win: Add and use GET_FUNCTION() and GET_FUNCTION_REQUIRED()

util/win/get_function.h

Index: util/win/get_function.h
diff --git a/util/win/get_function.h b/util/win/get_function.h
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+++ b/util/win/get_function.h
@@ -0,0 +1,109 @@
+// Copyright 2015 The Crashpad Authors. All rights reserved.
+//
+// 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.
+
+#ifndef CRASHPAD_UTIL_WIN_GET_FUNCTION_H_
+#define CRASHPAD_UTIL_WIN_GET_FUNCTION_H_
+
+#include <windows.h>
+
+//! \file
+
+namespace crashpad {
+namespace internal {
+
+//! \brief Returns a function pointer to a named function in a library.
+//!
+//! Do not call this directly, use the GET_FUNCTION() or GET_FUNCTION_REQUIRED()
+//! macros instead.
+//!
+//! \param[in] library The library to search in.
+//! \param[in] function The function to search for. If a leading `::` is
+//!     present, it will be stripped.
+//! \param[in] required If `true`, require the function to resolve by `DCHECK`.
+//!
+//! \return A pointer to the requested function on success. If \a required is
+//!     `true`, triggers a `DCHECK` assertion on failure, otherwise, `nullptr`
+//!     on failure.
+FARPROC GetFunctionInternal(
+    const wchar_t* library, const char* function, bool required);
+
+//! \copydoc GetFunctionInternal
+template <typename FunctionType>
+FunctionType* GetFunction(
+    const wchar_t* library, const char* function, bool required) {
+  return reinterpret_cast<FunctionType*>(
+      internal::GetFunctionInternal(library, function, required));
+}
+
+}  // namespace internal
+}  // namespace crashpad
+
+//! \brief Returns a function pointer to a named function in a library without
+//!     requiring that it be found.
+//!
+//! If the library or function cannot be found, this will return `nullptr`. This
+//! macro is intended to be used to access functions that may not be available
+//! at runtime.
+//!
+//! This macro returns a properly-typed function pointer. It is expected to be
+//! used in this way:
+//! \code
+//!     static const auto get_named_pipe_client_process_id =
+//!         GET_FUNCTION(L"kernel32.dll", ::GetNamedPipeClientProcessId);
+//!     if (get_named_pipe_client_process_id) {
+//!       BOOL rv = get_named_pipe_client_process_id(pipe, &client_process_id);
+//!     }
+//! \endcode

[scottmg, 2015/10/19 17:28:06]

Just note here that they use ::LoadLibrary(), as it can't be used in DllMain().

+//!
+//! \param[in] library The library to search in.
+//! \param[in] function The function to search for. A leading `::` is
+//!     recommended when a wrapper function of the same name is present.
+//!
+//! \return A pointer to the requested function on success, or `nullptr` on
+//!     failure.
+//!
+//! \sa GET_FUNCTION_REQUIRED
+#define GET_FUNCTION(library, function)                  \
+    crashpad::internal::GetFunction<decltype(function)>( \
+        library, #function, false)
+
+//! \brief Returns a function pointer to a named function in a library,
+//!     requiring that it be found.
+//!
+//! If the library or function cannot be found, this will trigger a `DCHECK`
+//! assertion. This macro is intended to be used to access functions that are
+//! always expected to be available at runtime but which are not present in any
+//! import library.
+//!
+//! This macro returns a properly-typed function pointer. It is expected to be
+//! used in this way:
+//! \code
+//!     static const auto nt_query_object =
+//!         GET_FUNCTION_REQUIRED(L"ntdll.dll", ::NtQueryObject);
+//!     NTSTATUS status =
+//!         nt_query_object(handle, type, &info, info_length, &return_length);
+//! \endcode
+//!
+//! \param[in] library The library to search in.
+//! \param[in] function The function to search for. A leading `::` is
+//!     recommended when a wrapper function of the same name is present.
+//!
+//! \return A pointer to the requested function.
+//!
+//! \sa GET_FUNCTION
+#define GET_FUNCTION_REQUIRED(library, function)         \
+    crashpad::internal::GetFunction<decltype(function)>( \
+        library, #function, true)
+
+#endif  // CRASHPAD_UTIL_WIN_GET_FUNCTION_H_