What are sockets?

Learn the socket concept and its role in network communication.

What are sockets?

A socket is an endpoint for communication between processes or over a network. In POSIX, sockets are also file descriptors; they work with read and write.

Two main types exist: SOCK_STREAM (TCP, reliable connection) and SOCK_DGRAM (UDP, connectionless datagram). Sockets are defined in <sys/socket.h>. A server follows socket, bind, listen and accept; a client uses socket and connect.

Socket addresses

Each socket is bound to an address. AF_UNIX uses a local file path, AF_INET uses an IPv4 network. The address structures are struct sockaddr_un and struct sockaddr_in respectively.

In this lesson we use AF_UNIX; it has no network dependency and is fast on the same machine. Socket functions take the address as struct sockaddr *; so you need to cast. Clearing the address struct with memset is a good habit.

Exercise Print socket types

Print two socket types, one per line, with printf: printf("SOCK_STREAM\n"); and printf("SOCK_DGRAM\n");.

Show example solution

#include <stdio.h>
#include <sys/socket.h>

int main(void) {
    printf("SOCK_STREAM\n");
    printf("SOCK_DGRAM\n");
    return 0;
}

Test yourself

Which protocol does SOCK_STREAM correspond to?
- TCP
- UDP
- ICMP
- ARP
Show answer

TCP — SOCK_STREAM corresponds to TCP's reliable connection.

Socket type

int fd = socket(AF_UNIX, ____, 0);

Show answer

int fd = socket(AF_UNIX, SOCK_STREAM, 0);

Match socket types

SOCK_STREAM

TCP reliable
SOCK_DGRAM

UDP connectionless
AF_UNIX

Local socket
AF_INET

IPv4 network

Next lesson: socket and bind

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