linking problem when calling main.c in compilation unit - c

Question

Suppose that we call main.c in another c file in the compilation unit.
I understand that this is illegal since we can only call header file. However, why does it cause a bad linking but not a bad compiling when we make build everything?
The C file that calls main should be something like this:
#include "main.c"
int add (int x, int y) { return x+y; }
}

Answer 1

It is generally a bad idea to #include C files, but there is nothing strictly illegal about it. The #include directive effectively copies the source code directly into the current file. What you currently have can be made to work:
$ cat main.c
#include <stdlib.h>
#include <stdio.h>
int add(int, int);
int
main(int argc, char **argv)
{
int a = argc > 1 ? strtol(argv[1], NULL, 10) : 1;
int b = argc > 2 ? strtol(argv[2], NULL, 10) : 1;
printf("%d + %d = %d\n", a, b, add(a, b));
return 0;
}
$ cat add.c
#include "main.c"
int add(int x, int y) { return x + y; }
$ cc add.c
$ ./a.out
1 + 1 = 2
$ ./a.out 3 -5
3 + -5 = -2
If you are getting linkage problems, it is probably because you are trying to do something like:
$ cc main.c add.c
Which is problematic since you now have duplicate definitions of main
Possibly what you want to do is to remove the #include from the add.c and do something like:
$ cat add1.c
int add(int x, int y) { return x + y; }
$ cc -c add1.c
$ cc main.c add1.o