Experiments with const in C

const_test.c

#include <malloc.h>

#define ARRSZ 10

int main() {
  int x = 5;
  const int* xp = &x;

  int y = 5;
  int* yp = &y;

  //*xp = 6; // error: assignment of read-only location '*xp'
  x = 6; // ...but OK to assign a new value this way
  *yp = 6;
  printf("x = %1d,  y = %1d\n",*xp,*yp);

  // But we can still change the values directly!
  x = 4;
  y = 4;

  int x1 = 7;
  int y1 = 7;

  xp = &x1; // reassigning pointers is OK
  yp = &y1;

  xp = malloc(ARRSZ * sizeof x); // again, OK
  yp = malloc(ARRSZ * sizeof y);

  // *xp = 6; // error: assignment of read-only location '*xp'
  *yp = 6;

  // Seems we can't write to ANY location relative to pointer xp,
  // even though we are no longer pointing to the same memory

  // *(xp+1) = 6; // Can't write here either!
  *(yp+1) = 6;

  // Now, what if we define an int* pointer to the same location?

  int* xncp = (int*) xp; // OK! (the cast avoids a warning)
  *xncp = 8;
  printf("*xp = %1d,  *xncp = %1d\n",*xp,*xncp);

}