Why can't I convert 'char**' to a 'const char* const*' in C?

char **a;
const char** b = a;

char **a;
const char* const* b = a;

void f(const char* const* in) {
  // Only reads the data from in, does not write to it
}

void foo(const int*);

char *y;

char **a = &y; // a points to y
const char **b = a; // now b also points to y

// const protection has been violated, because:

const char x = 42; // x must never be modified
*b = &x; // the type of *b is const char *, so set it
         //     with &x which is const char* ..
         //     ..  so y is set to &x... oops;
*y = 43; // y == &x... so attempting to modify const
         //     variable.  oops!  undefined behavior!
cout << x << endl;

char **a;
const char* const* b = a;

const char **a;
const char* const* b = a;

char **a;
const char* const* b = (const char **)a;

#include <stdlib.h>
#include <stdio.h>
void foo(const char * const * bar)
{
    printf("bar %s nulln", bar ? "is not" : "is");
}

int main(int argc, char **argv)
{
    char **x = NULL;
    const char* const*y = x;
    foo(x);
    foo(y);
    return 0;
}

test.c(8) : warning C4100: 'argv' : unreferenced formal parameter
test.c(8) : warning C4100: 'argc' : unreferenced formal parameter

test.c(8) : warning C4100: 'argv' : unreferenced formal parameter
test.c(8) : warning C4100: 'argc' : unreferenced formal parameter

test2.c: In function `main':
test2.c:11: warning: initialization from incompatible pointer type
test2.c:12: warning: passing arg 1 of `foo' from incompatible pointer type

char c = 'c';
char *p = &c;
char **a = &p;

const char *bi = *a;
const char * const * b = &bi;

const volatile int *const serial_port = SERIAL_PORT;

char *const p = (char *) 0xb000;
//error: p = (char *) 0xc000;
char **q = (char **)&p;
*q = (char *)0xc000; // p is now 0xc000

void foo (const char* const& x);