diff --git a/HACKING b/HACKING
new file mode 100644
index 0000000000000000000000000000000000000000..4211d15a067ac4af657278d414d397fa26168c7a
--- /dev/null
+++ b/HACKING
@@ -0,0 +1,6 @@
+1. Preprocessor
+
+For variadic macros, stick with this C99-like syntax:
+
+#define DPRINTF(fmt, ...)                                       \
+    do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)