constant pointer vs pointer on a constant value

Question

What is the difference between the following declarations?

char * const a;
const char * a;

In order to understand the difference I wrote this small program:

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


int main (int argc, char **argv)
{
    char a = 'x';
    char b = 'y';

    char * const pc1 = &a;
    const char * pc2 = &a;

    printf ("Before\n");
    printf ("pc1=%p\n", pc1);
    printf ("*pc1=%c\n", *pc1);
    printf ("pc2=%p\n", pc2);
    printf ("*pc2=%c\n", *pc2);

    *pc1 = b;
/*     pc1 = &b; */

/*     *pc2 = b; */
    pc2 = &b;

    printf ("\n\n");

    printf ("After\n");
    printf ("pc1=%p\n", pc1);
    printf ("*pc1=%c\n", *pc1);
    printf ("pc2=%p\n", pc2);
    printf ("*pc2=%c\n", *pc2);

    return EXIT_SUCCESS;
}

I compiled the program (with gcc 3.4) and ran it. The output highlights the difference rather well:

Before
pc1=ffbfd7e7
*pc1=x
pc2=ffbfd7e7
*pc2=x


After
pc1=ffbfd7e7
*pc1=y
pc2=ffbfd7e6
*pc2=x

However, I had to write the small program to get the answer. In case I'm away from the machine (at an interview for instance), I wouldn't be able to answer the question.

Can someone please explain, by commenting the above example, how the const keyword operates?

Answer 1

char * const a;

means that the pointer is constant and immutable but the pointed data is not.
You could use const_cast(in C++) or c-style cast to cast away the constness in this case as data itself is not constant.

const char * a;

means that the pointed data cannot be written to using the pointer a. Using a const_cast(C++) or c-style cast to cast away the constness in this case causes Undefined Behavior.