Question

if I make a system call, say

struct hostent * gethostbyaddr (const char *addr, int len, int family );

and it returns a struct*, do i need to copy the struct to somewhere I “own” instead of using the pointer directly?

Answer 1

The documentation of gethostbyaddr reads (see Notes section):

The functions gethostbyname() and gethostbyaddr() may return pointers
to static data, which may be overwritten by later calls. Copying the
struct hostent does not suffice, since it contains pointers; a deep
copy is required.

It means that you need to copy the struct if you want to be sure that it won’t be overwritten by subsequent calls to gethostbyaddr.

But you really need to consider using getaddrinfo since gethostbyaddr is obsolete. With getaddrinfo you will not be asking this question because you’ll need to allocate memory for the result yourself.