const can protect the pointed-to data, the pointer itself, or both. Read the declaration right to left.

Declaration	Read as	Change `*p`?	Change `p`?
`int *p`	pointer to int	Yes	Yes
`const int p` (= `int const p`)	pointer to const int	No	Yes
`int *const p`	const pointer to int	Yes	No
`const int *const p`	const pointer to const int	No	No

C

#include <stdio.h>

int main(void) {
    int a = 1, b = 2;

    const int *to_const = &a;     /* read-only view */
    to_const = &b;                /* OK: pointer can move */

    int *const fixed = &a;        /* pointer is fixed */
    *fixed = 10;                  /* OK: data can change */

    const int *const both = &b;
    printf("%d %d %d\n", *to_const, a, *both);
    return 0;
}

Output

2 10 2

C — both assignments are compile errors

int main(void) {
    int a = 1, b = 2;
    const int *to_const = &a;
    *to_const = 5;                /* error: read-only */
    int *const fixed = &a;
    fixed = &b;                   /* error: pointer is const */
    return 0;
}

const in function parameters

C

#include <stdio.h>
#include <ctype.h>

/* const promises: "I will only read the characters" */
int count_upper(const char *s) {
    int n = 0;
    for (; *s; s++)                 /* moving the pointer is fine */
        if (isupper((unsigned char)*s)) n++;
    return n;
}

int main(void) {
    char name[] = "Hello World";
    printf("%d\n", count_upper(name));   /* char * -> const char * is implicit */
    printf("%d\n", count_upper("ABC"));
    return 0;
}

Output

2
3

What is the difference between const char *p, char const *p and char *const p?

Can a const variable be modified through a pointer?