Introduce libseat_set_log_handler

This allows libseat users to register a custom logging function.

include/libseat.h

@@ -135,4 +135,29 @@ int libseat_get_fd(struct libseat *seat);
  */
 int libseat_dispatch(struct libseat *seat, int timeout);
 
+/*
+ * A log level.
+ */
+enum libseat_log_level {
+	LIBSEAT_LOG_LEVEL_SILENT = 0,
+	LIBSEAT_LOG_LEVEL_ERROR = 1,
+	LIBSEAT_LOG_LEVEL_INFO = 2,
+	LIBSEAT_LOG_LEVEL_DEBUG = 3,
+	LIBSEAT_LOG_LEVEL_LAST,
+};
+
+/*
+ * A function that handles log messages.
+ */
+typedef void (*libseat_log_func)(enum libseat_log_level level, const char *format, va_list args);
+
+/*
+ * Sets the handler for log messages.
+ *
+ * The handler will be called for each message whose level is lower or equal
+ * to the current log level. If the handler is NULL, the handler is reset to
+ * the default.
+ */
+void libseat_set_log_handler(libseat_log_func handler);
+
 #endif

include/log.h

@@ -3,6 +3,8 @@
 
 #include <stdarg.h>
 
+#include "libseat.h"
+
 #ifdef __GNUC__
 #define ATTRIB_PRINTF(start, end) __attribute__((format(printf, start, end)))
 #else
@@ -41,14 +43,6 @@
 #define log_debug(str)
 #endif
 
-enum libseat_log_level {
-	LIBSEAT_LOG_LEVEL_SILENT = 0,
-	LIBSEAT_LOG_LEVEL_ERROR = 1,
-	LIBSEAT_LOG_LEVEL_INFO = 2,
-	LIBSEAT_LOG_LEVEL_DEBUG = 3,
-	LIBSEAT_LOG_LEVEL_LAST,
-};
-
 void log_init(enum libseat_log_level level);
 void _logf(enum libseat_log_level level, const char *fmt, ...) ATTRIB_PRINTF(2, 3);