/*
- * Copyright 2012 Facebook, Inc.
+ * Copyright 2015 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
#include <type_traits>
#include <utility>
#include <boost/noncopyable.hpp>
+#include <boost/type_traits.hpp>
namespace folly {
struct ProducerConsumerQueue : private boost::noncopyable {
typedef T value_type;
- // size must be >= 2
+ // size must be >= 2.
+ //
+ // Also, note that the number of usable slots in the queue at any
+ // given time is actually (size-1), so if you start with an empty queue,
+ // isFull() will return true after size-1 insertions.
explicit ProducerConsumerQueue(uint32_t size)
: size_(size)
, records_(static_cast<T*>(std::malloc(sizeof(T) * size)))
// We need to destruct anything that may still exist in our queue.
// (No real synchronization needed at destructor time: only one
// thread can be doing this.)
- if (!std::has_trivial_destructor<T>::value) {
- int read = readIndex_;
- int end = writeIndex_;
+ if (!boost::has_trivial_destructor<T>::value) {
+ size_t read = readIndex_;
+ size_t end = writeIndex_;
while (read != end) {
records_[read].~T();
if (++read == size_) {
return false;
}
+ // move (or copy) the value at the front of the queue to given variable
bool read(T& record) {
auto const currentRead = readIndex_.load(std::memory_order_relaxed);
if (currentRead == writeIndex_.load(std::memory_order_acquire)) {
return true;
}
+ // pointer to the value at the front of the queue (for use in-place) or
+ // nullptr if empty.
+ T* frontPtr() {
+ auto const currentRead = readIndex_.load(std::memory_order_relaxed);
+ if (currentRead == writeIndex_.load(std::memory_order_acquire)) {
+ // queue is empty
+ return nullptr;
+ }
+ return &records_[currentRead];
+ }
+
+ // queue must not be empty
+ void popFront() {
+ auto const currentRead = readIndex_.load(std::memory_order_relaxed);
+ assert(currentRead != writeIndex_.load(std::memory_order_acquire));
+
+ auto nextRecord = currentRead + 1;
+ if (nextRecord == size_) {
+ nextRecord = 0;
+ }
+ records_[currentRead].~T();
+ readIndex_.store(nextRecord, std::memory_order_release);
+ }
+
+ bool isEmpty() const {
+ return readIndex_.load(std::memory_order_consume) ==
+ writeIndex_.load(std::memory_order_consume);
+ }
+
bool isFull() const {
auto nextRecord = writeIndex_.load(std::memory_order_consume) + 1;
if (nextRecord == size_) {
return true;
}
+ // * If called by consumer, then true size may be more (because producer may
+ // be adding items concurrently).
+ // * If called by producer, then true size may be less (because consumer may
+ // be removing items concurrently).
+ // * It is undefined to call this from any other thread.
+ size_t sizeGuess() const {
+ int ret = writeIndex_.load(std::memory_order_consume) -
+ readIndex_.load(std::memory_order_consume);
+ if (ret < 0) {
+ ret += size_;
+ }
+ return ret;
+ }
+
private:
const uint32_t size_;
T* const records_;
- std::atomic<int> readIndex_;
- std::atomic<int> writeIndex_;
+ std::atomic<unsigned int> readIndex_;
+ std::atomic<unsigned int> writeIndex_;
};
}