Log window: stream the hook's logs over IPC into an in-app Log panel

Add a shared log ring (common/coop/log_ring.hpp): a lossy multi-producer /
single-consumer ring named coop_log_<pid>. The hook logs from several threads,
so producers claim a slot with fetch_add and publish each record with a
release store of its sequence; the consumer reads in order and tolerates
losing the oldest lines if it falls a whole ring behind.

The DLL's logf() now formats once and pushes every line to the ring (the file
trace stays as an opt-in mirror); the worker attaches the ring right after IPC
connect so bring-up is captured. The host (IpcServer) creates the ring at
injection time and exposes drain_logs(); a new LogPanel pulls new lines each
frame into a bounded rolling buffer and renders them with auto-scroll, a
filter, and clear. Added to the View menu (and UiState.show_log).

Verified against Phantom Brave via coop_audio_probe, which now also creates the
ring and drains it: the full hook bring-up trace streamed over IPC. All four
tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -125,6 +125,11 @@ Done:
   control is disabled when input forwarding is off, and the Audio panel notes when
   the render-hook is off (mirroring then uses loopback). Defaults to all-on so
   behavior is unchanged unless you toggle something.
+- **In-app Log window. ✅** The injected DLL streams its log lines to the host
+  over a shared log ring (`coop_log_<pid>`, a lossy multi-producer ring), and the
+  host shows them in a **Log** window with auto-scroll, a text filter, and clear.
+  Replaces tailing `%TEMP%\coop_hook.log` (which stays as an opt-in file mirror).
+  `coop_audio_probe` drains and prints the same stream headless.
 
 Future work, roughly in priority order:
 

common/include/coop/log_ring.hpp

@@ -0,0 +1,128 @@
+// Shared-memory log channel: the injected hook (coop_hook.dll) streams its log
+// lines to the host (coop_host.exe), which shows them in a Log window. Separate
+// mapping from the input/status SharedBlock, named coop_log_<pid>.
+//
+// Lossy multi-producer / single-consumer ring: the hook logs from several threads
+// (worker, audio render thread, window thread), so producers claim a slot with an
+// atomic fetch_add and publish each record with a release store of its sequence;
+// the host consumer reads in order and tolerates losing the oldest lines if it
+// ever falls a whole ring behind (fine for diagnostics). POD + version-locked.
+#pragma once
+
+#include <atomic>
+#include <cstdint>
+#include <cstring>
+#include <string>
+
+namespace coop
+{
+
+// 'CLOG' little-endian.
+inline constexpr std::uint32_t kLogRingMagic = 0x474F4C43u;
+inline constexpr std::uint32_t kLogRingVersion = 1;
+
+// Per-pid mapping name, mirroring the other channels: coop_log_<pid>.
+inline constexpr wchar_t kLogRingPrefix[] = L"Local\\coop_log_";
+
+inline constexpr std::uint32_t kLogMsgLen = 192;  // chars per line (incl. NUL)
+inline constexpr std::uint32_t kLogCapacity = 1024; // ring records
+
+struct LogRecord
+{
+	std::atomic<std::uint64_t> seq; // 0 = empty; else (global index + 1) once written
+	std::uint32_t pid;
+	std::uint32_t level; // reserved (0)
+	std::uint64_t millis; // producer timestamp (GetTickCount64)
+	char text[kLogMsgLen];
+};
+
+struct LogRing
+{
+	std::uint32_t magic;
+	std::uint32_t version;
+	std::uint32_t capacity; // number of records
+	std::uint32_t msg_len;  // kLogMsgLen (sanity)
+	std::atomic<std::uint64_t> write_index; // total records ever claimed (free-running)
+	std::uint8_t reserved[32];
+	// LogRecord records[capacity] follows immediately.
+};
+
+static_assert(std::atomic<std::uint64_t>::is_always_lock_free,
+              "log ring needs a lock-free 64-bit atomic for cross-process use");
+
+inline constexpr std::size_t log_ring_total_size(std::uint32_t capacity)
+{
+	return sizeof(LogRing) + static_cast<std::size_t>(capacity) * sizeof(LogRecord);
+}
+
+inline LogRecord* log_ring_records(LogRing* r)
+{
+	return reinterpret_cast<LogRecord*>(reinterpret_cast<std::uint8_t*>(r) + sizeof(LogRing));
+}
+
+// Host: stamp a freshly created (zero-filled) mapping. Records start empty (seq 0).
+inline void log_ring_init(LogRing& r, std::uint32_t capacity)
+{
+	r.capacity = capacity;
+	r.msg_len = kLogMsgLen;
+	r.write_index.store(0, std::memory_order_relaxed);
+	std::memset(r.reserved, 0, sizeof(r.reserved));
+	r.version = kLogRingVersion;
+	r.magic = kLogRingMagic; // last
+}
+
+inline bool log_ring_valid(const LogRing& r)
+{
+	return r.magic == kLogRingMagic && r.version == kLogRingVersion && r.capacity != 0 &&
+	       r.msg_len == kLogMsgLen;
+}
+
+// Producer (hook): append a line. Multi-producer safe.
+inline void log_ring_push(LogRing& r, std::uint32_t pid, std::uint64_t millis, const char* text)
+{
+	const std::uint64_t idx = r.write_index.fetch_add(1, std::memory_order_acq_rel);
+	LogRecord& rec = log_ring_records(&r)[idx % r.capacity];
+	rec.pid = pid;
+	rec.level = 0;
+	rec.millis = millis;
+	std::strncpy(rec.text, text, kLogMsgLen - 1);
+	rec.text[kLogMsgLen - 1] = '\0';
+	rec.seq.store(idx + 1, std::memory_order_release); // publish: record is ready
+}
+
+// Consumer (host): emit each new record since `cursor` (advanced in place). Skips
+// records lost to ring wrap; stops at an in-flight record and retries next call.
+template <typename F>
+inline void log_ring_drain(LogRing& r, std::uint64_t& cursor, F&& emit)
+{
+	const std::uint64_t w = r.write_index.load(std::memory_order_acquire);
+	if (w <= cursor)
+	{
+		return;
+	}
+	const std::uint64_t lo = (w > r.capacity) ? (w - r.capacity) : 0;
+	std::uint64_t i = cursor < lo ? lo : cursor; // skip records already overwritten
+	LogRecord* recs = log_ring_records(&r);
+	for (; i < w; ++i)
+	{
+		LogRecord& rec = recs[i % r.capacity];
+		const std::uint64_t s = rec.seq.load(std::memory_order_acquire);
+		if (s == i + 1)
+		{
+			emit(rec); // ready
+		}
+		else if (s <= i)
+		{
+			break; // slot not written for this generation yet (in-flight); retry later
+		}
+		// s > i + 1: overwritten before we read it; skip (lost)
+	}
+	cursor = i;
+}
+
+inline std::wstring log_ring_name(unsigned long target_pid)
+{
+	return std::wstring(kLogRingPrefix) + std::to_wstring(target_pid);
+}
+
+} // namespace coop

hook/src/debug_log.cpp

@@ -1,12 +1,15 @@
 #define _CRT_SECURE_NO_WARNINGS
 #include "debug_log.hpp"
 
+#include <atomic>
 #include <cstdarg>
 #include <cstdio>
 #include <mutex>
 
 #include <windows.h>
 
+#include "coop/log_ring.hpp"
+
 namespace coop::hook
 {
 
@@ -16,6 +19,7 @@ namespace
 std::mutex g_log_mutex;
 FILE* g_log_file = nullptr;
 bool g_log_tried = false;
+std::atomic<coop::LogRing*> g_log_ring{nullptr};
 
 // Logging is opt-in so an injected DLL doesn't write to disk in normal use.
 // Enable it by setting the COOP_HOOK_LOG environment variable for the target, or
@@ -60,27 +64,37 @@ FILE* log_file_locked()
 
 } // namespace
 
+void set_log_ring(coop::LogRing* ring)
+{
+	g_log_ring.store(ring, std::memory_order_release);
+}
+
 void logf(const char* fmt, ...)
 {
-	std::scoped_lock lock(g_log_mutex);
+	// Format the line once.
-	FILE* f = log_file_locked();
+	char line[coop::kLogMsgLen];
-	if (f == nullptr)
-	{
-		return;
-	}
-
-	SYSTEMTIME st;
-	GetLocalTime(&st);
-	std::fprintf(f, "[%02u:%02u:%02u.%03u pid=%lu] ", st.wHour, st.wMinute, st.wSecond, st.wMilliseconds,
-	             GetCurrentProcessId());
-
 	va_list args;
 	va_start(args, fmt);
-	std::vfprintf(f, fmt, args);
+	std::vsnprintf(line, sizeof(line), fmt, args);
 	va_end(args);
 
-	std::fputc('\n', f);
+	// Stream to the host's Log window over the shared ring (the primary sink).
+	if (coop::LogRing* ring = g_log_ring.load(std::memory_order_acquire))
+	{
+		coop::log_ring_push(*ring, GetCurrentProcessId(), GetTickCount64(), line);
+	}
+
+	// Also mirror to the file when the opt-in trace is enabled.
+	std::scoped_lock lock(g_log_mutex);
+	FILE* f = log_file_locked();
+	if (f != nullptr)
+	{
+		SYSTEMTIME st;
+		GetLocalTime(&st);
+		std::fprintf(f, "[%02u:%02u:%02u.%03u pid=%lu] %s\n", st.wHour, st.wMinute, st.wSecond,
+		             st.wMilliseconds, GetCurrentProcessId(), line);
 		std::fflush(f);
+	}
 }
 
 } // namespace coop::hook

hook/src/debug_log.hpp

@@ -1,14 +1,21 @@
-// Lightweight file logger for diagnosing the injected hook from inside a game.
+// Lightweight logger for the injected hook. We can't see stdout from an injected
-//
+// DLL, so each line is (a) streamed to the host over the shared log ring for the
-// We can't see stdout from an injected DLL, so route diagnostics to a file in
+// in-app Log window, and (b) optionally written to %TEMP%\coop_hook.log (opt-in;
-// %TEMP%\coop_hook.log. Thread-safe, opened lazily, append-only. Intended for
+// see debug_log.cpp). Thread-safe; cheap enough to leave compiled in.
-// development / bring-up; cheap enough to leave compiled in.
 #pragma once
 
+namespace coop
+{
+struct LogRing;
+}
+
 namespace coop::hook
 {
 
-// Append a printf-style line to %TEMP%\coop_hook.log (prefixed with pid + time).
+// Append a printf-style line to the log ring (if attached) and the file (if on).
 void logf(const char* fmt, ...);
 
+// Attach/detach the host's shared log ring so lines stream to the Log window.
+void set_log_ring(coop::LogRing* ring);
+
 } // namespace coop::hook

hook/src/dllmain.cpp

@@ -14,6 +14,7 @@
 
 #include "audio_hook.hpp"
 #include "coop/audio_ring.hpp"
+#include "coop/log_ring.hpp"
 #include "coop/shared_memory.hpp"
 #include "debug_log.hpp"
 #include "focus_spoof.hpp"
@@ -27,6 +28,7 @@ namespace
 coop::hook::IpcClient g_ipc;
 std::atomic<bool> g_running{true};
 coop::SharedMemory g_audio_shm; // the host's audio ring, opened when present
+coop::SharedMemory g_log_shm;   // the host's log ring, opened when present
 
 DWORD WINAPI worker_thread(LPVOID)
 {
@@ -38,6 +40,24 @@ DWORD WINAPI worker_thread(LPVOID)
 		coop::hook::logf("worker_thread: IPC connect FAILED (no host mapping); exiting");
 		return 0;
 	}
+	// Attach the host's log ring first so the rest of bring-up streams to the Log
+	// window. The host creates it at injection time; it's normally already there.
+	{
+		const std::wstring log_name = coop::log_ring_name(GetCurrentProcessId());
+		if (g_log_shm.open(log_name, coop::log_ring_total_size(coop::kLogCapacity)))
+		{
+			auto* lr = g_log_shm.as<coop::LogRing>();
+			if (coop::log_ring_valid(*lr))
+			{
+				coop::hook::set_log_ring(lr);
+			}
+			else
+			{
+				g_log_shm.reset();
+			}
+		}
+	}
+
 	coop::hook::logf("worker_thread: IPC connected");
 
 	// The audio render-hook instantiates a COM enumerator on this thread.
@@ -161,6 +181,7 @@ BOOL APIENTRY DllMain(HMODULE module, DWORD reason, LPVOID reserved)
 		if (reserved == nullptr)
 		{
 			g_running.store(false, std::memory_order_relaxed);
+			coop::hook::set_log_ring(nullptr);
 			coop::hook::remove_focus_spoof();
 			coop::hook::remove_xinput_hooks();
 			coop::hook::remove_audio_hooks();

host/CMakeLists.txt

@@ -6,6 +6,7 @@ add_executable(coop_host WIN32
 	src/injection_panel.cpp
 	src/capture_panel.cpp
 	src/audio_panel.cpp
+	src/log_panel.cpp
 	src/ui/app_chrome.cpp
 	src/input/xinput_source.cpp
 	src/inject/process_list.cpp

host/src/injection_panel.hpp

@@ -42,6 +42,13 @@ public:
 		return server_.hook_status();
 	}
 
+	// Drain log lines the hook streamed (for the Log window). No-op if not active.
+	template <typename F>
+	void drain_logs(F&& emit)
+	{
+		server_.drain_logs(std::forward<F>(emit));
+	}
+
 private:
 	void refresh_processes();
 	void inject_selected();

host/src/ipc/ipc_server.cpp

@@ -22,6 +22,15 @@ bool IpcServer::start(unsigned long target_pid)
 
 	block_ = block;
 	target_pid_ = target_pid;
+
+	// Log ring: the injected hook opens this and streams its log lines back for the
+	// Log window. Best-effort -- the rest of the tool works without it.
+	if (log_shm_.create(log_ring_name(target_pid), log_ring_total_size(kLogCapacity)))
+	{
+		log_ring_ = log_shm_.as<LogRing>();
+		log_ring_init(*log_ring_, kLogCapacity);
+		log_cursor_ = 0;
+	}
 	return true;
 }
 
@@ -96,6 +105,9 @@ void IpcServer::stop()
 		block_ = nullptr;
 	}
 	shm_.reset();
+	log_ring_ = nullptr;
+	log_shm_.reset();
+	log_cursor_ = 0;
 	target_pid_ = 0;
 }
 

host/src/ipc/ipc_server.hpp

@@ -5,6 +5,7 @@
 #include <array>
 #include <cstdint>
 
+#include "coop/log_ring.hpp"
 #include "coop/protocol.hpp"
 #include "coop/shared_memory.hpp"
 #include "input/input_source.hpp"
@@ -56,6 +57,17 @@ public:
 	// reconciles on its next tick. No-op if not started.
 	void set_subsystem_enabled(std::uint32_t subsystem, bool enabled);
 
+	// Drain new log lines streamed by the hook, calling `emit(const LogRecord&)`
+	// for each. No-op if not started. Header-only so the callback can stay generic.
+	template <typename F>
+	void drain_logs(F&& emit)
+	{
+		if (log_ring_ != nullptr)
+		{
+			log_ring_drain(*log_ring_, log_cursor_, emit);
+		}
+	}
+
 	[[nodiscard]] bool running() const
 	{
 		return block_ != nullptr;
@@ -69,6 +81,10 @@ private:
 	SharedMemory shm_;
 	SharedBlock* block_ = nullptr;
 	unsigned long target_pid_ = 0;
+
+	SharedMemory log_shm_;          // shared log ring (named coop_log_<pid>)
+	LogRing* log_ring_ = nullptr;
+	std::uint64_t log_cursor_ = 0;  // consumer position into the log ring
 };
 
 } // namespace coop

host/src/log_panel.cpp

@@ -0,0 +1,75 @@
+#include "log_panel.hpp"
+
+#include <cstdio>
+#include <cstring>
+
+#include "imgui.h"
+
+#include "injection_panel.hpp"
+
+namespace coop
+{
+
+void LogPanel::add_line(const LogRecord& rec)
+{
+	if (first_millis_ == 0)
+	{
+		first_millis_ = rec.millis;
+	}
+	const double secs = static_cast<double>(rec.millis - first_millis_) / 1000.0;
+
+	char buf[256];
+	std::snprintf(buf, sizeof(buf), "[%8.3f] %s", secs, rec.text);
+	lines_.emplace_back(buf);
+	while (lines_.size() > kMaxLines)
+	{
+		lines_.pop_front();
+	}
+}
+
+void LogPanel::pull(InjectionPanel& injection)
+{
+	injection.drain_logs([this](const LogRecord& rec) { add_line(rec); });
+}
+
+void LogPanel::draw()
+{
+	ImGui::SetNextWindowPos(ImVec2(24, 760), ImGuiCond_FirstUseEver);
+	ImGui::SetNextWindowSize(ImVec2(720, 240), ImGuiCond_FirstUseEver);
+	ImGui::Begin("Log");
+
+	if (ImGui::Button("Clear"))
+	{
+		lines_.clear();
+		first_millis_ = 0;
+	}
+	ImGui::SameLine();
+	ImGui::Checkbox("Auto-scroll", &autoscroll_);
+	ImGui::SameLine();
+	ImGui::SetNextItemWidth(-1.0f);
+	ImGui::InputTextWithHint("##logfilter", "filter...", filter_, sizeof(filter_));
+
+	ImGui::Separator();
+	if (ImGui::BeginChild("loglines", ImVec2(0, 0), ImGuiChildFlags_None, ImGuiWindowFlags_HorizontalScrollbar))
+	{
+		const bool has_filter = filter_[0] != '\0';
+		for (const std::string& line : lines_)
+		{
+			if (has_filter && line.find(filter_) == std::string::npos)
+			{
+				continue;
+			}
+			ImGui::TextUnformatted(line.c_str());
+		}
+		// Stick to the bottom while new lines arrive (unless the user scrolled up).
+		if (autoscroll_ && ImGui::GetScrollY() >= ImGui::GetScrollMaxY() - 1.0f)
+		{
+			ImGui::SetScrollHereY(1.0f);
+		}
+	}
+	ImGui::EndChild();
+
+	ImGui::End();
+}
+
+} // namespace coop

host/src/log_panel.hpp

@@ -0,0 +1,36 @@
+// Log window: shows the log lines the injected hook streams over the shared log
+// ring (see coop/log_ring.hpp). Pull new lines each frame from the IPC server,
+// keep a bounded rolling history, and render them with autoscroll + a filter.
+#pragma once
+
+#include <cstdint>
+#include <deque>
+#include <string>
+
+#include "coop/log_ring.hpp"
+
+namespace coop
+{
+
+class InjectionPanel;
+
+class LogPanel
+{
+public:
+	// Pull any new lines the hook emitted (call once per frame before draw()).
+	void pull(InjectionPanel& injection);
+
+	void draw();
+
+private:
+	void add_line(const LogRecord& rec);
+
+	std::deque<std::string> lines_;
+	char filter_[96] = {};
+	bool autoscroll_ = true;
+	std::uint64_t first_millis_ = 0; // hook clock at the first line, for relative timestamps
+
+	static constexpr std::size_t kMaxLines = 2000;
+};
+
+} // namespace coop

host/src/main.cpp

@@ -20,6 +20,7 @@
 #include "imgui_layer.hpp"
 #include "injection_panel.hpp"
 #include "input/xinput_source.hpp"
+#include "log_panel.hpp"
 #include "ui/app_chrome.hpp"
 
 namespace
@@ -68,6 +69,7 @@ int run()
 	coop::InjectionPanel injection;
 	coop::AudioPanel audio;
 	coop::CapturePanel capture;
+	coop::LogPanel log;
 	if (!capture.init(window.device()))
 	{
 		MessageBoxW(nullptr, L"Failed to initialize the video mirror.", L"CoopAllTheThings", MB_ICONERROR);
@@ -91,6 +93,7 @@ int run()
 
 		imgui.begin_frame();
 		stats.tick(ImGui::GetIO().DeltaTime * 1000.0f);
+		log.pull(injection); // drain hook log lines even while the Log window is hidden
 
 		if (ImGui::IsKeyPressed(ImGuiKey_F1, false))
 		{
@@ -120,6 +123,10 @@ int run()
 			{
 				capture.draw_ui(stats);
 			}
+			if (ui.show_log)
+			{
+				log.draw();
+			}
 		}
 		else
 		{

host/src/ui/app_chrome.cpp

@@ -24,6 +24,7 @@ float draw_main_menu_bar(UiState& ui, const FrameStats& stats)
 		ImGui::MenuItem("Injection", nullptr, &ui.show_injection);
 		ImGui::MenuItem("Video mirror", nullptr, &ui.show_video);
 		ImGui::MenuItem("Audio mirror", nullptr, &ui.show_audio);
+		ImGui::MenuItem("Log", nullptr, &ui.show_log);
 		ImGui::Separator();
 		ImGui::MenuItem("Debug details", nullptr, &ui.debug_details);
 		ImGui::EndMenu();

host/src/ui/app_chrome.hpp

@@ -18,6 +18,7 @@ struct UiState
 	bool show_injection = true;
 	bool show_video = true;
 	bool show_audio = true;
+	bool show_log = true;
 	bool debug_details = false; // off = general status; on = full diagnostics
 };
 

tools/audio_probe/main.cpp

@@ -20,6 +20,7 @@
 #include <windows.h>
 
 #include "coop/audio_ring.hpp"
+#include "coop/log_ring.hpp"
 #include "coop/protocol.hpp"
 #include "coop/shared_memory.hpp"
 
@@ -124,6 +125,16 @@ int wmain(int argc, wchar_t** argv)
 	}
 	block->magic = coop::kProtocolMagic;
 
+	// Log ring (host's role): the hook opens this and streams its log lines back.
+	coop::SharedMemory log_shm;
+	coop::LogRing* log_ring = nullptr;
+	std::uint64_t log_cursor = 0;
+	if (log_shm.create(coop::log_ring_name(pid), coop::log_ring_total_size(coop::kLogCapacity)))
+	{
+		log_ring = log_shm.as<coop::LogRing>();
+		coop::log_ring_init(*log_ring, coop::kLogCapacity);
+	}
+
 	// Enable the hook's file trace (%TEMP%\coop_hook.log) for this debug session.
 	{
 		wchar_t dir[MAX_PATH] = {};
@@ -259,6 +270,14 @@ int wmain(int argc, wchar_t** argv)
 		            e.installed ? "ON " : "off", static_cast<unsigned long long>(e.calls));
 	}
 
+	// Drain the IPC log ring to verify the hook streams its logs to the host.
+	if (log_ring != nullptr)
+	{
+		std::printf("\nHook log (streamed over IPC):\n");
+		coop::log_ring_drain(*log_ring, log_cursor,
+		                     [](const coop::LogRecord& rec) { std::printf("  %s\n", rec.text); });
+	}
+
 	std::printf("\nDone. Leaving the hook loaded in the game.\n");
 	block->magic = 0; // invalidate so a late hook read won't trust stale data
 	return 0;