…you can never go back.

After being shown that you can declare a global variable, as one does…

int g_globalVariable;

…and then make it be treated as a read-only variable to other files by extern-ing it as a const:

extern int const g_globalVariable;

…of course I wondered what the compiler did if you went the other way:

// main.c
#include <stdio.h>

void function (void);

int const c_Value = 0;

int main()
{
    printf("Hello World\n");
    
    printf ("c_Value: %d\n", c_Value);
    
    function ();
    
    printf ("c_Value: %d\n", c_Value);

    return 0;
}

// function.c
#include <stdio.h>

// Extern as a non-const.
extern int c_Value;

void function ()
{
    c_Value++;
}

Above, main.c contains a global const variable, but function.c tries to extern it as non-const.

But when I run the code…

Hello World
c_Value: 0


...Program finished with exit code 139
Press ENTER to exit console.

…the compiler does not complain, but we get a crash. Looking at this in a debugger shows more detail:

Program received signal SIGSEGV, Segmentation fault.
0x00005555555551ef in function () at Function.c:11
11          c_Value++;

I am unfamiliar with the inner workings on whatever compiler this Online C Compiler – online editor is using, but I suspect I’d see similar results doing this on any system with memory protection. Go back to the early days (like OS-9 on a 6809 computer, or even on a 68000 without an MMU) and … maybe it just allows it and it modifies something it shouldn’t?

We can file this away in the “don’t do this” category.

Until next time…