【发布时间】：2021-04-23 20:23:32
【问题描述】：
我正在尝试在我的 C/C++ 应用程序中使用 Boost.PropertyTree 解析一个简单的 JSON 字符串。
{"header":{"version":42,"source":1,"destination":2},"coffee":"colombian"}
这是我在 C/C++ 多线程应用程序中的设置方式（手动定义 JSON 字符串来演示问题）。
ParseJson.cpp
#ifdef __cplusplus
extern "C"
{
#endif
#include "ParseJson.hpp"
#ifdef __cplusplus
}
#endif
#include <iostream>
#include <sstream>
#include <string>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>
using boost::property_tree::ptree;
using boost::property_tree::read_json;
using boost::property_tree::write_json;
extern "C" MyStruct * const parseJsonMessage(char * jsonMessage, unsigned int const messageLength) {
MyStruct * myStruct = new MyStruct();
// Create empty property tree object.
ptree tree;
if (myStruct != nullptr) {
try {
// Create an istringstream from the JSON message.
std::string jsonMessageString("{\"header\":{\"version\":42,\"source\":1,\"destination\":2},\"coffee\":\"colombian\"}"); // doesn't work
std::istringstream isStreamJson(jsonMessageString);
// Parse the JSON into the property tree.
std::cout << "Reading JSON ..." << jsonMessageString << "...";
read_json(isStreamJson, tree);
std::cout << " Done!" << std::endl;
// Get the values from the property tree.
printf("version: %d\n", tree.get<int>("header.version"));
printf("source: %d\n", tree.get<int>("header.source"));
printf("coffee: %s\n", tree.get<std::string>("coffee").c_str());
}
catch (boost::property_tree::ptree_bad_path badPathException) {
std::cout << "Exception caught for bad path: " << badPathException.what() << std::endl;
return nullptr;
}
catch (boost::property_tree::ptree_bad_data badDataException) {
std::cout << "Exception caught for bad data: " << badDataException.what() << std::endl;
return nullptr;
}
catch (std::exception exception) {
std::cout << "Exception caught when parsing message into Boost.Property tree: " << exception.what() << std::endl;
return nullptr;
}
}
return myStruct;
}
read_json() 调用似乎已完成，但从属性树中检索解析数据的 get() 调用失败：
Reading JSON ...{"header":{"version":42,"source":1,"destination":2},"coffee":"colombian"}... Done!
Exception caught for bad path: No such node (header.version)
我在 RHEL 7 上使用 Boost 1.53（编译器是 gcc/g++ 版本 4.8.5），并且我已经尝试了 post 中提到的与 Boost.PropertyTree 和多线程相关的两个建议。我已经为项目全局定义了BOOST_SPIRIT_THREADSAFE 编译定义。我还尝试了为该帖子建议的原子交换解决方案。这些都对症状没有任何影响。
奇怪的是，我可以使用另一个 public methods 来手动获取 Boost.Property 树的值：
std::cout << "front.key: " << tree.front().first << std::endl;
std::cout << "front.front.key: " << tree.front().second.front().first << std::endl;
std::cout << "front.front.value: " << tree.front().second.front().second.get_value_optional<std::string>() << std::endl;
这表明 JSON 已被实际解析：
front.key: header
front.front.key: version
front.front.value: 42
注意，我必须使用 std::string 来获取 header.version 值，因为尝试使用 get_value_optional<int>() 也会崩溃。
但是，这种手动方法不可扩展；我的应用程序需要接受几个更复杂的 JSON 结构。
当我尝试更复杂的 JSON 字符串时，这些字符串也被成功解析，但使用 get() 方法访问值同样失败，这一次导致程序崩溃。这是我从崩溃中提取的 GDB 回溯之一，但我对 Boost 不够熟悉，无法从中获得任何有用的信息：
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffebfff700 (LWP 7176)]
0x00007ffff5aa8200 in std::locale::locale(std::locale const&) () from /lib64/libstdc++.so.6
Missing separate debuginfos, use: debuginfo-install boost-system-1.53.0-28.el7.x86_64 boost-thread-1.53.0-28.el7.x86_64 bzip2-libs-1.0.6-13.el7.x86_64 elfutils-libelf-0.176-5.el7.x86_64 elfutils-libs-0.176-5.el7.x86_64 glibc-2.17-292.el7.x86_64 keyutils-libs-1.5.8-3.el7.x86_64 krb5-libs-1.15.1-37.el7_7.2.x86_64 libattr-2.4.46-13.el7.x86_64 libcap-2.22-10.el7.x86_64 libcom_err-1.42.9-16.el7.x86_64 libgcc-4.8.5-39.el7.x86_64 libselinux-2.5-14.1.el7.x86_64 libstdc++-4.8.5-39.el7.x86_64 openssl-libs-1.0.2k-19.el7.x86_64 pcre-8.32-17.el7.x86_64 systemd-libs-219-67.el7_7.2.x86_64 xz-libs-5.2.2-1.el7.x86_64 zlib-1.2.7-18.el7.x86_64
(gdb) bt
#0 0x00007ffff5aa8200 in std::locale::locale(std::locale const&) () from /lib64/libstdc++.so.6
#1 0x00007ffff5ab6051 in std::basic_ios<char, std::char_traits<char> >::imbue(std::locale const&) () from /lib64/libstdc++.so.6
#2 0x000000000041e322 in boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, int>::get_value(std::string const&) ()
#3 0x000000000041c5b2 in boost::optional<int> boost::property_tree::basic_ptree<std::string, std::string, std::less<std::string> >::get_value_optional<int, boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, int> >(boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, int>) const ()
#4 0x000000000041aa61 in boost::enable_if<boost::property_tree::detail::is_translator<boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, int> >, int>::type boost::property_tree::basic_ptree<std::string, std::string, std::less<std::string> >::get_value<int, boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, int> >(boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, int>) const ()
#5 0x000000000041985d in int boost::property_tree::basic_ptree<std::string, std::string, std::less<std::string> >::get_value<int>() const ()
#6 0x0000000000418673 in int boost::property_tree::basic_ptree<std::string, std::string, std::less<std::string> >::get<int>(boost::property_tree::string_path<std::string, boost::property_tree::id_translator<std::string> > const&) const ()
#7 0x0000000000414f4a in parseJsonMessage ()
#8 0x000000000040d8cd in ProcessThread () at ../../src/Processing.c:906
#9 0x00007ffff7bc6ea5 in start_thread () from /lib64/libpthread.so.0
#10 0x00007ffff55538cd in clone () from /lib64/libc.so.6
FWIW，我尝试将此代码放入一个简单的（单线程）main.cpp：
#include <iostream>
#include <sstream>
#include <string>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>
using boost::property_tree::ptree;
using boost::property_tree::read_json;
using boost::property_tree::write_json;
int main(int numArgs, char * const * const args) {
ptree tree;
try {
// Create an istringstream from the JSON message.
std::string jsonMessageString("{\"header\":{\"version\":42,\"source\":1,\"destination\":2},\"coffee\":\"colombian\"}");
std::istringstream isStreamJson(jsonMessageString);
// Parse the JSON into the property tree.
std::cout << "Reading JSON..." << jsonMessageString << "...";
read_json(isStreamJson, tree);
std::cout << " Done!" << std::endl;
// Print what we parsed.
std::cout << "version: " << tree.get<int>("header.version") << std::endl;
std::cout << "source: " << tree.get<int>("header.source") << std::endl;
std::cout << "coffee: " << tree.get<std::string>("coffee") << std::endl;
}
catch (boost::property_tree::ptree_bad_path badPathException) {
std::cout << "Exception caught for bad path: " << badPathException.what() << std::endl;
return -1;
}
catch (boost::property_tree::ptree_bad_data badDataException) {
std::cout << "Exception caught for bad data: " << badDataException.what() << std::endl;
return -1;
}
catch (std::exception exception) {
std::cout << "Exception caught when parsing message into Boost.Property tree: " << exception.what() << std::endl;
return -1;
}
std::cout << "Program completed!" << std::endl;
return 0;
}
这段代码运行良好：
bash-4.2$ g++ -std=c++11 main.cpp -o main.exe
bash-4.2$ ./main.exe
Reading JSON...{"header":{"version":42,"source":1,"destination":2},"coffee":"colombian"}... Done!
version: 42
source: 1
coffee: colombian
Program completed!
那么，为什么 Boost.PropertyTree get() 方法不适用于多线程应用程序？ 多线程应用程序是 C 和 C++ 代码混合的事实是否可以引起问题？我看到我的特定编译器版本（GCC 4.8.5）没有explicitly verified这个Boost库......这可能是编译器问题吗？还是 Boost 1.53 版本有问题？
基于提供的答案的更新：
诚然，我的 parseJsonMessage 方法的原始代码很混乱（数十次调试迭代的产物，并删除了与问题无关的代码）。下面是一个更精简的版本，没有分心（以及可能的红鲱鱼）：
#ifdef __cplusplus
extern "C"
{
#endif
#include "DirectIpRev3.hpp"
#ifdef __cplusplus
}
#endif
#include <iostream>
#include <sstream>
#include <string>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>
using boost::property_tree::ptree;
using boost::property_tree::read_json;
using boost::property_tree::write_json;
extern "C" void parseJsonMessage2() {
// Create empty property tree object.
ptree tree;
std::string jsonMessageString("{\"header\":{\"version\":42,\"source\":1,\"destination\":2},\"coffee\":\"colombian\"}"); //doesn't work
std::istringstream isStreamJson(jsonMessageString);
try {
read_json(isStreamJson, tree);
std::cout << tree.get<int>("header.version") << std::endl;
std::cout << tree.get<int>("header.source") << std::endl;
std::cout << tree.get<std::string>("coffee") << std::endl;
}
catch (boost::property_tree::ptree_bad_path const & badPathException) {
std::cerr << "Exception caught for bad path: " << badPathException.what() << std::endl;
}
catch (boost::property_tree::ptree_bad_data const & badDataException) {
std::cerr << "Exception caught for bad data: " << badDataException.what() << std::endl;
}
catch (std::exception const & exception) {
std::cerr << "Exception caught when parsing message into Boost.Property tree: " << exception.what() << std::endl;
}
}
在我的多线程程序中运行这个压缩函数会产生异常：
Exception caught when parsing message into Boost.Property tree: <unspecified file>(1): expected object or array
没有异常处理，它会打印更多信息：
terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::property_tree::json_parser::json_parser_error> >'
what(): <unspecified file>(1): expected object or array
我仍然不太确定是什么导致了这里的失败，但我倾向于按照建议使用 nlohmann。
【问题讨论】：
-
不错的更新。如果可以的话，我会再次 +1。
标签： c++ multithreading boost boost-propertytree