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