Line of code clarification

Question

I'm reading a book about bufferoverflows and shellcode, and in the book there is this code below.

I understand most of it except the purpose of buffer = command +strlen(command); .

When I use memset() on the buffer doesn't it overwrite what I stored there previously with command+strlen(command) ? Can someone clarify it for me?

Answer 1

When one of the operands of + is a pointer then C does pointer arithmetic.

The result or pointer + number is a pointer value that points to the value with index number . It is equivalent to &pointer[number] .

So, in this case:

buffer = command + strlen(command);

is equivalent to

buffer = &command[strlen(command)];

So buffer will point to the string terminator in command , which is just the right place if you want to concatenate something to the command string.