the socket

[nadji]

hello, I want to develop a program that interact with the browser using the socket but I can't understand the difference between the two structure

struct sockaddr {  }

and

struct sockaddr_in {   }

and why casting sockaddr_in to sockaddr

[cas]

"struct sockaddr" is a generic address, while "struct sockaddr_in" is an IPv4 address.

and why casting sockaddr_in to sockaddr

It allows functions like accept(2) to work for any address type (that is, you don't need accept_ipv4(), accept_ipv6(), and so on). All of the struct sockaddr types will have a common beginning sequence which contains the address family, so accept(2) can simply peek at that to decide how to proceed.

[christop]

It's a nice example of class inheritance in C. In OOP terms, sockaddr is the base class, and sockaddr_in and sockaddr_in6 (and others like sockaddr_un) are derived classes.

Here's a good page that explains what these structures are in more depth: struct sockaddr and pals