Question

Today I had an interview there they asked me can we include .c file to a source file?
I said yes. Because few years back I saw the same in some project where they have include .c file. But just now I was trying the same.

abc.c

#include<stdio.h>
void abc()
{ printf("From ABC() \n"); }

main.c

#include<stdio.h>
#include "abc.c"
int main()
{   void abc();
    return 0;
}

Getting an error:

D:\Embedded\...\abc.c :- multiple definition of 'abc'

Where is it going wrong?

I wrote an abc.h file (the body of abc.h is { extern void abc(void); }),
and included the file in abc.c (commenting out #include abc.c). Worked fine.

Answer 1

Do it as follows:

abc.c:

#include <stdio.h>
void abc()
{printf("From ABC() \n");}

main.c:

#include<stdio.h>
#include "abc.c"
int main()
{   
    abc();
    return 0;
}

(no need for the header file)

Then, to compile, you’d only compile main.c. Do not attempt to compile both abc.c and main.c, because then you’d have the abc() function defined twice.

You need to understand that #include is basically “copy-paste”, nothing more. If you tell it #include "abc.c", it will simply take the contents of abc.c, and “paste” them in your main.c file. Therefore, using the above for main.c, after the preprocessor processes it, your main.c will look like this (I’m ignoring the #include <stdio.h>s):

#include<stdio.h>
#include <stdio.h>
void abc()
{printf("From ABC() \n");}
int main()
{   
    abc();
    return 0;
}

which is a valid program.

That said you should generally not do this; you should compile all your .c files separately and only then link them together.