How to Get Clang LSP Working With Zig

14 Mar 2025

The script below allows you to build C/C++ with a build.zig file and integrate it with a Clang LSP.

It took me a little while to set this up so I figured I’d post it online to save whoever reads this a bit of effort (and possibly to remind myself in the future).

Essentially it collects CDB (compilation database) fragments into a compile_commands.json file, which is what the Clang LSP looks for. Any C/C++ files compiled with -gen-cdb-fragments-path=cdb-frags will be included.

The following example adds zig build cdb which just creates compile_commands.json. It also adds the cdb build step as a dependency to the install step, so it will also create compile_commands.json whenever you execute zig build or zig build run.

const std = @include("std");

pub fn build(b: *std.Build) void {
    // Build step
    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});
    const exe = b.addExecutable(.{
        .name = "example_proj",
        .target = target,
        .optimize = optimize,
    });
    exe.linkLibCpp();
    exe.addCSourceFiles(.{
        .files = &.{
            "src/main.cpp",
        },
        .flags = &.{
            "-gen-cdb-fragment-path", "cdb-frags",
        },
    });
	b.installArtifact(exe);

    // Step to create compile_commands.json for the debugger
    const cdb_step = b.step("cdb", "Compile CDB fragments into compile_commands.json");
    cdb_step.makeFn = collect_cdb_fragments;
    cdb_step.dependOn(&exe.step);

    b.getInstallStep().dependOn(cdb_step);
}

fn collect_cdb_fragments(_: *std.Build.Step, _: std.Progress.Node) !void {
    const outf = try std.fs.cwd().createFile("compile_commands.json", .{});
    defer outf.close();
    try outf.writeAll("[");

    var dir = std.fs.cwd().openDir("cdb-frags", .{ .iterate = true }) catch {
        std.debug.print("Failed to open ./cdb-frags/", .{});
        return;
    };
    defer dir.close();

    var iter = dir.iterate();
    while (try iter.next()) |entry| {
        const fpath = try std.fmt.allocPrint(std.heap.page_allocator, "cdb-frags/{s}", .{entry.name});
        const f = try std.fs.cwd().openFile(fpath, .{});
        const contents = try f.reader().readAllAlloc(std.heap.page_allocator, 1024 * 1024);
        try outf.seekFromEnd(0);
        try outf.writeAll(contents);
    }

    try outf.writeAll("]");
}