Ja.

README.md

@@ -51,3 +51,15 @@ For a protocol-only test that does not create a window:
 
 Set `RKT_WEBVIEW_PRG` to test a backend executable outside the test program's
 directory.
+
+### Backend logging
+
+Diagnostics written through `utils.h` use one-line structured stderr records:
+
+```text
+json:{"topic":"webview-backend","level":"info","elapsed":0.125,"message":"..."}
+```
+
+Stdout remains reserved for protocol messages. Unstructured stderr output from
+Qt or third-party libraries may still occur and should be treated as diagnostic
+text.

STDIO-PROTOCOL.md

@@ -69,3 +69,18 @@ A normal shutdown sends `CMD_QUIT`, waits for its result, closes the child's
 stdin, and waits for the child process. If the Racket process terminates
 unexpectedly, the operating system closes the pipe; EOF on stdin makes the Qt
 backend close all windows and quit.
+
+## Structured stderr logging
+
+The backend writes diagnostic logging to stderr, never to stdout. Log entries
+created through `utils.h` are emitted as one line beginning with `json:` followed
+by a compact JSON object:
+
+```text
+json:{"topic":"webview-backend","level":"debug","elapsed":1.234,"message":"..."}
+```
+
+Newlines and other control characters in the message are JSON escaped, so one
+C++ log call always produces one physical stderr line. Consumers should also
+accept unstructured stderr lines because Qt, Qt WebEngine, or another library
+may write diagnostics directly.

utils.cpp

@@ -1,17 +1,36 @@
-
 #include "utils.h"
+
+#include <QJsonDocument>
+#include <QJsonObject>
+#include <QString>
+
 #include <chrono>
+#include <cstdarg>
+#include <cstdio>
+#include <vector>
 
 typedef struct {
     std::chrono::time_point<std::chrono::system_clock> start;
 } timer;
 
-
-
 static int _log_level = LOG_INFO;
 static timer *_timer = nullptr;
 
+static QString formattedMessage(const char *format, va_list args)
+{
+    va_list countArgs;
+    va_copy(countArgs, args);
+    const int length = std::vsnprintf(nullptr, 0, format, countArgs);
+    va_end(countArgs);
 
+    if (length < 0) {
+        return QString::fromUtf8(format);
+    }
+
+    std::vector<char> buffer(static_cast<size_t>(length) + 1);
+    std::vsnprintf(buffer.data(), buffer.size(), format, args);
+    return QString::fromUtf8(buffer.data(), length);
+}
 
 extern "C" {
 int logLevel()
@@ -27,27 +46,48 @@ void setLogLevel(int l)
 const char *logIndicator(int l)
 {
     switch(l) {
-    case LOG_ERROR:   return "ERROR  ";
-    case LOG_INFO:    return "INFO   ";
-    case LOG_DEBUG:   return "DEBUG  ";
-    case LOG_WARNING: return "WARNING";
+    case LOG_ERROR:   return "error";
+    case LOG_INFO:    return "info";
+    case LOG_DEBUG:   return "debug";
+    case LOG_WARNING: return "warning";
     }
-    return "UNKNOWN";
+    return "unknown";
 }
 
-void logElapsed()
+double logElapsedSeconds()
 {
     if (_timer == nullptr) {
         _timer = new timer;
         _timer->start = std::chrono::system_clock::now();
     }
 
-    auto c = std::chrono::system_clock::now();
-    auto duration = c - _timer->start;
-    auto milliseconds
+    const auto current = std::chrono::system_clock::now();
+    const auto milliseconds
         = std::chrono::duration_cast<std::chrono::milliseconds>(
-              duration)
+              current - _timer->start)
               .count();
-    fprintf(stderr, "%8.3lf: ", milliseconds/ 1000.0);
+    return milliseconds / 1000.0;
+}
+
+void logMessage(int level, const char *format, ...)
+{
+    if (logLevel() < level) {
+        return;
+    }
+
+    va_list args;
+    va_start(args, format);
+    const QString message = formattedMessage(format, args);
+    va_end(args);
+
+    QJsonObject object;
+    object.insert(QStringLiteral("topic"), QStringLiteral("webview-backend"));
+    object.insert(QStringLiteral("level"), QString::fromLatin1(logIndicator(level)));
+    object.insert(QStringLiteral("elapsed"), logElapsedSeconds());
+    object.insert(QStringLiteral("message"), message);
+
+    const QByteArray json = QJsonDocument(object).toJson(QJsonDocument::Compact);
+    std::fprintf(stderr, "json:%s\n", json.constData());
+    std::fflush(stderr);
 }
 }

utils.h

@@ -29,6 +29,7 @@ inline std::string basedir(const std::string &path)
     }
     return r;
 }
+
 extern "C" {
 #ifdef RKTWEBVIEW_PRG_EXE
 #undef RKTWEBVIEW_EXPORT
@@ -37,8 +38,9 @@ extern "C" {
 
 RKTWEBVIEW_EXPORT int logLevel();
 RKTWEBVIEW_EXPORT void setLogLevel(int l);
-RKTWEBVIEW_EXPORT void logElapsed();
+RKTWEBVIEW_EXPORT double logElapsedSeconds();
 RKTWEBVIEW_EXPORT const char *logIndicator(int l);
+RKTWEBVIEW_EXPORT void logMessage(int level, const char *format, ...);
 }
 
 #define LOG_ERROR 1
@@ -46,28 +48,22 @@ RKTWEBVIEW_EXPORT const char *logIndicator(int l);
 #define LOG_INFO 3
 #define LOG_DEBUG 4
 
-#define MKLOGSTMT(level, code) if (logLevel() >= level) { fprintf(stderr, "%s: ", logIndicator(level));logElapsed();code;fflush(stderr); }
-#define MKL0(level, msg) MKLOGSTMT(level, fprintf(stderr, msg))
-#define MKL1(level, msg, a) MKLOGSTMT(level, fprintf(stderr, msg, a))
-#define MKL2(level, msg, a, b) MKLOGSTMT(level, fprintf(stderr, msg, a, b))
-#define MKL3(level, msg, a, b, c) MKLOGSTMT(level, fprintf(stderr, msg, a, b, c))
+#define ERROR0(msg) logMessage(LOG_ERROR, msg)
+#define WARN0(msg) logMessage(LOG_WARNING, msg)
+#define INFO0(msg) logMessage(LOG_INFO, msg)
+#define DEBUG0(msg) logMessage(LOG_DEBUG, msg)
 
-#define ERROR0(msg) MKL0(LOG_ERROR, msg)
-#define WARN0(msg) MKL0(LOG_WARNING, msg)
-#define INFO0(msg)  MKL0(LOG_INFO, msg)
-#define DEBUG0(msg) MKL0(LOG_DEBUG, msg)
+#define ERROR1(msg, a) logMessage(LOG_ERROR, msg, a)
+#define WARN1(msg, a) logMessage(LOG_WARNING, msg, a)
+#define INFO1(msg, a) logMessage(LOG_INFO, msg, a)
+#define DEBUG1(msg, a) logMessage(LOG_DEBUG, msg, a)
 
-#define ERROR1(msg, a) MKL1(LOG_ERROR, msg, a)
-#define WARN1(msg, a)  MKL1(LOG_WARNING, msg, a)
-#define INFO1(msg, a)  MKL1(LOG_INFO, msg, a)
-#define DEBUG1(msg, a) MKL1(LOG_DEBUG, msg, a)
+#define ERROR2(msg, a, b) logMessage(LOG_ERROR, msg, a, b)
+#define WARN2(msg, a, b) logMessage(LOG_WARNING, msg, a, b)
+#define INFO2(msg, a, b) logMessage(LOG_INFO, msg, a, b)
+#define DEBUG2(msg, a, b) logMessage(LOG_DEBUG, msg, a, b)
 
-#define ERROR2(msg, a, b) MKL2(LOG_ERROR, msg, a, b)
-#define WARN2(msg, a, b)  MKL2(LOG_WARNING, msg, a, b)
-#define INFO2(msg, a, b)  MKL2(LOG_INFO, msg, a, b)
-#define DEBUG2(msg, a, b) MKL2(LOG_DEBUG, msg, a, b)
-
-#define INFO3(msg, a, b, c) MKL3(LOG_INFO, msg, a, b, c)
+#define INFO3(msg, a, b, c) logMessage(LOG_INFO, msg, a, b, c)
 
 inline void do_free_data(rkt_data_t *d)
 {