spawn

    fn spawn(config: std.Thread.SpawnConfig, comptime f: anytype, args: anytype) SpawnError!WasiThreadImpl {
        if (config.allocator == null) {
            @panic("an allocator is required to spawn a WASI thread");
        }

        // Wrapping struct required to hold the user-provided function arguments.
        const Wrapper = struct {
            args: @TypeOf(args),
            fn entry(ptr: usize) void {
                const w: *@This() = @ptrFromInt(ptr);
                const bad_fn_ret = "expected return type of startFn to be 'u8', 'noreturn', 'void', or '!void'";
                switch (@typeInfo(@typeInfo(@TypeOf(f)).@"fn".return_type.?)) {
                    .noreturn, .void => {
                        @call(.auto, f, w.args);
                    },
                    .int => |info| {
                        if (info.bits != 8) {
                            @compileError(bad_fn_ret);
                        }
                        _ = @call(.auto, f, w.args); // WASI threads don't support exit status, ignore value
                    },
                    .error_union => |info| {
                        if (info.payload != void) {
                            @compileError(bad_fn_ret);
                        }
                        @call(.auto, f, w.args) catch |err| {
                            std.debug.print("error: {s}\n", .{@errorName(err)});
                            if (@errorReturnTrace()) |trace| {
                                std.debug.dumpErrorReturnTrace(trace);
                            }
                        };
                    },
                    else => {
                        @compileError(bad_fn_ret);
                    },
                }
            }
        };

        var stack_offset: usize = undefined;
        var tls_offset: usize = undefined;
        var wrapper_offset: usize = undefined;
        var instance_offset: usize = undefined;

        // Calculate the bytes we have to allocate to store all thread information, including:
        // - The actual stack for the thread
        // - The TLS segment
        // - `Instance` - containing information about how to call the user's function.
        const map_bytes = blk: {
            // start with atleast a single page, which is used as a guard to prevent
            // other threads clobbering our new thread.
            // Unfortunately, WebAssembly has no notion of read-only segments, so this
            // is only a best effort.
            var bytes: usize = std.wasm.page_size;

            bytes = std.mem.alignForward(usize, bytes, 16); // align stack to 16 bytes
            stack_offset = bytes;
            bytes += @max(std.wasm.page_size, config.stack_size);

            bytes = std.mem.alignForward(usize, bytes, __tls_align());
            tls_offset = bytes;
            bytes += __tls_size();

            bytes = std.mem.alignForward(usize, bytes, @alignOf(Wrapper));
            wrapper_offset = bytes;
            bytes += @sizeOf(Wrapper);

            bytes = std.mem.alignForward(usize, bytes, @alignOf(Instance));
            instance_offset = bytes;
            bytes += @sizeOf(Instance);

            bytes = std.mem.alignForward(usize, bytes, std.wasm.page_size);
            break :blk bytes;
        };

        // Allocate the amount of memory required for all meta data.
        const allocated_memory = try config.allocator.?.alloc(u8, map_bytes);

        const wrapper: *Wrapper = @ptrCast(@alignCast(&allocated_memory[wrapper_offset]));
        wrapper.* = .{ .args = args };

        const instance: *Instance = @ptrCast(@alignCast(&allocated_memory[instance_offset]));
        instance.* = .{
            .thread = .{ .memory = allocated_memory, .allocator = config.allocator.? },
            .tls_offset = tls_offset,
            .stack_offset = stack_offset,
            .raw_ptr = @intFromPtr(wrapper),
            .call_back = &Wrapper.entry,
            .original_stack_pointer = __get_stack_pointer(),
        };

        const tid = spawnWasiThread(instance);
        // The specification says any value lower than 0 indicates an error.
        // The values of such error are unspecified. WASI-Libc treats it as EAGAIN.
        if (tid < 0) {
            return error.SystemResources;
        }
        instance.thread.tid.store(tid, .seq_cst);

        return .{ .thread = &instance.thread };
    }