undefined reference error in VScode

Question

I'm testing the how to use extern in C ,so I create three files for main.c, test.c, headfile.h . I want to declare variable and function in headfile.h,define in the test.c ,then print out the variable and call function at the main.c It works successfully by using Dev c++,however, when I put the exact same files into VScode it show errors that there are undefined reference to variables

the error messages enter image description here

main.c

#include <stdio.h>
#include <stdlib.h>
#include"D:\My Documents\Desktop\CODE\c\VScode\externTest\headfile.h"
int gVar = 1;

int main(void)
{
    extern float a;

    printf("a = %f\n",a);
    printf("gVar = %d\n",gVar);
    printf("aa = %d\n",aa);
    printf("bb = %f\n",bb);
    function ();
    system("pause");
    return 0;
}

test.c

#include <stdio.h>
#include "D:\My Documents\Desktop\CODE\c\VScode\externTest\headfile.h" 
float a = 100;
int aa = 200;
float bb = 300;

void function (void){
    printf("yeh you got it!!\n");
    extern int gVar;
    gVar++;
    printf("gVar in test.c function = %d",gVar);
}

headfile.h

extern int aa;
extern float bb;
void function(void);

Answer 1

It looks like your main.c file is not being linked with test.c. I have been able to reproduce the exact same error message using the following compilation commands:

$ gcc main.c
/tmp/ccVqEXL5.o: In function `main':
main.c:(.text+0x8): undefined reference to `a'
main.c:(.text+0x38): undefined reference to `aa'
main.c:(.text+0x51): undefined reference to `bb'
main.c:(.text+0x69): undefined reference to `function'
collect2: error: ld returned 1 exit status

To fix this, simply add your test.c file to the compilation by doing gcc main.c test.c.