使用malloc时堆与堆栈内存

Question

In following code:

int main(void)
{
    char* a = malloc(sizeof(char));
    a[0] = 'a';
    a[1] = 'b';
    a[2] = 'c';
    printf("%s", a);
}

Acc. to me a[0] is being allocated memory from heap while a[1] and a[2] are on the stack. I get the output of following code as abc . How is that happening, if stack and heap are supposed to two different segments of memory I must get only a as the output as I have allocated memory in heap for only one char.

Answer 1

You're only allocating room on the heap for one char in a , but you're writing three. The other two don't magically go to the stack; they just overwrite some other probably-important value that happens to be after the one you did allocate. Your program is simple enough that it didn't blow up in your face, but it's still Undefined Behavior to do this. C doesn't have bounds checking, so it won't save you from this mistake.