What is the meaning of "sin_addr.s_addr " and "inet_addr"?

Question

What exactly is difference between sin_addr.s_addr and inet_addr?

addr.sin_addr.s_addr = inet_addr("127.0.0.1");

is what i am using in my programming - what does this achieve?

Answer 1

sin_addr is the IP address in the socket (the socket structure also contains other data, such as a port). The type of sin_addr is a union, so it can be accessed in three different ways : as s_un_b (four 1-byte integers), s_un_w (two 2-bytes integers) or as s_addr (one 4-bytes integer).

inet_addr converts an IPv4 address from a string in dotted decimal representation to an integer. This function is deprecated because it does not support IPv6, use inet_pton instead.

So basically, the line about which you are asking loads into the socket the IP address 127.0.0.1, meaning the local host.

Answer 2

addr.sin_addr.s_addr

s_addr is variable that holds the information about the address we agree to accept. So, in this case i put INADDR_ANY because i would like to accept connections from any internet address. This case is used about server example. In a client example i could NOT accept connections from ANY ADDRESS.

ServAddr.sin_addr.s_addr = htonl(INADDR_ANY);

The inet_addr() and inet_network() functions return numbers suitable for use as Internet addresses and Internet network numbers, respectively.

They are different things, as you can see if you put INADDR_ANY in .s_addr your connection will accept all incoming addresses, in your case you're specifying to accept localhost.

This works both for client and for server, server will use INADDR_ANY (if want to accept all incoming connections) and client should just specify one concrete address

fonts: https://wiki.netbsd.org/examples/socket_programming/#index1h3 and man inet_addr