it is not already defined (like as an environment variable). It behaves almost like wrapping the assignment in ifndef. John Gramm-Cunning has a good explanation of this subtle difference.

Here’s an example of ?= in action.


The INSTALL_HOST macro will have the value “” unless someone specifies it as an environment variable when invoking make. This makes its value easily overridable by users calling your Makfile, like this:

$ make install

I find it pretty handy.

However, I have one gripe. Make in general is tough to learn because its keywords and operators are not Googlable. Say I find the ?= operator in a Makefile and I want to look it up. Normally, I would just punch it into Google, but that just gives you a bunch of junk results. Not a huge deal, but it’s tough to find good docs out there for this reason

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  1. Awesome! I didnt know about that! Thanks for writing about it!

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