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* Copyright (C) 2013 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
* 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 <hardware/sensors.h>
#include <pthread.h>
* Fixed-size circular queue, with an API developed around the sensor HAL poll() method.
* Poll() takes a pointer to a buffer, which is written by poll() before it returns.
* This class can provide a pointer to a spot in its internal buffer for poll() to
* write to, instead of using an intermediate buffer and a memcpy.
* Thread safety:
* Reading can be done safely after grabbing the mutex lock, while poll() writing in a separate
* thread without a mutex lock. But there can only be one writer at a time.
class SensorEventQueue {
int mCapacity;
int mStart; // start of readable region
int mSize; // number of readable items
sensors_event_t* mData;
pthread_cond_t mSpaceAvailableCondition;
explicit SensorEventQueue(int capacity);
// Returns length of region, between zero and min(capacity, requestedLength). If there is any
// writable space, it will return a region of at least one. Because it must return
// a pointer to a contiguous region, it may return smaller regions as we approach the end of
// the data array.
// Only call while holding the lock.
// The region is not marked internally in any way. Subsequent calls may return overlapping
// regions. This class expects there to be exactly one writer at a time.
int getWritableRegion(int requestedLength, sensors_event_t** out);
// After writing to the region returned by getWritableRegion(), call this to indicate how
// many records were actually written.
// This increases size() by count.
// Only call while holding the lock.
void markAsWritten(int count);
// Gets the number of readable records.
// Only call while holding the lock.
int getSize();
// Returns pointer to the first readable record, or NULL if size() is zero.
// Only call this while holding the lock.
sensors_event_t* peek();
// This will decrease the size by one, freeing up the oldest readable event's slot for writing.
// Only call while holding the lock.
void dequeue();
// Blocks until space is available. No-op if there is already space.
// Returns true if it had to wait.
bool waitForSpace(pthread_mutex_t* mutex);