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C-language

C: const is not constant!

const does not mean that the variable is constant. In this post, we explore the true meaning of the const qualifier and its implications.

C: const is not constant!
c code and the output showing modification of a const variable.

In C, const is a type qualifier that is used to specify that a variable's value cannot be modified after it has been initialized. The const type of variable is a read-only variable, meaning that it can be read but cannot be changed.

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This understanding of the const keyword is misleading!

The reassignment of c to X in the following code is not allowed!

#include <stdio.h>

int main() {
    volatile const char c = 'A';

    c = 'X';

    printf(" &c: %p |    c: %c\n", &c, c);

    return 0;
}
Experiment #1
❯ gcc main.c
main.c:6:7: error: cannot assign to variable 'c' with const-qualified type 'const volatile char'
    c = 'X';
    ~ ^
main.c:4:25: note: variable 'c' declared const here
    volatile const char c = 'A';
    ~~~~~~~~~~~~~~~~~~~~^~~~~~~
1 error generated.

So, a const variable cannot be modified?

Now try this

What do you think the output of the following code will be?

#include <stdio.h>

int main() {
    volatile const char c = 'A';
    char *ptr = (char *)&c;

    printf(" &c: %p |    c: %c\n", &c, c);

    *ptr = 'X';

    printf(" &c: %p |    c: %c\n", &c, c);
    printf("ptr: %p | *ptr: %c\n", ptr, *ptr);

    return 0;
}
Experiment #2