Undefined reference variable using extern keyword

Question

I have two C files in the same folder:

file1.c

#include <stdio.h>

int a;

int main()
{
    a = 1;
    printf("%d",a);
    return 0;
}

file2.c

#include <stdio.h>

extern int a;

int main()
{
    a = 1;
    printf("%d",a);
    return 0;
}

I run file2.c on vscode and its return:

C:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/12.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe:C:\Users\Duong\AppData\Local\Temp\cc8Tiaj4.o:file2.c:(.rdata$.refptr.a[.refptr.a]+0x0): undefined reference to `a'
collect2.exe: error: ld returned 1 exit status

I have try many way sincluding add header file, but it doesn't work at all.

Can someone tell me if my code or my compiler has any problem

Answer 1

1. You need to compile and link both files.
2. You can't have two functions with the same name. As you have main function defined in both files it proves that you do not link them together.

file1.c

#include <stdio.h>

int a;

file2.c

#include <stdio.h>

extern int a;

int main()
{
    a = 1;
    printf("%d",a);
    return 0;
}

Then compile and link them from command line

gcc -o main file1.c file2.c

Visual studio code is not for a very beginners as it requires some configuration and understanding how thing work.

Install Eclipse CDT which manages the projects for you without problems

Answer 2

No, your compiler is telling you the truth. In pru2.c you have a main() routine (as in pru1.c) so I must assume both are different programs.

You have defined a variable in pru2.c that will be found in another compilation unit (module) of the same program (a process cannot access the memory of another, so you can acces no variables of another, different program) For that variable to be viewable, both, file1.c and file2.c need to be compiled together (or better said, linked together) into a single problem. But that cannot be done, because you have one main() function on each, and when trying to link both compilation units together, you will get a linker error telling you that you have two definitions for main in the same program (which cannot be)

Just change the name of main() routine in one of the files to something different (you will not use it) and compile both modules together into the same program

cc -o pru2 file1.c file2.c

As the main() functions in both files are the same, just delete main() in file1.c makint it to appear as:

int a; /* just one variable in this compilation unit */

while file2.c remains the same:

#include <stdio.h>
extern int a;
int main()
{
   a = 1;
   printf("a = %d\n", a);
}

and then you can compile them separately:

cc -c file1.c                  # compilations
cc -c file2.c
cc -o program file1.o file2.o  # this is the linker 

or all together:

cc -o program file1.c file2.c