diff --git a/src/lib9/encodefmt.c b/src/lib9/encodefmt.c
new file mode 100644
index 0000000..9a5cbfc
--- /dev/null
+++ b/src/lib9/encodefmt.c
@@ -0,0 +1,69 @@
+#include <lib9.h>
+
+int
+encodefmt(Fmt *f)
+{
+	char *out;
+	char *buf;
+	int len;
+	int ilen;
+	int rv;
+	uchar *b;
+	char obuf[64];	// rsc optimization
+
+	if(!(f->flags&FmtPrec) || f->prec < 1)
+		goto error;
+
+	b = va_arg(f->args, uchar*);
+
+	ilen = f->prec;
+	f->prec = 0;
+	f->flags &= ~FmtPrec;
+	switch(f->r){
+	case '<':
+		len = (8*ilen+4)/5 + 3;
+		break;
+	case '[':
+		len = (8*ilen+5)/6 + 4;
+		break;
+	case 'H':
+		len = 2*ilen + 1;
+		break;
+	default:
+		goto error;
+	}
+
+	if(len > sizeof(obuf)){
+		buf = malloc(len);
+		if(buf == nil)
+			goto error;
+	} else
+		buf = obuf;
+
+	// convert
+	out = buf;
+	switch(f->r){
+	case '<':
+		rv = enc32(out, len, b, ilen);
+		break;
+	case '[':
+		rv = enc64(out, len, b, ilen);
+		break;
+	case 'H':
+		rv = enc16(out, len, b, ilen);
+		break;
+	default:
+		rv = -1;
+		break;
+	}
+	if(rv < 0)
+		goto error;
+
+	fmtstrcpy(f, buf);
+	if(buf != obuf)
+		free(buf);
+	return 0;
+
+error:
+	return fmtstrcpy(f, "<encodefmt>");
+}