C Pointer to Const

const int *ptr1;  // Preferred by convention
int const *ptr2;  // Syntactically identical

int val1 = 10;
int val2 = 20;

const int *ptr = &val1;

int read_val = *ptr; // PERMITTED: Reading the value
ptr = &val2;         // PERMITTED: Reassigning the pointer to a new address

// *ptr = 30;        // ILLEGAL: Error - assignment of read-only location

int mutable_var = 42;
const int *ptr = &mutable_var;

mutable_var = 100;   // PERMITTED: The original variable is not const
// *ptr = 100;       // ILLEGAL: Cannot mutate through the pointer-to-const

#include <stddef.h>

// The const qualifier guarantees the memory 'str' points to will not be mutated
size_t string_length(const char *str) {
    size_t len = 0;
    while (*str != '\0') {
        len++;
        str++; // PERMITTED: Modifying the pointer itself to traverse memory
        // *str = 'A'; // ILLEGAL: Cannot modify the underlying characters
    }
    return len;
}

int val = 10;

// Pointer to Const
const int *ptr1 = &val;
ptr1++;       // Allowed
// *ptr1 = 5; // Error

// Const Pointer
int * const ptr2 = &val;
// ptr2++;    // Error
*ptr2 = 5;    // Allowed