space is a flag.  e.g.

#include <u.h>
#include <libc.h>

void
main(void)
{
	print("% d\n", 40);
	print("% d\n", -40);
	exits("");
}

Reference: /n/sources/patch/applied/spaceflag
Date: Tue May  5 22:18:54 CES 2009
Signed-off-by: quanstro@quanstro.net

--- /sys/include/libc.h	Tue May  5 22:18:08 2009
+++ /sys/include/libc.h	Tue May  5 22:18:05 2009
@@ -207,6 +207,7 @@
 #pragma	varargck	type	"n"	int*
 #pragma	varargck	type	"p"	uintptr
 #pragma	varargck	type	"p"	void*
+#pragma	varargck	flag	' '
 #pragma	varargck	flag	','
 #pragma	varargck	flag	'h'
 #pragma varargck	type	"<"	void*