diff --git a/libusb/libusb.h b/libusb/libusb.h
index e8e1201..15bd0d5 100644
--- a/libusb/libusb.h
+++ b/libusb/libusb.h
@@ -25,6 +25,12 @@
 #define LIBUSB_H
 
 #ifdef _MSC_VER
+// Disable warning 4200 for [0].
+#pragma warning(push)
+#pragma warning(disable: 4200)
+#endif
+
+#ifdef _MSC_VER
 /* on MS environments, the inline keyword is available in C++ only */
 #if !defined(__cplusplus)
 #define inline __inline
@@ -1938,4 +1944,8 @@ void LIBUSB_CALL libusb_hotplug_deregister_callback(libusb_context *ctx,
 }
 #endif
 
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
+
 #endif
diff --git a/libusb/os/threads_posix.c b/libusb/os/threads_posix.c
index 9769f58..46f6db7 100644
--- a/libusb/os/threads_posix.c
+++ b/libusb/os/threads_posix.c
@@ -20,11 +20,6 @@
  */
 
 #if defined(__linux__) || defined(__OpenBSD__)
-# if defined(__linux__)
-#  define _GNU_SOURCE
-# else
-#  define _BSD_SOURCE
-# endif
 # include <unistd.h>
 # include <sys/syscall.h>
 #elif defined(__APPLE__)
diff --git a/libusb/os/windows_usb.c b/libusb/os/windows_usb.c
index 63357b1..51ce55d 100644
--- a/libusb/os/windows_usb.c
+++ b/libusb/os/windows_usb.c
@@ -2142,6 +2142,7 @@ static int windows_handle_events(struct libusb_context *ctx, struct pollfd *fds,
 			windows_handle_callback(transfer, io_result, io_size);
 		} else {
 			usbi_err(ctx, "could not find a matching transfer for fd %x", fds[i]);
+			usbi_mutex_unlock(&ctx->open_devs_lock);
 			return LIBUSB_ERROR_NOT_FOUND;
 		}
 	}