[libc] add windows implementation

Author: SchrodingerZhu

libc/config/windows/entrypoints.txt

@@ -101,6 +101,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     # time.h entrypoints
     libc.src.time.time
     libc.src.time.clock_getres
+
+    # unistd.h entrypoints
+    libc.src.unistd.getentropy
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/config/windows/headers.txt

@@ -6,4 +6,5 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.errno
     libc.include.fenv
     libc.include.math
+    libc.include.unistd
 )

libc/src/unistd/windows/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_entrypoint_object(
+  getentropy
+  SRCS
+    getentropy.cpp
+  HDRS
+    ../getentropy.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.hdr.errno_macros
+    libc.src.errno.errno
+)

libc/src/unistd/windows/getentropy.cpp

@@ -0,0 +1,42 @@
+//===-- Windows implementation of getentropy ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/unistd/getentropy.h"
+#include "hdr/errno_macros.h"
+#include "src/__support/common.h"
+#include "src/errno/libc_errno.h"
+
+#define WIN32_LEAN_AND_MEAN
+#include <Windows.h>
+#include <bcrypt.h>
+#include <ntstatus.h>
+#pragma comment(lib, "bcrypt.lib")
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, getentropy, (void *buffer, size_t length)) {
+  __try {
+    // check the length limit
+    if (length > 256)
+      __leave;
+
+    NTSTATUS result = ::BCryptGenRandom(nullptr, static_cast<PUCHAR>(buffer),
+                                        static_cast<ULONG>(length),
+                                        BCRYPT_USE_SYSTEM_PREFERRED_RNG);
+
+    if (result == STATUS_SUCCESS)
+      return 0;
+
+  } __except (EXCEPTION_EXECUTE_HANDLER) {
+    // no need to handle exceptions specially
+  }
+
+  libc_errno = EIO;
+  return -1;
+}
+} // namespace LIBC_NAMESPACE_DECL