add logging

src/stk_log.c

@@ -0,0 +1,14 @@
+#include "stk_log.h"
+#include <stdarg.h>
+#include <stdio.h>
+
+void stk_log_file(FILE *fp, const char *fmt, ...)
+{
+	va_list args;
+	va_start(args, fmt);
+
+	vfprintf(fp, fmt, args);
+	fputc('\n', fp);
+
+	va_end(args);
+}