Prepend \\.\ to configured serial line string, to allow easy access to ports

above COM9.

[originally from svn r7345]
[this svn revision also touched putty-wishlist]

doc/config.but

@@ -2976,9 +2976,7 @@ serial line you want PuTTY to talk to, if your computer has more
 than one serial port.
 
 On Windows, the first serial line is called \i\cw{COM1}, and if there
-is a second it is called \cw{COM2}, and so on. A serial line with
-a name other than \cw{COM1} to \cw{COM9} can be specified by prefixing
-its name with \cw{\\\\.\\} - for instance, \cw{\\\\.\\COM10}.
+is a second it is called \cw{COM2}, and so on.
 
 This configuration setting is also visible on the Session panel,
 where it replaces the \q{Host Name} box (see \k{config-hostname}) if

windows/winser.c

@@ -221,8 +221,39 @@ static const char *serial_init(void *frontend_handle, void **backend_handle,
 	logevent(serial->frontend, msg);
     }
 
-    serport = CreateFile(cfg->serline, GENERIC_READ | GENERIC_WRITE, 0, NULL,
-			 OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
+    {
+	/*
+	 * Munge the string supplied by the user into a Windows filename.
+	 *
+	 * Windows supports opening a few "legacy" devices (including
+	 * COM1-9) by specifying their names verbatim as a filename to
+	 * open. (Thus, no files can ever have these names. See
+	 * <http://msdn2.microsoft.com/en-us/library/aa365247.aspx>
+	 * ("Naming a File") for the complete list of reserved names.)
+	 *
+	 * However, this doesn't let you get at devices COM10 and above.
+	 * For that, you need to specify a filename like "\\.\COM10".
+	 * This is also necessary for special serial and serial-like
+	 * devices such as \\.\WCEUSBSH001. It also works for the "legacy"
+	 * names, so you can do \\.\COM1 (verified as far back as Win95).
+	 * See <http://msdn2.microsoft.com/en-us/library/aa363858.aspx>
+	 * (CreateFile() docs).
+	 *
+	 * So, we believe that prepending "\\.\" should always be the
+	 * Right Thing. However, just in case someone finds something to
+	 * talk to that doesn't exist under there, if the serial line
+	 * contains a backslash, we use it verbatim. (This also lets
+	 * existing configurations using \\.\ continue working.)
+	 */
+	char *serfilename =
+	    dupprintf("%s%s",
+		      strchr(cfg->serline, '\\') ? "" : "\\\\.\\",
+		      cfg->serline);
+	serport = CreateFile(serfilename, GENERIC_READ | GENERIC_WRITE, 0, NULL,
+			     OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
+	sfree(serfilename);
+    }
+
     if (serport == INVALID_HANDLE_VALUE)
 	return "Unable to open serial port";