Socket basics
- C++ course
- Advanced
- Lesson 55/83
- 5 min
Learn socket programming basics in C++.
What is a socket?
A socket is an endpoint for communication between two programs. It is used to send and receive data over a network. In C++ the POSIX socket API provides the socket() function, which creates a socket and returns a file descriptor (int).
Example: int sock = socket(AF_INET, SOCK_STREAM, 0);. Here AF_INET means the IPv4 address family, SOCK_STREAM means a reliable TCP connection, and 0 picks the default protocol. A negative return value signals an error. After creating the socket you use connect (client) or bind + listen (server). This lesson focuses on socket creation only.
AF_INET and the socket descriptor
AF_INET specifies the IPv4 address family. When a socket is created the returned int is a file descriptor. This value is used in all subsequent socket operations (connect, bind, send, recv).
Example: int sock = socket(AF_INET, SOCK_STREAM, 0); if (sock < 0) { /* error */ }. A negative value means an error. To close the socket you call close(sock);. A socket must always be closed when done; otherwise a resource leak occurs. This is like file operations: open, use, close.
Practice Create a socket
Add #include <sys/socket.h>. Create a TCP socket with int sock = socket(AF_INET, SOCK_STREAM, 0);. Print the status with std::cout << "Socket ready";.
Starter code
#include <iostream>
// Add the socket header and create a socket
int main() {
// Add the socket call and output
return 0;
}Sample solution
#include <iostream>
#include <sys/socket.h>
int main() {
int sock = socket(AF_INET, SOCK_STREAM, 0);
std::cout << "Socket ready";
return 0;
}Expected output
Socket ready
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which protocol does SOCK_STREAM correspond to?
- UDP
- ICMP
- TCP Correct answer
- ARP
Explanation: SOCK_STREAM means a reliable TCP connection.
2 Match each piece to its role
| Term | Meaning |
|---|---|
AF_INET |
IPv4 address family |
SOCK_STREAM |
TCP stream |
socket() |
Creates a socket |
close(sock) |
Closes the socket |
3 Complete the socket type
int sock = socket(AF_INET, ____, 0);int sock = socket(AF_INET, SOCK_STREAM, 0);Explanation: SOCK_STREAM gives a reliable TCP based stream.