diff -r d45cfe9072cd sys/src/libc/fmt/vsnprint.c
--- a/sys/src/libc/fmt/vsnprint.c	Sat May 30 13:03:05 2020 +0200
+++ b/sys/src/libc/fmt/vsnprint.c	Sat May 30 14:31:13 2020 -0700
@@ -2,19 +2,27 @@
 #include <libc.h>
 
 int
+fmtbufinit(Fmt *f, char *buf, int len)
+{
+	if(len <= 0)
+		return -1;
+	f->runes = 0;
+	f->start = buf;
+	f->to = buf;
+	f->stop = buf + len - 1;
+	f->flush = nil;
+	f->farg = nil;
+	f->nfmt = 0;
+	return 0;
+}
+
+int
 vsnprint(char *buf, int len, char *fmt, va_list args)
 {
 	Fmt f;
 
-	if(len <= 0)
+	if(fmtbufinit(&f, buf, len) == -1)
 		return -1;
-	f.runes = 0;
-	f.start = buf;
-	f.to = buf;
-	f.stop = buf + len - 1;
-	f.flush = nil;
-	f.farg = nil;
-	f.nfmt = 0;
 	f.args = args;
 	dofmt(&f, fmt);
 	*(char*)f.to = '\0';