Choosing different header file depending on instructions from Makefile

Question

I am trying to reorganize someone's code and I seem to have hit a wall. I have a function int fnc() in fnc.c that is called by either main1.c or main2.c. An executable is compiled from either main1.c or main2.c using a makefile:

main1: main1.o fnc.o
    gcc main1.o fnc.o -o main1.x

main2: main2.o fnc.o
    gcc main2.o fnc.o -o main2.x

main1.o: main1.c
    gcc -c main1.c

main2.o: main2.c
    gcc -c main2.c

fnc.o:
    gcc -c fnc.c

The only problem is, there are two header files header1.h and header2.h to be included in the preamble of fnc.c depending on whether main1.x or main2.x is compiled:

#include "header1.h"

int fnc(){
    // do stuff
}

Is it possible to select which header to be loaded depending on whether one types make main1 or make main2?

Answer 1

You can use the -D compiler flag to define a symbol that will be visible to preprocessor. And then to check it with the preprocessor #ifdef directive. Such as:

Makefile:

main1: main1.o fnc1.o
    gcc main1.o fnc1.o -o main1.x

main2: main2.o fnc2.o
    gcc main2.o fnc2.o -o main2.x

main1.o: main1.c
    gcc -c main1.c

main2.o: main2.c
    gcc -c main2.c

fnc1.o:
    gcc -o fnc1.o -c fnc.c -DCONFIG1

fnc2.o:
    gcc -o fnc2.o -c fnc.c -DCONFIG2

And your source:

#ifdef CONFIG1
#include "header1.h"
#elif defined(CONFIG2)
#include "header2.h"
#else
#error "blah"
#endif

int fnc(){
    // do stuff
}

• Note - this is neither tested nor an optimal solution. Just giving the idea