Why I can't reference the implement of function defined in .h file?

IDE: VS code

gcc version: 8.1.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)

I new in C , now I have three files:

Get_para.h

#ifndef _GETPARA_H_
#define _GETPARA_H_
extern double C;
extern double dEarthAngularVelocity;
...
extern void calParameter();
#endif

and Get_para.c, which is the implement of Get_para.h

#include <math.h>
#define _USE_MATH_DEFINES
#define pi M_PI

double C = 3e8;
double dEarthAngularVelocity = 7.29210e-5;
...

void calParameter(){
...
}

then, I want to include Get_para.h in test.c and call calParameter function which is implemented in Get_para.c

#include <stdio.h>
#include "Get_para.h"

int main(){
   calParameter();
   printf("%lf\n",dSemiMajorAxis);
}

I use ‘run code’ in VS,the command in terminal is:

if ($?) { gcc test.c -o test } ; if ($?) { .\test }

the output is:

C:\Users\bob\AppData\Local\Temp\ccuWLUIl.o:test.c:(.text+0xe): undefined reference to `calParameter'
C:\Users\bob\AppData\Local\Temp\ccuWLUIl.o:test.c:(.rdata$.refptr.dSemiMajorAxis[.refptr.dSemiMajorAxis]+0x0): undefined reference to `dSemiMajorAxis'
collect2.exe: error: ld returned 1 exit status

but I want to just include the "Get_para.h" then I can use the implement of them in "Get_para.c".I search this in google , others’ code didn’t work on my computer. Now I guess the problem is the parameters of gcc, but can’t figure out what is it or what knowledge of C I need to know to solve this problem.

>Solution :

You have to compile the implementation of the function too:

gcc Get_para.c test.c -o test

Consider learning a build system, like cmake.

#define _USE_MATH_DEFINES

For a macro like this to work, it has to be defined before any includes.

#define _USE_MATH_DEFINES
#include <math.h>

#define _GETPARA_H_

It’s not valid to define your macros with leading _ followed by an upper case letter. Such identifiers are reserved. For example, use:

#define GETPARA_H_