C programming: char * const and const char *

Difference between char * const and const char *?

const char *: char* const is a constant pointer to a character array. It means that the pointer itself is a constant, and cannot be reassigned to point to another address, but the contents of the array it points to can be modified.

char * const: char* const is a C type that represents a constant pointer to a non-constant character. This means that the memory address pointed to by the pointer cannot be changed, but the value stored at that memory address can be changed.

Example const char *:

#include<stdio.h>

void print_string(const char *str) {
    while (*str != '\0') {
        printf("%c", *str);
        str++;
    }
    printf("\n");
}

int main() {
    const char *text = "Hello, World!";
    print_string(text);
    return 0;
}

In the above example print_string() takes a const char * parameter “str”, which means that the function promises not to modify the contents of the string pointed to by "str". The while loop in print_string() uses pointer arithmetic to traverse the string, printing each character one at a time.

In main() function, a const char * variable message is declared and initialized with the string literal "Hello, World!". print_string() is then called with message as the argument. Message is a constant string, so the function cannot modify it.

Example char * const:

#include <stdio.h>
int main() {
	 // create a constant pointer to a string
    char * const text = "Hello, World!";
    // this line would result in a compilation error, 
	// since myString is a constant pointer
    //  text = "Goodbye, World!"; 
	printf("%s\n", text); 
    return 0;
}

Output:

Hello, World!

In the above example, "text" is a constant pointer to a string literal "Hello, World!". Since the pointer is declared as const, it cannot be reassigned to point to a different location. We will get an error if we uncomment the line that tries to reassign "text".

We have not declared the string as a const, so we can still modify the contents of the string that "text" points to.