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 #include <sys/system_properties.h> 33 34 // Cached system property lookup. For code that needs to read the same property multiple times, 35 // this class helps optimize those lookups. 36 class CachedProperty { 37 public: 38 // The lifetime of `property_name` must be greater than that of this CachedProperty. CachedProperty(const char * property_name)39 explicit CachedProperty(const char* property_name) 40 : property_name_(property_name), 41 prop_info_(nullptr), 42 cached_area_serial_(0), 43 cached_property_serial_(0), 44 is_read_only_(strncmp(property_name, "ro.", 3) == 0), 45 read_only_property_(nullptr) { 46 cached_value_[0] = '\0'; 47 } 48 49 // Returns true if the property has been updated (based on the serial rather than the value) 50 // since the last call to Get. DidChange()51 bool DidChange() { 52 uint32_t initial_property_serial_ = cached_property_serial_; 53 Get(); 54 return (cached_property_serial_ != initial_property_serial_); 55 } 56 57 // Returns the current value of the underlying system property as cheaply as possible. 58 // The returned pointer is valid until the next call to Get. It is the caller's responsibility 59 // to provide a lock for thread-safety. Get()60 const char* Get() { 61 // Do we have a `struct prop_info` yet? 62 if (prop_info_ == nullptr) { 63 // `__system_property_find` is expensive, so only retry if a property 64 // has been created since last time we checked. 65 uint32_t property_area_serial = __system_property_area_serial(); 66 if (property_area_serial != cached_area_serial_) { 67 prop_info_ = __system_property_find(property_name_); 68 cached_area_serial_ = property_area_serial; 69 } 70 } 71 72 if (prop_info_ != nullptr) { 73 // Only bother re-reading the property if it's actually changed since last time. 74 uint32_t property_serial = __system_property_serial(prop_info_); 75 if (property_serial != cached_property_serial_) { 76 __system_property_read_callback(prop_info_, &CachedProperty::Callback, this); 77 } 78 } 79 if (is_read_only_ && read_only_property_ != nullptr) { 80 return read_only_property_; 81 } 82 return cached_value_; 83 } 84 85 private: 86 const char* property_name_; 87 const prop_info* prop_info_; 88 uint32_t cached_area_serial_; 89 uint32_t cached_property_serial_; 90 char cached_value_[PROP_VALUE_MAX]; 91 bool is_read_only_; 92 const char* read_only_property_; 93 Callback(void * data,const char *,const char * value,uint32_t serial)94 static void Callback(void* data, const char*, const char* value, uint32_t serial) { 95 CachedProperty* instance = reinterpret_cast<CachedProperty*>(data); 96 instance->cached_property_serial_ = serial; 97 // Read only properties can be larger than PROP_VALUE_MAX, but also never change value or 98 // location, thus we return the pointer from the shared memory directly. 99 if (instance->is_read_only_) { 100 instance->read_only_property_ = value; 101 } else { 102 strlcpy(instance->cached_value_, value, PROP_VALUE_MAX); 103 } 104 } 105 }; 106