I'm trying to put the int 100 in a variable malloced 1, why doesn't this program crash?

Question

Ok, so I'm screwing around with C trying to break stuff in a somewhat safe manner, like calling malloc(100000000000000) and having my computer run out of memory for a microsecond, and I thought, what would happen if I tried to put too much info in a variable, so I did this...

#include <stdio.h>
#include <stdlib.h>
int main(int argc, const char * argv[]) {
    int *p;
    p = malloc(1);
    *p = 100;
    printf("%d", *p);
    free(p);
    printf("%d", *p);
    printf("Hello, World!\n");
    return 0;
}

This does two things I didn't expect.

• First, it prints p twice. I though p would "vanish" after I freed it. (Though on second thought, I may have to let the OS run a while before it vanished.)
• Second, I expected it to crash. I'm putting 100, a 4 byte int in a box that I think is supposed to be 1 byte. Does malloc(1) malloc one byte? Or is it more like malloc(sizeof(int))? Why does this work? And, assuming the compiler would let me, how would I cause the buffer overflow (is that the right name here?) that I'm trying to cause?

Pertinent computer info:

• OS: Mac OS X 10.10.latest
• compiler: Xcode, so clang/llvm