The size of the C array given by the variable

Today I found some code that confused me. He did something like this:

#include <stdio.h>

int main(int argc, char **argv) {
    int x = 5;
    int foo[x];

    foo[0] = 33;
    printf("%d\n", foo[0]);
    return 0;
}

My question is: why does this work?

The array foois on the stack, so how can it be deployed to x?

I would expect something like this:

#include <stdio.h>

int main(int argc, char **argv) {
    int x = 5;
    int foo[] = malloc(sizeof(int)*x);

    foo[0] = 33;
    printf("%d\n", foo[0]);
    free(foo);
    return 0;
}

Not that it's prettier or something, but, I'm just curious.