Windows Socket API Reference

Socket API Reference

Overview

Functions

Structures

Constants

Examples

The Windows Socket (Winsock) API provides a standard interface for network programming on Windows platforms. It enables developers to create, configure, and manage connections over TCP/IP, UDP, and other protocols.

Key Concepts

Socket – An endpoint for communication.
WSAStartup – Initializes Winsock.
bind, listen, accept – Server-side operations.
connect, send, recv – Client-side operations.
IOCTL, select, WSAPoll – Advanced I/O control.

Socket Functions

Function	Description
`WSAStartup`	Initializes Winsock library.
`WSACleanup`	Terminates Winsock usage.
`socket`	Creates a new socket.
`bind`	Associates a socket with a local address.
`listen`	Marks a socket as passive to accept connections.
`accept`	Accepts an incoming connection.
`connect`	Initiates a connection to a remote socket.
`send`	Sends data on a connected socket.
`recv`	Receives data from a connected socket.
`sendto`	Sends a datagram to a specific address.
`recvfrom`	Receives a datagram and captures the source address.
`setsockopt`	Sets socket options.
`getsockopt`	Retrieves socket options.
`select`	Monitors multiple sockets for activity.
`WSAPoll`	Polls sockets for events (modern alternative to select).
`ioctlsocket`	Controls mode of a socket (e.g., non‑blocking).
`getsockname`	Retrieves the local address of a socket.
`getpeername`	Retrieves the remote address of a connected socket.
`shutdown`	Disables sends and/or receives on a socket.

Important Structures

typedef struct sockaddr_in {
    short   sin_family;   // AF_INET
    u_short sin_port;     // Port number (network byte order)
    struct  in_addr sin_addr; // IPv4 address
    char    sin_zero[8];
} SOCKADDR_IN;

typedef struct in_addr {
    union {
        struct { BYTE s_b1,s_b2,s_b3,s_b4; } S_un_b;
        struct { WORD s_w1,s_w2; } S_un_w;
        ULONG S_addr;
    } S_un;
} IN_ADDR;

Additional structures include WSADATA, FD_SET, and WSAPOLLFD.

Common Constants

#define AF_INET        2
#define SOCK_STREAM    1   // TCP
#define SOCK_DGRAM     2   // UDP
#define IPPROTO_TCP    6
#define IPPROTO_UDP    17
#define INVALID_SOCKET ((SOCKET)(~0))
#define SOCKET_ERROR   (-1)

Simple TCP Echo Server (C++)

#include <winsock2.h>
#pragma comment(lib, "Ws2_32.lib")

int main() {
    WSADATA wsa;
    SOCKET listenSock, clientSock;
    sockaddr_in server, client;
    int clientSize = sizeof(client);
    char buffer[512];
    int recvSize;

    WSAStartup(MAKEWORD(2,2), &wsa);
    listenSock = socket(AF_INET, SOCK_STREAM, 0);

    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(27015);
    bind(listenSock, (sockaddr*)&server, sizeof(server));
    listen(listenSock, 3);

    clientSock = accept(listenSock, (sockaddr*)&client, &clientSize);
    while((recvSize = recv(clientSock, buffer, sizeof(buffer), 0)) > 0) {
        send(clientSock, buffer, recvSize, 0);
    }

    closesocket(clientSock);
    closesocket(listenSock);
    WSACleanup();
    return 0;
}

For a full walkthrough, see the Microsoft Winsock tutorial.