Why does CC not see my function definition in header?

Question

I'm writing a simple application in ANSI C. I am using GCC in a Unix environment.

I have the following sample application:

    //main.c

#include "foo.h"

int main()
{

int result;

result = add(1,5);

return0;
}

Header:

  //foo.h
    #ifndef FOO_H_INCLUDED
    #define FF_H_INCLUDED

    int add(int a, int b);

    #endif

Implementation:

//foo.c

int add(int a, int b)
{
return a+b;
}

I am compiling my program with the following command:

 cc main.c -o main.o

The compiler complains that 'reference to add is undefined'. Is this a linking problem? How do properly make use of my header?

Thanks!

Answer 1

You need to compile both your source files together:

cc main.c foo.c -o main

Also, in this case, -o produces an executable, so calling it main.o can be misleading.

---

Yet another tidbit, though unrelated to the question: the #ifndef and #define in foo.h don't match.

Answer 2

The header is not your current problem. Your current problem is that you're not compiling the add function definition in foo.c .

Try

cc main.c foo.c -o main.o

Answer 3

If you are trying to compile main.c into an assembled object file, you need to prevent gcc from trying to link. This is done via

cc -c main.c -o main.o

You can compile all other object files, then when you have all of your object files ready, you simply do

cc main.o obj1.o anotherOBJ.o -o myExecutableBinary

Answer 4

"undefined reference" is a linker error, not a compiler error.

The compiler sees the declaration in the header, but you have not compiled or linked the definition in foo.c. Your title uses the term definition incorrectly.