Question

我正在玩c ++ 17和插件，我遇到了一个我无法解决的错误。在下面的MWE中，我可以调用一个带std::any的本地函数，当我尝试读取内容时，一切都按预期工作。当我通过插件（dlopen）加载这个完全相同的函数时，它正确地看到了any上的类型，但它不能std::any_cast内容。

在弄清楚导致此错误的原因时，将非常感谢任何帮助。

这是我的环境，MWE和产生的错误。

>> g++ --version

g++ (GCC) 7.1.1 20170526 (Red Hat 7.1.1-2)
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

>> scons --version

SCons by Steven Knight et al.:
    script: v2.5.1.rel_2.5.1:3735:9dc6cee5c168[MODIFIED], 2016/11/03 14:02:02, by bdbaddog on mongodog
    engine: v2.5.1.rel_2.5.1:3735:9dc6cee5c168[MODIFIED], 2016/11/03 14:02:02, by bdbaddog on mongodog
    engine path: ['/usr/lib/scons/SCons']
Copyright (c) 2001 - 2016 The SCons Foundation

>> tree

.
├── SConstruct
└── src
    ├── main.cpp
    ├── plugin.cpp
    └── SConscript

1 directory, 4 files

>> cat SConstruct

SConscript('src/SConscript', variant_dir='build', duplicate=0)

>> cat src/SConscript

env = Environment()
env.Append(CXXFLAGS=['-std=c++17'])
plugin = env.SharedLibrary('plugin', 'plugin.cpp')
Install('../lib', plugin)
driver_env = env.Clone()
driver_env.Append(LIBS=['dl', 'stdc++fs'])
driver = driver_env.Program('driver', 'main.cpp')
Install('../bin', driver)

>> cat src/plugin.cpp

#include <any>
#include <iostream>
using namespace std;
extern "C" {
int plugin(any& context) {
    cout << "    Inside Plugin" << endl;
    cout << "    Has Value? " << context.has_value() << endl;
    cout << "    Type Name: " << context.type().name() << endl;
    cout << "    Value: " << any_cast<double>(context) << endl;
}
}

>> cat src/main.cpp

#include <functional>
#include <iostream>
#include <stdexcept>
#include <any>
#include <experimental/filesystem>
#include <dlfcn.h>

using namespace std;
using namespace std::experimental;

function< void(any&) > loadplugin(string filename) {
    function< void(any&) > plugin;
    filesystem::path library_path(filename);
    filesystem::path library_abspath = canonical(library_path);
    void * libraryHandle = dlopen(library_abspath.c_str(), RTLD_NOW);
    if (!libraryHandle) {
        throw runtime_error("ERROR: Could not load the library");
    }
    plugin = (int(*) (any&))dlsym(libraryHandle, "plugin");
    if (!plugin) {
        throw runtime_error("ERROR: Could not load the plugin");
    }
    return plugin;
}

int local(any& context) {
    cout << "    Inside Local" << endl;
    cout << "      Has Value? " << context.has_value() << endl;
    cout << "      Type Name: " << context.type().name() << endl;
    cout << "      Value: " << any_cast<double>(context) << endl;
}

int main(int argc, char* argv[]) {
    cout << "  Resolving Paths..." << endl;
    filesystem::path full_path = filesystem::system_complete( argv[0] ).parent_path();
    filesystem::path plugin_path(full_path/".."/"lib"/"libplugin.so");
    cout << "  Creating Context..." << endl;
    any context(.1);
    cout << "  Loading Plugin..." << endl;
    function<void(any&) > plugin = loadplugin(plugin_path.string());
    cout << "  Calling Local..." << endl;
    local(context);
    cout << "  Calling Plugin..." << endl;
    plugin(context);
}

>> scons

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
g++ -o build/main.o -c -std=c++17 src/main.cpp
g++ -o build/driver build/main.o -ldl -lstdc++fs
Install file: "build/driver" as "bin/driver"
g++ -o build/plugin.os -c -std=c++17 -fPIC src/plugin.cpp
g++ -o build/libplugin.so -shared build/plugin.os
Install file: "build/libplugin.so" as "lib/libplugin.so"
scons: done building targets.

>> tree

.
├── bin
│   └── driver
├── build
│   ├── driver
│   ├── libplugin.so
│   ├── main.o
│   └── plugin.os
├── lib
│   └── libplugin.so
├── SConstruct
└── src
    ├── main.cpp
    ├── plugin.cpp
    └── SConscript

4 directories, 10 files

>> bin/driver

  Resolving Paths...
  Creating Context...
  Loading Plugin...
  Calling Local...
    Inside Local
      Has Value? 1
      Type Name: d
      Value: 0.1
  Calling Plugin...
    Inside Plugin
    Has Value? 1
    Type Name: d
terminate called after throwing an instance of 'std::bad_any_cast'
  what():  bad any_cast
    Value: Aborted (core dumped)

Answer 1

libstdc ++的any依赖于同一模板实例化的地址在程序中是相同的，这意味着你需要take precautions if you are using dlopen：

编译器必须发出[带有模糊链接的对象，比如模板   实例化]在需要他们出席的任何翻译单元中，   然后依靠链接和加载过程来确保   其中只有一个在最终可执行文件中处于活动状态。随着静态   链接所有这些符号在链接时解析，但有   动态链接，在加载时进一步解决。你必须   确保解析共享库中的对象   可执行文件和其他共享库中的对象。


对于与共享库链接的程序，无需其他预防措施。

您无法使用-Bsymbolic选项创建共享库，因为这会阻止上述解决方案。

如果使用dlopen从共享库中显式加载代码，则必须执行多项操作。首先，从中导出全局符号   通过将其与-E标志链接来执行（您必须指定   如果您以通常的方式调用链接器，则为-Wl,-E   来自编译器驱动程序，g ++）。你还必须做外部   加载库中的符号可用于后续库   向RTLD_GLOBAL提供dlopen标记。符号解析   可以立即或懒惰。

为什么在dlopen函数中传递std :: any的std :: any_cast会引发错误

1 个答案: