Why do I get a linker error while I try to compile a C file?

I've had this error for weeks I already made a post about it but it wasn't very clear.
So I am calling a function from a a header file myBmpGris.h and the functions are implemented on the file myBmpGris.c . Here is my main file:

#include<stdio.h>
#include<stdlib.h>
#include "myBmpGris.h"

int main(){

    char * image_name = "image_carre.bmp";
    BmpImg image = readBmpImage(image_name);

 return 0;

I compile by using ggc main.c and I get this error message :

Undefined symbols for architecture x86_64:
"_readBmpImage", referenced from:
_main in main-1c453a.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I read a lot of posts about the same error message but none of the answers seem to apply to my case. I'm kind of desperate because a lot of my programs give me the same error. What should I do ?