Linux 动态加载共享库时出现未定义符号错误
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Getting undefined symbol error while dynamic loading of shared library
提问by Yuvi
I am getting undefined symbolerror while loading library dynamically.
Here is my code snippet that generates this error :
undefined symbol动态加载库时出错。这是我生成此错误的代码片段：
int main ()
{
void *lib_handle = NULL;
MyClass* (*create)();
void (*destroy)(MyClass*);
char *error;
lib_handle = dlopen ("./libshared.so", RTLD_LAZY);
if (lib_handle == NULL)
{
fprintf(stderr, "%s\n", dlerror());
exit(1);
}
create = (MyClass* (*)()) dlsym(lib_handle, "create_object");
if ((error = dlerror()) != NULL)
{
fprintf(stderr, "%s\n", error);
exit(1);
}
destroy = (void (*)(MyClass*)) dlsym(lib_handle, "destroy_object");
MyClass *myClass = (MyClass*) create;
destroy(myClass);
dlclose(lib_handle);
}
But when I load library simply by commenting above code and exporting library path everything works like charm.
但是当我简单地通过注释上面的代码和导出库路径来加载库时，一切都像魅力一样。
For dynamic linking I am using the following command on command prompt.
对于动态链接，我在命令提示符下使用以下命令。
g++ -Wl,--export-dynamic shared_user.cpp -ldl
g++ -Wl,--export-dynamic shared_user.cpp -ldl
Any help would be appreciated.
任何帮助，将不胜感激。
回答by FrankH.
You're very likely seeing Name Manglingin action here.
您很可能会在这里看到Name Mangling 的运行情况。
If you want to use dlopen()/ dlsym()with C++ shared libraries, you either need to:
如果要将dlopen()/dlsym()与 C++ 共享库一起使用，则需要：
- declare the functions you want to lookup through
dlsym()asextern "C" { ... }so that the C++ compiler creates unmanglednames for them.
This is only possible if the function you try to access is a non-member or static member function, and not overloaded (only a single signature); C++ can't create unmangled names in other situations.
If one requested the compiler to do so viaextern "C" { ... }and it's possible to create an unmangled name, it ends up verbatim in the ELF symbol table. You can then look it up usingdlsym()exactly like you would for any C function. - Find out what the mangledname of the function is, and use that in your
dlsym()call.
- 声明要查找的函数
dlsym()，extern "C" { ... }以便 C++ 编译器为它们创建未混淆的名称。
这仅在您尝试访问的函数是非成员或静态成员函数且未重载（只有单个签名）时才有可能；在其他情况下，C++ 无法创建未混淆的名称。
如果要求编译器这样做，extern "C" { ... }并且可以创建一个未混淆的名称，则它最终会在 ELF 符号表中逐字逐句。然后，您可以dlsym()像查找任何 C 函数一样使用它来查找它。 - 找出函数的损坏名称，并在您的
dlsym()调用中使用它。
The latter you can do via the nmutility. For example:
后者您可以通过该nm实用程序进行。例如：
$ nm libstdc++.a | grep -v '^ ' | grep unexpected
0000000000000000 T __cxa_call_unexpected
0000000000000000 T _ZN10__cxxabiv112__unexpectedEPFvvE
0000000000000000 T _ZSt10unexpectedv
0000000000000000 T _ZSt14set_unexpectedPFvvE
0000000000000000 D _ZN10__cxxabiv120__unexpected_handlerE
These are the manglednames, what the C++ compiler has actually put into the ELF object. If you use the -Coption to request nmto demanglethe names for you, you get:
这些都是缺胳膊少腿的名字，有什么C ++编译器实际上已经投入了ELF对象。如果您使用的-C选项要求nm，以还原函数你的名字，你会得到：
$ nm -C libstdc++.a | grep -v '^ ' | grep unexpected
0000000000000000 T __cxa_call_unexpected
0000000000000000 T __cxxabiv1::__unexpected(void (*)())
0000000000000000 T std::unexpected()
0000000000000000 T std::set_unexpected(void (*)())
0000000000000000 D __cxxabiv1::__unexpected_handler
That means for this lib, if you'd want to get the function pointer to std::unexpected()from it, you'd have to request dlsym(hdl, "_ZN10__cxxabiv112__unexpectedEPFvvE");to make the lookup succeed.
这意味着对于这个库，如果你想从中获取函数指针std::unexpected()，你必须请求dlsym(hdl, "_ZN10__cxxabiv112__unexpectedEPFvvE");使查找成功。
