When the -S option is given to man(1) and the requested manual page

name is not found and the requested architecture is unknown, complain
about the architecture rather than about the manual page name:

$ man -S vax cpu
man: Unknown architecture "vax".
$ man -S sparc64 foobar
man: No entry for foobar in the manual.

Friendlier error message suggested by jmc@, who also OK'ed the patch.

1 files changed, 54 insertions, 0 deletions

diff --git a/arch.c b/arch.c
new file mode 100644
index 00000000..56b937ec
--- /dev/null
+++ b/arch.c
@@ -0,0 +1,54 @@
+/*	$Id: arch.c,v 1.14 2019/03/04 13:01:57 schwarze Exp $ */
+/*
+ * Copyright (c) 2017, 2019 Ingo Schwarze <schwarze@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#include "config.h"
+
+#include <string.h>
+
+#include "roff.h"
+
+int
+arch_valid(const char *arch, enum mandoc_os os)
+{
+	const char *openbsd_arch[] = {
+		"alpha", "amd64", "arm64", "armv7", "hppa", "i386",
+		"landisk", "loongson", "luna88k", "macppc", "mips64",
+		"octeon", "sgi", "socppc", "sparc64", NULL
+	};
+	const char *netbsd_arch[] = {
+		"acorn26", "acorn32", "algor", "alpha", "amiga",
+		"arc", "atari",
+		"bebox", "cats", "cesfic", "cobalt", "dreamcast",
+		"emips", "evbarm", "evbmips", "evbppc", "evbsh3", "evbsh5",
+		"hp300", "hpcarm", "hpcmips", "hpcsh", "hppa",
+		"i386", "ibmnws", "luna68k",
+		"mac68k", "macppc", "mipsco", "mmeye", "mvme68k", "mvmeppc",
+		"netwinder", "news68k", "newsmips", "next68k",
+		"pc532", "playstation2", "pmax", "pmppc", "prep",
+		"sandpoint", "sbmips", "sgimips", "shark",
+		"sparc", "sparc64", "sun2", "sun3",
+		"vax", "walnut", "x68k", "x86", "x86_64", "xen", NULL
+	};
+	const char **arches[] = { NULL, netbsd_arch, openbsd_arch };
+	const char **arch_p;
+
+	if ((arch_p = arches[os]) == NULL)
+		return 1;
+	for (; *arch_p != NULL; arch_p++)
+		if (strcmp(*arch_p, arch) == 0)
+			return 1;
+	return 0;
+}