adb: implement adb_writev.

Change-Id: I55258c155d7b07368ebb45577b2e01ca804cf258
Test: adb_test
Test: python test_device.py
diff --git a/sysdeps/uio.h b/sysdeps/uio.h
new file mode 100644
index 0000000..d06ef89
--- /dev/null
+++ b/sysdeps/uio.h
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <sys/types.h>
+
+#if defined(_WIN32)
+
+// Layout of this struct must match struct WSABUF (verified via static assert in sysdeps_win32.cpp)
+struct adb_iovec {
+    size_t iov_len;
+    void* iov_base;
+};
+
+ssize_t adb_writev(int fd, const adb_iovec* iov, int iovcnt);
+
+#else
+
+#include <sys/uio.h>
+using adb_iovec = struct iovec;
+#define adb_writev writev
+
+#endif
+
+#pragma GCC poison writev