GNU make's -j option

Question

Ever since I learned about -j I\'ve used -j8 blithely. The other day I was compiling an atlas installation and the make failed. Eventually I tracked it down to things being ma

Answer 1

Here's an example of a problem that I ran into when I started using parallel builds. I have a target called "fresh" that I use to rebuild the target from scratch (a "fresh" build). In the past, I coded the "fresh" target by simply indicating "clean" and then "build" as dependencies.

build: ## builds the default target
clean: ## removes generated files
fresh: clean build ## works for -j1 but fails for -j2

That worked fine until I started using parallel builds, but with parallel builds, it attempts to do both "clean" and "build" simultaneously. So I changed the definition of "fresh" as follows in order to guarantee the correct order of operations.

fresh:
    $(MAKE) clean
    $(MAKE) build

This is fundamentally just a matter of specifying dependencies correctly. The trick is that parallel builds are more strict about this than are single-threaded builds. My example demonstrates that a list of dependencies for given target does not necessarily indicate the order of execution.