1 /* 2 * Copyright (C) 2017 The Android Open Source Project 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * * Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * * Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in 12 * the documentation and/or other materials provided with the 13 * distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS 22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED 23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29 #pragma once 30 31 #include <string.h> 32 33 #define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_ 34 #include <sys/_system_properties.h> 35 36 #include "private/bionic_lock.h" 37 38 class CachedProperty { 39 public: CachedProperty(const char * property_name)40 CachedProperty(const char* property_name) 41 : property_name_(property_name), 42 prop_info_(nullptr), 43 cached_area_serial_(0), 44 cached_property_serial_(0) { 45 cached_value_[0] = '\0'; 46 } 47 Get()48 const char* Get() { 49 lock_.lock(); 50 51 // Do we have a `struct prop_info` yet? 52 if (prop_info_ == nullptr) { 53 // `__system_property_find` is expensive, so only retry if a property 54 // has been created since last time we checked. 55 uint32_t property_area_serial = __system_property_area_serial(); 56 if (property_area_serial != cached_area_serial_) { 57 prop_info_ = __system_property_find(property_name_); 58 cached_area_serial_ = property_area_serial; 59 } 60 } 61 62 if (prop_info_ != nullptr) { 63 // Only bother re-reading the property if it's actually changed since last time. 64 uint32_t property_serial = __system_property_serial(prop_info_); 65 if (property_serial != cached_property_serial_) { 66 __system_property_read_callback(prop_info_, &CachedProperty::Callback, this); 67 } 68 } 69 70 lock_.unlock(); 71 return cached_value_; 72 } 73 74 private: 75 Lock lock_; 76 const char* property_name_; 77 const prop_info* prop_info_; 78 uint32_t cached_area_serial_; 79 uint32_t cached_property_serial_; 80 char cached_value_[PROP_VALUE_MAX]; 81 Callback(void * data,const char *,const char * value,uint32_t serial)82 static void Callback(void* data, const char*, const char* value, uint32_t serial) { 83 CachedProperty* instance = reinterpret_cast<CachedProperty*>(data); 84 instance->cached_property_serial_ = serial; 85 strcpy(instance->cached_value_, value); 86 } 87 }; 88