sharing-sockets-with-so-reuseport-in-zig

name	sharing-sockets-with-so-reuseport-in-zig
description	Writes Zig code for sharing TCP/UDP sockets between unrelated processes using SO_REUSEPORT. Use when implementing multi-process socket sharing, zero-downtime restarts, load balancing across processes, or when user mentions SO_REUSEPORT, reuseport, socket sharing, or graceful restart in a Zig project.

Sharing Sockets with SO_REUSEPORT in Zig

Write Zig code that shares sockets between unrelated processes using SO_REUSEPORT. Use this skill when building multi-process servers, implementing zero-downtime deployments, or distributing connections across independent Zig processes.

Quick Start

Minimal SO_REUSEPORT TCP server in Zig -- each process independently binds to the same port:

const std = @import("std");
const posix = std.posix;

pub fn main() !void {
    const address = try std.net.Address.parseIp("0.0.0.0", 8080);

    const sockfd = try posix.socket(
        posix.AF.INET,
        posix.SOCK.STREAM | posix.SOCK.CLOEXEC,
        posix.IPPROTO.TCP,
    );
    defer posix.close(sockfd);

    // SO_REUSEPORT MUST be set BEFORE bind()
    try posix.setsockopt(sockfd, posix.SOL.SOCKET, posix.SO.REUSEPORT, &std.mem.toBytes(@as(c_int, 1)));
    try posix.setsockopt(sockfd, posix.SOL.SOCKET, posix.SO.REUSEADDR, &std.mem.toBytes(@as(c_int, 1)));

    try posix.bind(sockfd, &address.any, address.getOsSockLen());
    try posix.listen(sockfd, 128);

    std.debug.print("Listening on :8080 (pid={})\n", .{std.os.linux.getpid()});

    while (true) {
        var client_addr: posix.sockaddr = undefined;
        var addr_len: posix.socklen_t = @sizeOf(posix.sockaddr);
        const client_fd = try posix.accept(sockfd, &client_addr, &addr_len, posix.SOCK.CLOEXEC);
        defer posix.close(client_fd);

        const response = "HTTP/1.1 200 OK\r\nContent-Length: 2\r\n\r\nOK";
        _ = try posix.write(client_fd, response);
    }
}

Compile and run multiple instances: each process independently accepts connections, kernel distributes via hash.

Critical Rules

1. SO_REUSEPORT before bind() -- mandatory ordering; setting after bind has no effect
2. ALL sockets must set SO_REUSEPORT -- including the very first one; otherwise subsequent binds fail with EADDRINUSE
3. Same effective UID -- kernel enforces all processes in a reuseport group share the same EUID
4. Use raw posix API -- std.net.Address.listen() with reuse_address = true sets both SO_REUSEADDR and SO_REUSEPORT, but for explicit control use posix.setsockopt() directly

Zig Socket API Cheat Sheet

Option value conversion

posix.setsockopt() takes []const u8. Convert typed values:

// Integer options (c_int)
try posix.setsockopt(fd, posix.SOL.SOCKET, posix.SO.REUSEPORT,
    &std.mem.toBytes(@as(c_int, 1)));

// Struct options
const timeout = posix.timeval{ .tv_sec = 5, .tv_usec = 0 };  // .sec/.usec in Zig 0.14+
try posix.setsockopt(fd, posix.SOL.SOCKET, posix.SO.RCVTIMEO,
    std.mem.asBytes(&timeout));

Key constants

Constant	Path	Value (x86_64)
SO.REUSEPORT	posix.SO.REUSEPORT	15
SO.REUSEADDR	posix.SO.REUSEADDR	2
SOL.SOCKET	posix.SOL.SOCKET	1
AF.INET	posix.AF.INET	2
SOCK.STREAM	posix.SOCK.STREAM	1
SOCK.CLOEXEC	posix.SOCK.CLOEXEC	0o2000000

Address creation

// From string
const addr = try std.net.Address.parseIp("0.0.0.0", 8080);

// From raw bytes
const addr = std.net.Address.initIp4(.{ 127, 0, 0, 1 }, 8080);

// Pass to bind: &addr.any and addr.getOsSockLen()

Common Patterns

Zero-Downtime Restart

1. New process starts, sets SO_REUSEPORT, binds, listens -- joins existing reuseport group
2. Kernel splits incoming connections across old + new processes (hash-based)
3. Signal old process to stop accepting (close listen fd)
4. Old process drains in-flight requests, then exits

Gotcha: During listener count change, in-flight TCP handshakes may get RST'd (~2-3 per million connections). On Linux 5.14+, enable socket migration: sysctl net.ipv4.tcp_migrate_req=1.

High-Level API Alternative

const address = try std.net.Address.parseIp("0.0.0.0", 8080);
var server = try address.listen(.{
    .reuse_address = true,  // sets BOTH SO_REUSEADDR and SO_REUSEPORT
    .kernel_backlog = 128,
});
defer server.deinit();
const conn = try server.accept();  // returns Server.Connection

Note: reuse_address = true enables SO_REUSEPORT on Linux. This is a known naming inconsistency (see zig#24838).

Detailed References

• SO_REUSEPORT internals, security, eBPF steering: See SO_REUSEPORT.md
• Zig socket API details, raw syscalls, error handling: See ZIG-SOCKET-API.md
• Full code examples (multi-process server, graceful restart, UDP): See EXAMPLES.md