c++ Global Variables Across Multiple Files

You could simply use M_PI from the include (there are other constants too).

Edit: your setup is correct. I got a working minmal example:

globals.h

extern double g_tst;

globals.cpp

#include "globals.h"
double g_tst = 4.0;

main.cpp

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

int main()
{
fprintf (stderr, "g_tst  = %lf \n", g_tst);
return 0;
}

The problem is within your buildsystem

See wikipedia

g_pi musst not be declared extern in one translation unit. You could use a small #define for this

in globals.cpp

#define MY_EXTERN_CPP

in /*globals.h

#ifdef MY_EXTERN_CPP
    #define MY_CONFIGURATION_EXTERN
#else
    #define MY_CONFIGURATION_EXTERN extern
#endif

MY_CONFIGURATION_EXTERN double g_pi;

so g_pi will be extern in all translation units you include it except globals.cpp

I think the problem is that you've got #include gobals.h instead of #include globals.h. This would give you the undefined references because it isn't inserting globals.h. The C++ precompiler doesn't fail when it can't find a header file. Instead you get an undefined reference message at compilation.

The order of linking might be the problem. Try to link the global object file as the last one.