sizeof operator (in C)

[Strogian]

Is it legal to use the sizeof operator anywhere in a C program, except in #if statements? This book says that sizeof is a compile-time operator, so would that mean I couldn't use it if the compiler can't know the size of that object? For example:

#include <stdio.h>

void dosomething(int);

int main()
{
	srand();
	dosomething(rand());
	return 0;
}

void dosomething(int x)
{
	char s[x];
	printf("%d\n", sizeof s);
}

I tried doing something like this, but ended up with a Segmentation fault. I wasn't sure why I got one, so I just decided to ask the question here. Thanks in advance.