cptable: error out on incorrect usage

diff --git a/codepage/cptable.pl b/codepage/cptable.pl
index 5f6f538..416e730 100755 (executable)
--- a/codepage/cptable.pl
+++ b/codepage/cptable.pl
@@ -5,7 +5,7 @@
 # matching.)
 #
 # Usage:
-#      cptable.pl UnicodeData filesystem-cp.txt console-cp.txt output.bin
+#      cptable.pl UnicodeData filesystem-cp.txt console-cp.txt output.cp
 #
 # Note: for the format of the UnicodeData file, see:
 # http://www.unicode.org/Public/UNIDATA/UCD.html
@@ -13,6 +13,10 @@
 
 ($ucd, $cpfs, $cpco, $cpout) = @ARGV;
 
+if (!defined($cpout)) {
+    die "Usage: $0 UnicodeData fs-cp.txt console-cp.txt output.cp\n";
+}
+
 %ucase   = ();
 %lcase   = ();
 %tcase   = ();