Writing a HTTP Server in Zig

🖊️ 🔖 code zig 💬 0

I continue my Zig adventure by following up an echo server with a HTTP server.

I’ve been doing web development the majority of my career. Yet I never really thought too much about HTTP servers, much less what it would take to implement one. So it made perfect sense for me when I started to learn Zig to build one. The problem space is a nice mix of socket programming and string handling.

The source is available on Github.

To start the server:

zig run http.zig

I’m not super confident that any of the following code is “good Zig” but here it is anyways.

Constants: Errors and Mime-Types

I defined program-wide custom errors at the top where they are easy to reference. As well as an anonymous struct of structs that maps file extensions to mime-type strings. This is important later.

const std = @import("std");
const net = std.net;
const fs = std.fs;
const mem = std.mem;
const expect = std.testing.expect;

pub const ServeFileError = error{

const mimeTypes = .{
    .{ ".html", "text/html" },
    .{ ".css", "text/css" },
    .{ ".png", "image/png" },
    .{ ".jpg", "image/jpeg" },
    .{ ".gif", "image/gif" },

I really do like how Zig does error handling. The error-tuples reminds me of Golang, but without the annoying need to handle them explicitly every time they are returned.

The Main Loop

This is the main loop off the program. I probably could have factored it more.

It starts off with some pretty standard socket programming: listening on an address and then setting up a loop to listen for data on that address. The Zig standard library seems well designed here.

I also encountered my first browser behavioral peculiarity. It would seem that (at least Firefox) attempts to open a connect to the remote server of the target of an anchor tag when the user hovers over it with their mouse. Presumably this is to optimize load speed in anticipation of a click. However no data is actually sent until the user clicks, and will time out after about 10 seconds. This required a special case in this code.

The majority of the rest of the code is string parsing/formatting, followed by sending the result down the socket to the browser.

pub fn main() !void {
    std.debug.print("Starting server\n", .{});
    const self_addr = try net.Address.resolveIp("", 4206);
    var listener = try self_addr.listen(.{ .reuse_address = true });
    std.debug.print("Listening on {}\n", .{self_addr});

    while (listener.accept()) |conn| {
        std.debug.print("Accepted connection from: {}\n", .{conn.address});
        var recv_buf: [4096]u8 = undefined;
        var recv_total: usize = 0;
        while (conn.stream.read(recv_buf[recv_total..])) |recv_len| {
            if (recv_len == 0) break;
            recv_total += recv_len;
            if (mem.containsAtLeast(u8, recv_buf[0..recv_total], 1, "\r\n\r\n")) {
        } else |read_err| {
            return read_err;
        const recv_data = recv_buf[0..recv_total];
        if (recv_data.len == 0) {
            // Browsers (or firefox?) attempt to optimize for speed
            // by opening a connection to the server once a user highlights
            // a link, but doesn't start sending the request until it's
            // clicked. The request eventually times out so we just
            // go agane.
            std.debug.print("Got connection but no header!\n", .{});
        const header = try parseHeader(recv_data);
        const path = try parsePath(header.requestLine);
        const mime = mimeForPath(path);
        const buf = openLocalFile(path) catch |err| {
            if (err == error.FileNotFound) {
                _ = try conn.stream.writer().write(http404());
            } else {
                return err;
        std.debug.print("SENDING----\n", .{});
        const httpHead =
            "HTTP/1.1 200 OK \r\n" ++
            "Connection: close\r\n" ++
            "Content-Type: {s}\r\n" ++
            "Content-Length: {}\r\n" ++
        _ = try conn.stream.writer().print(httpHead, .{ mime, buf.len });
        _ = try conn.stream.writer().write(buf);
    } else |err| {
        std.debug.print("error in accept: {}\n", .{err});

Parsing the header

This is straight string parsing. While the std library has some nice inclusions, coming from Python this still seems verbose and difficult. But perhaps that’s not a fair comparison.

I used structs here to give some structure to the return type of the parseHeader function.

const HeaderNames = enum {

const HTTPHeader = struct {
    requestLine: []const u8,
    host: []const u8,
    userAgent: []const u8,

    pub fn print(self: HTTPHeader) void {
        std.debug.print("{s} - {s}\n", .{

pub fn parseHeader(header: []const u8) !HTTPHeader {
    var headerStruct = HTTPHeader{
        .requestLine = undefined,
        .host = undefined,
        .userAgent = undefined,
    var headerIter = mem.tokenizeSequence(u8, header, "\r\n");
    headerStruct.requestLine = headerIter.next() orelse return ServeFileError.HeaderMalformed;
    while (headerIter.next()) |line| {
        const nameSlice = mem.sliceTo(line, ':');
        if (nameSlice.len == line.len) return ServeFileError.HeaderMalformed;
        const headerName = std.meta.stringToEnum(HeaderNames, nameSlice) orelse continue;
        const headerValue = mem.trimLeft(u8, line[nameSlice.len + 1 ..], " ");
        switch (headerName) {
            .Host => headerStruct.host = headerValue,
            .@"User-Agent" => headerStruct.userAgent = headerValue,
    return headerStruct;

At least we have slices!

Parsing the Request Path

Again, this is normal string parsing. We do ensure that the browser is only performing a GET over HTTP/1.1

pub fn parsePath(requestLine: []const u8) ![]const u8 {
    var requestLineIter = mem.tokenizeScalar(u8, requestLine, ' ');
    const method = requestLineIter.next().?;
    if (!mem.eql(u8, method, "GET")) return ServeFileError.MethodNotSupported;
    const path = requestLineIter.next().?;
    if (path.len <= 0) return error.NoPath;
    const proto = requestLineIter.next().?;
    if (!mem.eql(u8, proto, "HTTP/1.1")) return ServeFileError.ProtoNotSupported;
    if (mem.eql(u8, path, "/")) {
        return "/index.html";
    return path;

Reading the Local File

The File API seems to be well thought out in Zig. Here we translate the requested path into a local file - or else return an error.FileNotFound which we can easily translate into a 404 status higher up the call stack.

pub fn openLocalFile(path: []const u8) ![]u8 {
    const localPath = path[1..];
    const file = fs.cwd().openFile(localPath, .{}) catch |err| switch (err) {
        error.FileNotFound => {
            std.debug.print("File not found: {s}\n", .{localPath});
            return error.FileNotFound;
        else => return err,
    defer file.close();
    std.debug.print("file: {}\n", .{file});
    const memory = std.heap.page_allocator;
    const maxSize = std.math.maxInt(usize);
    return try file.readToEndAlloc(memory, maxSize);

Speaking of 404s, this is what that looks like:

pub fn http404() []const u8 {
    return "HTTP/1.1 404 NOT FOUND \r\n" ++
        "Connection: close\r\n" ++
        "Content-Type: text/html; charset=utf8\r\n" ++
        "Content-Length: 9\r\n" ++
        "\r\n" ++
        "NOT FOUND";

Detecting the mime-type.

Nothing too interesting here, but necessary:

pub fn mimeForPath(path: []const u8) []const u8 {
    const extension = std.fs.path.extension(path);
    inline for (mimeTypes) |kv| {
        if (mem.eql(u8, extension, kv[0])) {
            return kv[1];
    return "application/octet-stream";


Originally I was writing tests inline adjacent to the functions they were testing. I think I might like doing that, for smaller files with a focused purpose. But for this project I moved the files out to test_http.zig for clarity. They can be run with zig test test_http.zig.

Final Thoughts

This was an extremely fun exercise to lean more Zig. The cool thing about an HTTP server is that there is so much to implement but a lot of it isn’t very complex. However, I think I’m going to leave this one here.