Add default stderr logger for logging to nullptr.

This is useful for debugging a function that doesn't have a logger
available. It should not be used in production code, since it outputs to
stderr.

toxcore/ccompat.h

@@ -43,9 +43,9 @@
 #endif
 
 #ifdef __GNUC__
-#define GNU_PRINTF __attribute__((__format__(__printf__, 6, 7)))
+#define GNU_PRINTF(f, a) __attribute__((__format__(__printf__, f, a)))
 #else
-#define GNU_PRINTF
+#define GNU_PRINTF(f, a)
 #endif
 
 #endif /* CCOMPAT_H */

toxcore/logger.c

@@ -39,6 +39,42 @@ struct Logger {
 };
 
 
+static const char *logger_level_name(LOGGER_LEVEL level)
+{
+    switch (level) {
+        case LOG_TRACE:
+            return "TRACE";
+
+        case LOG_DEBUG:
+            return "DEBUG";
+
+        case LOG_INFO:
+            return "INFO";
+
+        case LOG_WARNING:
+            return "WARNING";
+
+        case LOG_ERROR:
+            return "ERROR";
+    }
+
+    return "<unknown>";
+}
+
+static void logger_stderr_handler(void *context, LOGGER_LEVEL level, const char *file, int line, const char *func,
+                                  const char *message, void *userdata)
+{
+    // GL stands for "global logger".
+    fprintf(stderr, "[GL] %s %s:%d(%s): %s\n", logger_level_name(level), file, line, func, message);
+}
+
+static const Logger logger_stderr = {
+    logger_stderr_handler,
+    nullptr,
+    nullptr,
+};
+
+
 /**
  * Public Functions
  */
@@ -59,10 +95,14 @@ void logger_callback_log(Logger *log, logger_cb *function, void *context, void *
     log->userdata = userdata;
 }
 
-void logger_write(Logger *log, LOGGER_LEVEL level, const char *file, int line, const char *func, const char *format,
-                  ...)
+void logger_write(const Logger *log, LOGGER_LEVEL level, const char *file, int line, const char *func,
+                  const char *format, ...)
 {
-    if (!log || !log->callback) {
+    if (!log) {
+        log = &logger_stderr;
+    }
+
+    if (!log->callback) {
         return;
     }
 

toxcore/logger.h

@@ -50,6 +50,9 @@ typedef void logger_cb(void *context, LOGGER_LEVEL level, const char *file, int
  */
 Logger *logger_new(void);
 
+/**
+ * Frees all resources associated with the logger.
+ */
 void logger_kill(Logger *log);
 
 /**
@@ -59,10 +62,16 @@ void logger_kill(Logger *log);
 void logger_callback_log(Logger *log, logger_cb *function, void *context, void *userdata);
 
 /**
- * Main write function. If logging disabled does nothing.
+ * Main write function. If logging is disabled, this does nothing.
+ *
+ * If the logger is NULL, this writes to stderr. This behaviour should not be
+ * used in production code, but can be useful for temporarily debugging a
+ * function that does not have a logger available. It's essentially
+ * fprintf(stderr, ...), but with timestamps and source location.
  */
 void logger_write(
-    Logger *log, LOGGER_LEVEL level, const char *file, int line, const char *func, const char *format, ...) GNU_PRINTF;
+    const Logger *log, LOGGER_LEVEL level, const char *file, int line, const char *func,
+    const char *format, ...) GNU_PRINTF(6, 7);
 
 
 #define LOGGER_WRITE(log, level, ...) \