windows: Use architecture-specific size limit for WriteFile calls.

Makefile.am

@@ -32,7 +32,7 @@ if LINUX
 libserialport_la_SOURCES += linux.c linux_termios.c linux_termios.h
 endif
 if WIN32
-libserialport_la_SOURCES += windows.c
+libserialport_la_SOURCES += windows.c windows_ddk.c windows_ddk.h
 endif
 if MACOSX
 libserialport_la_SOURCES += macosx.c

libserialport_internal.h

@@ -47,6 +47,7 @@
 	static const GUID name = { l,w1,w2,{ b1,b2,b3,b4,b5,b6,b7,b8 } }
 #include <usbioctl.h>
 #include <usbiodef.h>
+#include "windows_ddk.h"
 #else
 #include <limits.h>
 #include <termios.h>

serialport.c

@@ -793,6 +793,10 @@ SP_API enum sp_return sp_blocking_write(struct sp_port *port, const void *buf,
 			RETURN_FAIL("SetCommTimeouts() failed");
 	}
 
+	/* Reduce count if it exceeds the WriteFile limit. */
+	if (count > WRITEFILE_MAX_SIZE)
+		count = WRITEFILE_MAX_SIZE;
+
 	/* Start write. */
 	if (WriteFile(port->hdl, buf, count, NULL, &port->write_ovl)) {
 		DEBUG("Write completed immediately");
@@ -921,6 +925,10 @@ SP_API enum sp_return sp_nonblocking_write(struct sp_port *port,
 			RETURN_FAIL("SetCommTimeouts() failed");
 	}
 
+	/* Reduce count if it exceeds the WriteFile limit. */
+	if (count > WRITEFILE_MAX_SIZE)
+		count = WRITEFILE_MAX_SIZE;
+
 	/* Copy data to our write buffer. */
 	buf_bytes = min(port->write_buf_size, count);
 	memcpy(port->write_buf, buf, buf_bytes);

windows_ddk.c

@@ -0,0 +1,28 @@
+/*
+ * This file is part of the libserialport project.
+ *
+ * Copyright (C) 2019 Martin Ling <martin-libserialport@earth.li>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * We can't include DDK headers in serialport.c because they conflict with
+ * user space headers. So this file exists to isolate the definitions which
+ * use the kernel declarations.
+ */
+
+#include <ddk/wdm.h>
+
+const size_t WRITEFILE_MAX_SIZE = ((65535 - sizeof(MDL)) / sizeof(ULONG_PTR)) * PAGE_SIZE;

windows_ddk.h

@@ -0,0 +1,25 @@
+/*
+ * This file is part of the libserialport project.
+ *
+ * Copyright (C) 2019 Martin Ling <martin-libserialport@earth.li>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef LIBSERIALPORT_WINDOWS_DDK_H
+#define LIBSERIALPORT_WINDOWS_DDK_H
+
+SP_PRIV extern const size_t WRITEFILE_MAX_SIZE;
+
+#endif