108 lines
4.3 KiB
C++
108 lines
4.3 KiB
C++
/*
|
|
* Copyright (C) 2017 The Android Open Source Project
|
|
* All rights reserved.
|
|
*
|
|
* Redistribution and use in source and binary forms, with or without
|
|
* modification, are permitted provided that the following conditions
|
|
* are met:
|
|
* * Redistributions of source code must retain the above copyright
|
|
* notice, this list of conditions and the following disclaimer.
|
|
* * Redistributions in binary form must reproduce the above copyright
|
|
* notice, this list of conditions and the following disclaimer in
|
|
* the documentation and/or other materials provided with the
|
|
* distribution.
|
|
*
|
|
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
|
|
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
|
|
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
|
|
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
|
|
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
|
|
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
|
|
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
|
|
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
|
|
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
|
|
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
|
|
* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
* SUCH DAMAGE.
|
|
*/
|
|
|
|
#pragma once
|
|
|
|
#include <string.h>
|
|
|
|
#define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_
|
|
#include <sys/_system_properties.h>
|
|
|
|
// Cached system property lookup. For code that needs to read the same property multiple times,
|
|
// this class helps optimize those lookups.
|
|
class CachedProperty {
|
|
public:
|
|
// The lifetime of `property_name` must be greater than that of this CachedProperty.
|
|
explicit CachedProperty(const char* property_name)
|
|
: property_name_(property_name),
|
|
prop_info_(nullptr),
|
|
cached_area_serial_(0),
|
|
cached_property_serial_(0),
|
|
is_read_only_(strncmp(property_name, "ro.", 3) == 0),
|
|
read_only_property_(nullptr) {
|
|
cached_value_[0] = '\0';
|
|
}
|
|
|
|
// Returns true if the property has been updated (based on the serial rather than the value)
|
|
// since the last call to Get.
|
|
bool DidChange() {
|
|
uint32_t initial_property_serial_ = cached_property_serial_;
|
|
Get();
|
|
return (cached_property_serial_ != initial_property_serial_);
|
|
}
|
|
|
|
// Returns the current value of the underlying system property as cheaply as possible.
|
|
// The returned pointer is valid until the next call to Get. It is the caller's responsibility
|
|
// to provide a lock for thread-safety.
|
|
const char* Get() {
|
|
// Do we have a `struct prop_info` yet?
|
|
if (prop_info_ == nullptr) {
|
|
// `__system_property_find` is expensive, so only retry if a property
|
|
// has been created since last time we checked.
|
|
uint32_t property_area_serial = __system_property_area_serial();
|
|
if (property_area_serial != cached_area_serial_) {
|
|
prop_info_ = __system_property_find(property_name_);
|
|
cached_area_serial_ = property_area_serial;
|
|
}
|
|
}
|
|
|
|
if (prop_info_ != nullptr) {
|
|
// Only bother re-reading the property if it's actually changed since last time.
|
|
uint32_t property_serial = __system_property_serial(prop_info_);
|
|
if (property_serial != cached_property_serial_) {
|
|
__system_property_read_callback(prop_info_, &CachedProperty::Callback, this);
|
|
}
|
|
}
|
|
if (is_read_only_ && read_only_property_ != nullptr) {
|
|
return read_only_property_;
|
|
}
|
|
return cached_value_;
|
|
}
|
|
|
|
private:
|
|
const char* property_name_;
|
|
const prop_info* prop_info_;
|
|
uint32_t cached_area_serial_;
|
|
uint32_t cached_property_serial_;
|
|
char cached_value_[PROP_VALUE_MAX];
|
|
bool is_read_only_;
|
|
const char* read_only_property_;
|
|
|
|
static void Callback(void* data, const char*, const char* value, uint32_t serial) {
|
|
CachedProperty* instance = reinterpret_cast<CachedProperty*>(data);
|
|
instance->cached_property_serial_ = serial;
|
|
// Read only properties can be larger than PROP_VALUE_MAX, but also never change value or
|
|
// location, thus we return the pointer from the shared memory directly.
|
|
if (instance->is_read_only_) {
|
|
instance->read_only_property_ = value;
|
|
} else {
|
|
strlcpy(instance->cached_value_, value, PROP_VALUE_MAX);
|
|
}
|
|
}
|
|
};
|