Does sockaddr_in make an address that is differrent than an ip address?

Question

This is what the sockaddr_in struct looks like:

struct sockaddr_in
{
    sa_family_t    sin_family; address family
    in_port_t      sin_port;   port in network byte order 
    struct in_addr sin_addr;   internet address 
};


struct in_addr
{
    uint32_t       s_addr;     address in network byte order 
};

From what I understand (and please correct me if I am wrong) the internet address (sin_addr.s_addr) is an ip address, but the sockaddr_in structure represents an overall structure for how a socket is set up.

If that is the case, and I wanted to connect to some process running on a server, would I just make a struct sockaddr_in, then change the parameters on it and pass it into a connect() system call, and magically my computer would just know how to connect to that computer?

Also, is there a system call in C that provides the ip address of the computer its running on?

Answer 1

A struct sockaddr_in contains all of the necessary data to connect to an IPv4 endpoint.

The sin_family family member should always be set to AF_INET. Because socket functions expect the address of a struct sockaddr, this tells those functions that the structure it is receiving is actually a struct sockaddr_in. This differentiates it from a struct sockaddr_un which is for Unix domain sockets and a struct sockaddr_in6 which is for IPv6 addresses, among others.

The sin_port member holds the port number of the remote machine, and the sin_addr member holds the IPv4 address of the remote machine.

So given those three fields, you have sufficient information to connect to a remote TCP/IP port.

Also, is there a system call in C that provides the ip address of the computer its running on?

For a connected socket, you can use getsockname to get the local IP and port of the connected socket. If you want to get all of the address on the local system, use getifaddrs on Linux and BSD.