GNU make: Generating automatic dependencies with generated header files

Question

So I followed the Advanced Auto-Dependency Generation paper --

Makefile:

SRCS := main.c foo.c

main: main.o foo.o

%.o: %.c
    $(CC) -MMD -         


        

Answer 1

Short answer: no. The recipe described in the paper is very clever, one of my favorites, but it's a sophisticated use of a crude tool. It takes advantage of the usual scheme in which all needed headers exist; what it tries to solve is the problem of determining which headers, if recently modified, require the given object file to be rebuilt. In particular, if the object file doesn't exist then it must be rebuilt-- and in that case there's no reason to worry about the header files because the compiler will surely find them.

Now header files are generated. So foo.h may not exist, so somebody will have to run the script to generate it, and only Make can do that. But Make can't know that foo.h is necessary without performing some analysis of main.c. But that really can't happen until Make starts to execute main-related rules (e.g main.o or main.o.d), which it cannot execute until after it has decided which targets it is going to build.

So we will have to use... recursive make! [Dun-dun-dunnnn!]

We can't achieve the paper's goal of avoiding reinvocation of Make, but we can at least avoid (some) unnecessary rebuilding. You could do something like the "Basic Auto-Dependencies" described in the paper; the paper describes the problems of that approach. Or you could use a command like the one in the "Advanced" recipe to generate a list of headers, then pass that to $(MAKE); this approach is tidy, but might call Make many times on the same header, depending on what your code tree looks like.