Global variable in a different library, C

Question

I am trying a global variable to hold my error message in C.

One library called Utils has:

#ifndef private_error_h
#define private_error_h

extern char error[1024];

__declspec(dllexport) void FillError(char* newError);
#define GetErr() error

#endif

File error.c:

#include "private_error.h"

char error[1024];
void FillError(char* newError) {
  // ...
}

Then I try to use it in a program:

#include "private_error.h"

int main() {
  FillError("General error");
  printf("%s\n", GetErr());
  return 0;
} 

It creates two variables with different addresses. How can I make the program use the variable from the Utils library?

I did manage to bypass this problem by changing GetErr to a function returning the string, but I am still wondering where the error is here.

Answer 1

You have to declare it in your header file as

extern char error[];

and in your code file (.c file) , declare it

char error[1024];

You are allocating it twice

Answer 2

Why not use function like this:

in .h:

char* GetErr();

and in .cpp:

char* GetErr() { return error; }

Try to avoid global variables. In file scope its pretty safe, but if you try to use it trough many files it can cause big errors.