Using m4 as a preprocessor for C (Part 1)

In a recent project I have been experimenting with alternative preprocessors for generating combinatorially exploding volumes of code via meta-programming — something the traditional C preprocessor does not do easily because it was designed to have robust termination qualities. m4 is particularly appealing because it is present on most POSIX compliant systems and rules for *.m4 files can be expressed in my Makefile directly using the shell (so I wouldn’t need to pull in a preprocessing library like Boost).

I couldn’t find many examples on the web which show very basic usage of m4 as a preprocessor, so I thought it would be useful to produce a very simple demonstration. Hypothetically wanting to avoid the regular C preprocessor for some reason, I have produced a testing C file like so;

#include 

define(`DEF', `3')

int main(int argc, char *argv[]) {
        printf("%d\n", DEF);
        return 0;
}

Preprocess it using m4:

$ m4 test.c.m4 > test.c
$ cat test.c
#include <stdio.h>



int main(int argc, char *argv[]) {
    printf("%dn", 3);
    return 0;
}

And finally, compile and test with clang;

$ clang test.c
$ ./a.out 
3

I will be writing a continuation to this post showing some more advanced things you can do with m4 that the traditional C preprocessor cannot, namely, automatically generating Duff’s device.