Makefile duplicate symbol for architecture x86_64 error? - c

Question

I am attempting to create a makefile for a simple program which relies on a simple file and a function stored in a c file. Here are the files:
function.c:
int random_fun(int n, int m)
{ int g;
n = n+m;
m=n/3;
g=n*m;
return g;
}
main.c:
#include <stdio.h>
#include "function.c"
int main()
{
int a, b;
printf("Enter numbers a, and b: ");
scanf("%d %d", &a, &b);
printf("Here is ur answer: %d", random_fun(a, b));
return 0;
}
And here is my makefile:
OBJS = main.o function.o
program: $(OBJS)
$(CC) -o $# $?
clean:
rm $(OBJS) program
Whenever I try type make, I get the following error:
duplicate symbol _random_fun in:
main.o
function.o
ld: 1 duplicate symbol for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
make: *** [program] Error 1"
Not sure what I am doing wrong. I can compile each code separately and main works. I was getting the same error for another project I was working on, so I tried with a very simple case involving only these 2 C files, and I get the same issues. I am fairly new to makefiles and what not, so bear with me if I am making a stupid mistake.

Answer 1

You should read about difference between definition and declaration in C.
As you're including function.c into your main.c, your function random_func is defined two times. Linker can't decide for you which one to use, so it errors out.
For your use case you should declare random_func in main.c or additional header file.

Answer 2

This is what happens with your files after preprocessing:
// function.c
int random_fun(int n, int m)
{ int g;
n = n+m;
m=n/3;
g=n*m;
return g;
}
-
// main.c
// contents of stdio.h goes first. I omit it for brevity
int random_fun(int n, int m)
{ int g;
n = n+m;
m=n/3;
g=n*m;
return g;
}
int main()
{
int a, b;
printf("Enter numbers a, and b: ");
scanf("%d %d", &a, &b);
printf("Here is ur answer: %d", random_fun(a, b));
return 0;
}
It means that now you have the same function in two separate files. When you compile both of them the linker sees two valid functions random_fun, it simply does not know which one to use.
There two ways to solve this problem.
Using header
In this case, you would need to create another file, e.g. function.h:
// random.h
int random_fun(int n, int m);
Then, in main.cyou include the header instead of .c:
#include <stdio.h>
#include "function.h" // <-- .h, not .c
int main()
{
int a, b;
printf("Enter numbers a, and b: ");
scanf("%d %d", &a, &b);
printf("Here is ur answer: %d", random_fun(a, b));
return 0;
}
This way you will have only one random_fun function across two files, the linker would not be confused.
Using extern keyword
In your main.c you can define the random_fun function as external. It basically says to a compiler that the function exists somewhere and it will be resolved later by a linker.
#include <stdio.h>
extern int random_fun(int n, int m);
int main()
{
int a, b;
printf("Enter numbers a, and b: ");
scanf("%d %d", &a, &b);
printf("Here is ur answer: %d", random_fun(a, b));
return 0;
}
Again, in this case, you will have just one random_fun function across two files and the linker would not be confused.
As a rule of thumb, I would say you never include .c files unless you absolutely need to. (I can hardly imagine when it may be needed.)