logging: Add handler pointer for logging

Default to old stderr logging method.
Add function pointer and userdata pointer as static globals.

lib/libunshield.h

@@ -32,7 +32,9 @@ typedef struct _Unshield Unshield;
 
 #define UNSHIELD_LOG_LEVEL_HIGHEST   4
 
+typedef void (UnshieldLogCallback)(void* userdata, int level, const char* file, int line, const char* format, ...);
 void unshield_set_log_level(int level);
+void unshield_set_log_handler(UnshieldLogCallback* handler, void* userdata);
 
 /*
    Open/close functions

lib/log.c

@@ -4,14 +4,24 @@
 
 /* evil static data */
 static int current_log_level = UNSHIELD_LOG_LEVEL_HIGHEST;
+static UnshieldLogCallback* current_log_handler = NULL;
+static void* current_log_handler_userdata = NULL;
 
 void unshield_set_log_level(int level)
 {
 	current_log_level = level;
 }
 
-void _unshield_log(int level, const char* file, int line, const char* format, ...)
+void unshield_set_log_handler(UnshieldLogCallback* handler, void* userdata)
+{
+	current_log_handler = handler;
+	current_log_handler_userdata = userdata;
+}
+
+void unshield_default_log_handler(void* userdata, int level, const char* file, int line, const char* format, ...)
 {
+	(void)userdata;
+
 	va_list ap;
 
 	if (level > current_log_level)
@@ -26,3 +36,14 @@ void _unshield_log(int level, const char* file, int line, const char* format, ..
 	fprintf(stderr, "\n");
 }
 
+void _unshield_log(int level, const char* file, int line, const char* format, ...)
+{
+	va_list ap;
+
+	if (current_log_handler == NULL)
+		unshield_set_log_handler(unshield_default_log_handler, NULL);
+
+	va_start(ap, format);
+	current_log_handler(current_log_handler_userdata, level, file, line, format, ap);
+	va_end(ap);
+}