C: why no & for strings in scanf() function? [duplicate]

Answer 1

A "string" in C is the address of a character buffer.
You want scanf to fill the memory in the buffer, which is pointed to by the variable.

In contrast, an int is a block of memory, not an address. In order for scanf to fill that memory, you need to pass its address.

Answer 2

Since the identifier buf in char buf[512]; degrades to a pointer to the first element of the array you don't need to specify a pointer to the buffer in the argument to scanf. By passing &buf the pointer passed to scanf is now a doubly-indirect pointer (it is a pointer to a pointer to the beginning of the buffer) and will likely cause the program to crash or other bad behaviour.