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// Copyright 2017 The Abseil Authors.
// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <atomic>
#include <cassert>
#include <cstdint>
#include <utility>
#ifdef _MSC_FULL_VER
namespace absl {
namespace base_internal {
template <typename T>
class AtomicHook;
// AtomicHook is a helper class, templatized on a raw function pointer type, for
// implementing Abseil customization hooks. It is a callable object that
// dispatches to the registered hook.
// A default constructed object performs a no-op (and returns a default
// constructed object) if no hook has been registered.
// Hooks can be pre-registered via constant initialization, for example,
// ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);
// and then changed at runtime via a call to Store().
// Reads and writes guarantee memory_order_acquire/memory_order_release
// semantics.
template <typename ReturnType, typename... Args>
class AtomicHook<ReturnType (*)(Args...)> {
using FnPtr = ReturnType (*)(Args...);
// Constructs an object that by default performs a no-op (and
// returns a default constructed object) when no hook as been registered.
constexpr AtomicHook() : AtomicHook(DummyFunction) {}
// Constructs an object that by default dispatches to/returns the
// pre-registered default_fn when no hook has been registered at runtime.
explicit constexpr AtomicHook(FnPtr default_fn)
: hook_(default_fn), default_fn_(default_fn) {}
explicit constexpr AtomicHook(FnPtr default_fn)
: hook_(kUninitialized), default_fn_(default_fn) {}
// Stores the provided function pointer as the value for this hook.
// This is intended to be called once. Multiple calls are legal only if the
// same function pointer is provided for each call. The store is implemented
// as a memory_order_release operation, and read accesses are implemented as
// memory_order_acquire.
void Store(FnPtr fn) {
bool success = DoStore(fn);
// Invokes the registered callback. If no callback has yet been registered, a
// default-constructed object of the appropriate type is returned instead.
template <typename... CallArgs>
ReturnType operator()(CallArgs&&... args) const {
return DoLoad()(std::forward<CallArgs>(args)...);
// Returns the registered callback, or nullptr if none has been registered.
// Useful if client code needs to conditionalize behavior based on whether a
// callback was registered.
// Note that atomic_hook.Load()() and atomic_hook() have different semantics:
// operator()() will perform a no-op if no callback was registered, while
// Load()() will dereference a null function pointer. Prefer operator()() to
// Load()() unless you must conditionalize behavior on whether a hook was
// registered.
FnPtr Load() const {
FnPtr ptr = DoLoad();
return (ptr == DummyFunction) ? nullptr : ptr;
static ReturnType DummyFunction(Args...) {
return ReturnType();
// Current versions of MSVC (as of September 2017) have a broken
// implementation of std::atomic<T*>: Its constructor attempts to do the
// equivalent of a reinterpret_cast in a constexpr context, which is not
// allowed.
// This causes an issue when building with LLVM under Windows. To avoid this,
// we use a less-efficient, intptr_t-based implementation on Windows.
// Return the stored value, or DummyFunction if no value has been stored.
FnPtr DoLoad() const { return hook_.load(std::memory_order_acquire); }
// Store the given value. Returns false if a different value was already
// stored to this object.
bool DoStore(FnPtr fn) {
FnPtr expected = default_fn_;
const bool store_succeeded = hook_.compare_exchange_strong(
expected, fn, std::memory_order_acq_rel, std::memory_order_acquire);
const bool same_value_already_stored = (expected == fn);
return store_succeeded || same_value_already_stored;
std::atomic<FnPtr> hook_;
// Use a sentinel value unlikely to be the address of an actual function.
static constexpr intptr_t kUninitialized = 0;
static_assert(sizeof(intptr_t) >= sizeof(FnPtr),
"intptr_t can't contain a function pointer");
FnPtr DoLoad() const {
const intptr_t value = hook_.load(std::memory_order_acquire);
if (value == kUninitialized) {
return default_fn_;
return reinterpret_cast<FnPtr>(value);
bool DoStore(FnPtr fn) {
const auto value = reinterpret_cast<intptr_t>(fn);
intptr_t expected = kUninitialized;
const bool store_succeeded = hook_.compare_exchange_strong(
expected, value, std::memory_order_acq_rel, std::memory_order_acquire);
const bool same_value_already_stored = (expected == value);
return store_succeeded || same_value_already_stored;
std::atomic<intptr_t> hook_;
const FnPtr default_fn_;
} // namespace base_internal
} // namespace absl