Pointers

Pointer To Const

A pointer to const allows reading through the pointer while protecting the pointed-to value from writes through that pointer.

const pointee `const int *ptr` means the `int` cannot be changed through `ptr`.
reassign pointer The pointer itself can still be changed to read a different object.

Pointer To Const

example
pointer_to_const.c
Replay: real traced execution (multi-file project)
#include <stdio.h>

int main(void) {
    int first = 4;
    int second = 6;
    int useSecond = 0;
    const int *ptr = &first;

    if (useSecond) {
        ptr = &second;
    }

    printf("read=%d\n", *ptr);
    return 0;
}

#include <stdio.h>

int main(void) {
    int first = 7;
    int second = 6;
    int useSecond = 0;
    const int *ptr = &first;

    if (useSecond) {
        ptr = &second;
    }

    printf("read=%d\n", *ptr);
    return 0;
}

#include <stdio.h>

int main(void) {
    int first = 10;
    int second = 6;
    int useSecond = 0;
    const int *ptr = &first;

    if (useSecond) {
        ptr = &second;
    }

    printf("read=%d\n", *ptr);
    return 0;
}

#include <stdio.h>

int main(void) {
    int first = 4;
    int second = 6;
    int useSecond = 1;
    const int *ptr = &first;

    if (useSecond) {
        ptr = &second;
    }

    printf("read=%d\n", *ptr);
    return 0;
}

  1. first ← 4, second ← 6, useSecond ← 0, ptr ← ⟨addr A⟩

    3int main(void) {4    int first→ 4 = 4; //@first=7, 105    int second→ 6 = 6;6    int useSecond→ 0 = 0; //@useSecond=17    const int *ptr→ ⟨addr A⟩ = &first4;89    if (useSecond) {10        ptr = &second;11    }1213    printf("read=%d\n", *ptr⟨addr A⟩);14    return 0;15}
    outputread=4

  1. first ← 7, second ← 6, useSecond ← 0, ptr ← ⟨addr A⟩

    3int main(void) {4    int first→ 7 = 7;5    int second→ 6 = 6;6    int useSecond→ 0 = 0;7    const int *ptr→ ⟨addr A⟩ = &first7;89    if (useSecond) {10        ptr = &second;11    }1213    printf("read=%d\n", *ptr⟨addr A⟩);14    return 0;15}
    outputread=7

  1. first ← 10, second ← 6, useSecond ← 0, ptr ← ⟨addr A⟩

    3int main(void) {4    int first→ 10 = 10;5    int second→ 6 = 6;6    int useSecond→ 0 = 0;7    const int *ptr→ ⟨addr A⟩ = &first10;89    if (useSecond) {10        ptr = &second;11    }1213    printf("read=%d\n", *ptr⟨addr A⟩);14    return 0;15}
    outputread=10

  1. first ← 4, second ← 6, useSecond ← 1, ptr ← ⟨addr A⟩

    3int main(void) {4    int first→ 4 = 4;5    int second→ 6 = 6;6    int useSecond→ 1 = 1;7    const int *ptr→ ⟨addr A⟩ = &first4;

  2. ptr ← ⟨addr B⟩

    9if (useSecond1) {10    ptr→ ⟨addr B⟩ = &second6;11}

  3. printf("read=%d ", *ptr);

    13    printf("read=%d\n", *ptr⟨addr B⟩);14    return 0;15}
    outputread=6