Can someone explain this code with structs please? (trying to learn Winsock 2)

Question

I have typically seen structs of the form

struct Employee {
int age;
char* name;
}

I was looking at Microsoft's "Getting Started" for Winsock 2, and saw this:

struct addrinfo *result = NULL,
                    *ptr = NULL,
                    hints;

ZeroMemory( &hints, sizeof(hints) );
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

What kind of struct is this? I figure the name of the struct is addrinfo , but then what type are *result , *ptr , or hints ?

Also, how is hints given a .ai_family/socktype/protocol when it never was coded previously?

Answer 1

It's a C-style way of declaring a variable as an instance of a struct. addrinfo is the struct that's defined elsewhere, and result is a pointer to an instance of one. Here is the actual definition of addrinfo

In modern C++, the following is equivalent:

addrinfo* result = NULL; // nullptr in C++11 and beyond
addrinfo* ptr = NULL; // nullptr in C++11 and beyond
addrinfo  hints;

Answer 2

struct addrinfo is a standard data structure to contain network address information. It is defined by eg POSIX, aka SingleUnix . You get it by including netdb.h or your OS' equivalent. Its fields are (at least):

int               ai_flags      Input flags. 
int               ai_family     Address family of socket. 
int               ai_socktype   Socket type. 
int               ai_protocol   Protocol of socket. 
socklen_t         ai_addrlen    Length of socket address. 
struct sockaddr  *ai_addr       Socket address of socket. 
char             *ai_canonname  Canonical name of service  location. 
struct addrinfo  *ai_next       Pointer to next in list. 

struct addrinfo *result = NULL, *ptr = NULL, hints;

result and ptr are pointers to struct addrinfo . hints is a stack-allocated struct addrinfo .

The Windows Socket API is a pure C API hence it uses C conventions. Structure variables in C must be declared explictly with struct . You cannot leave off the struct as in C++. They are also not initialized, hence the memset() ^W ZeroMemory()