Why does const extern give an error?

Question

The following code snippet works fine:

#include <stdio.h>

extern int foo; // Without constant
int foo = 42;

int main() 
{
    printf("%d\n",foo);
    return 0;
}

But the following code gives an error:

#include <stdio.h>

const extern int foo; // With constant
int foo = 42;

int main() 
{
    printf("%d\n",foo);
    return 0;
}

So why const externgives an error?

Answer 1

The standard says:
C11-§6.7 / 4

All declarations in the same scope that relate to the same object or function must indicate compatible types.

const intand intnot compatible for the same object fooin the same area.

Answer 2

These two declarations are contradictory:

const extern int foo; // With constant
int foo = 42;

The first declares it fooas const, and the second declares it as not const.

Error messages:

prog.cpp:4:5: error: conflicting declaration
   ‘int foo’ int foo = 42; 
         ^~~
 prog.cpp:3:18: note: previous declaration as ‘const int foo’
    const extern int foo; // With constant 
                     ^~~

Answer 3

You say foois const, and then try changing its constant with a different declaration. Do it and everything will be fine.

  const extern int foo; // With constant
  const int foo = 42;