diff --git a/include/plan9/libc.h b/include/plan9/libc.h
index 9c86df0c77..798e47095c 100644
--- a/include/plan9/libc.h
+++ b/include/plan9/libc.h
@@ -22,3 +22,7 @@ void	flagfn0(char*, char*, void(*fn)(void));
 void	flagfn1(char*, char*, void(*fn)(char*));
 void	flagfn2(char*, char*, void(*fn)(char*, char*));
 void	flagprint(int);
+
+// The libraries use size_t to avoid -Wconversion warnings from GCC
+// when calling standard library functions like memcpy.
+typedef unsigned long size_t;