# Socket Programming

> Low-level socket programming including BSD sockets, Winsock, and network byte manipulation

- Skill: `majiayu000/socket-programming-2` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/socket-programming-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/socket-programming-2/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/socket-programming-2

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# Socket Programming for Games

Master **low-level socket programming** for custom game networking.

## Socket Types

| Type | Protocol | Use Case |
|------|----------|----------|
| SOCK_STREAM | TCP | Reliable data |
| SOCK_DGRAM | UDP | Real-time |
| SOCK_RAW | Raw IP | Custom protocols |

## BSD Socket (C)

```c
#include <sys/socket.h>
#include <netinet/in.h>

int create_game_server(int port) {
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);

    struct sockaddr_in addr = {
        .sin_family = AF_INET,
        .sin_port = htons(port),
        .sin_addr.s_addr = INADDR_ANY
    };

    bind(sockfd, (struct sockaddr*)&addr, sizeof(addr));
    fcntl(sockfd, F_SETFL, O_NONBLOCK);

    return sockfd;
}
```

## Socket Options

```c
// Disable Nagle (reduce latency)
int flag = 1;
setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, &flag, sizeof(flag));

// Enable address reuse
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &flag, sizeof(flag));

// Set buffer size
int bufsize = 65536;
setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &bufsize, sizeof(bufsize));
```

## Byte Order

```c
// Network byte order
uint16_t port_net = htons(8080);
uint32_t ip_net = htonl(ip_host);

// Host byte order
uint16_t port_host = ntohs(port_net);
```

## Platform Differences

| Feature | BSD | Winsock |
|---------|-----|---------|
| Init | None | WSAStartup() |
| Close | close() | closesocket() |
| Error | errno | WSAGetLastError() |
| Non-block | fcntl() | ioctlsocket() |

## Troubleshooting

### Common Failure Modes

| Error | Root Cause | Solution |
|-------|------------|----------|
| EADDRINUSE | Port in use | SO_REUSEADDR |
| ECONNRESET | Peer closed | Handle gracefully |
| EMFILE | Too many fds | Increase ulimit |
| High latency | Nagle | TCP_NODELAY |

### Debug Checklist

```bash
# Check listening sockets
netstat -tlnp | grep game-server

# Trace syscalls
strace -e socket,bind,connect ./game-server

# Monitor traffic
tcpdump -i lo port 8080
```

## Unit Test Template

```c
void test_socket_creation() {
    int fd = create_game_server(8080);
    assert(fd >= 0);

    struct sockaddr_in addr;
    socklen_t len = sizeof(addr);
    getsockname(fd, (struct sockaddr*)&addr, &len);
    assert(ntohs(addr.sin_port) == 8080);

    close(fd);
}
```

## Resources

- `assets/` - Socket templates
- `references/` - Platform guides