The cache option is -c not -b.

Reference: /n/sources/patch/applied/httpfile-manusage
Date: Tue Jul 28 21:19:44 CES 2009
Signed-off-by: djeannot24@gmail.com

--- /sys/man/4/httpfile	Tue Jul 28 21:18:52 2009
+++ /sys/man/4/httpfile	Tue Jul 28 21:18:49 2009
@@ -7,7 +7,7 @@
 .B -9d
 ]
 [
-.B -b
+.B -c
 .I count
 ]
 [
@@ -69,7 +69,7 @@
 .BR /net.alt ).
 .PP
 The
-.B -b
+.B -c
 option sets the number of file blocks kept cached in memory (default 32).
 .SH EXAMPLE
 Mount an ISO image on a web server:
--- /sys/src/cmd/ip/httpfile.c	Tue Jul 28 21:19:01 2009
+++ /sys/src/cmd/ip/httpfile.c	Tue Jul 28 21:18:56 2009
@@ -33,7 +33,7 @@
 void
 usage(void)
 {
-	fprint(2, "usage: httpfile [-Dd] [-b count] [-f file] [-m mtpt] [-s srvname] [-x net] url\n");
+	fprint(2, "usage: httpfile [-Dd] [-c count] [-f file] [-m mtpt] [-s srvname] [-x net] url\n");
 	exits("usage");
 }