C++ 使用 gcc 构建共享库

Question

提问by Sergey K

SOLVED. See below for the corrections (labeled FIXED).

解决了。请参阅下面的更正（标记为 FIXED）。

I'm having trouble creating a shared library using gcc.

我在使用 gcc 创建共享库时遇到问题。

I created a little sample project that closely mirrors the structure of the actual project I'm working on. I've made it available as a tar.gz archive here:

我创建了一个小示例项目，它密切反映了我正在处理的实际项目的结构。我已将其作为 tar.gz 存档提供：

http://209.59.216.197/libtest.tar.gz

FIXED: I've made the fixed version available here:
http://209.59.216.197/libtest_fixed.tar.gz

固定：我已经在这里提供了固定版本：http :
//209.59.216.197/libtest_fixed.tar.gz

In this sample project, I have an application (app) that loads a shared library that I wrote (libshared.so) at runtime and calls a function that the shared library defines: function_inside_shared_lib().

在此示例项目中，我有一个应用程序 (app)，它在运行时加载我编写的共享库 (libshared.so) 并调用共享库定义的函数：function_inside_shared_lib()。

In turn, this shared library uses a function defined inside a static library (libstatic.a): function_inside_static_lib().

反过来，这个共享库使用在静态库 (libstatic.a) 中定义的函数：function_inside_static_lib()。

The problem is when I build the shared library, the symbol "function_inside_shared_lib" does not get exported. I examined the shared library using "nm" and the symbol wasn't there. I am wondering if the command I am using to create the shared library is correct:

问题是当我构建共享库时，符号“function_inside_shared_lib”没有被导出。我使用“nm”检查了共享库，但该符号不存在。我想知道我用来创建共享库的命令是否正确：

g++ -g -ggdb -fPIC -rdynamic -I ../static -c shared.cpp -o shared.o
g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so

FIXED: The correct commands are:

修正：正确的命令是：

g++ -g -ggdb -fPIC -rdynamic -I../static -c shared.cpp -o shared.o
g++ -g -ggdb -fPIC -rdynamic -shared -L../static -o libshared.so shared.o -lstatic

I tried these commands with and without -rdynamic, as well as with and without -fPIC. The results are always the same.

我在使用和不使用 -rdynamic 以及使用和不使用 -fPIC 的情况下尝试了这些命令。结果总是一样的。

I am using Ubuntu 10.04 (64-bit) with g++ version 4.4.3.

我正在使用带有 g++ 4.4.3 版的 Ubuntu 10.04（64 位）。

The full sample project follows. (Or you can download the archive using the link at the top of my post).

完整的示例项目如下。（或者您可以使用我帖子顶部的链接下载档案）。

serg@rodent:~/libtest$ ls
app shared static

serg@rodent:~/libtest$ ls
app 共享静态

Here are the three components:

以下是三个组成部分：

Component 1: A static library that defines a function called function_inside_static_lib().

组件 1：定义名为 function_inside_static_lib() 的函数的静态库。

This consists of the following:

这包括以下内容：

serg@rodent:~/libtest$ cd static/  
serg@rodent:~/libtest/static$ ls  
static.cpp  static.h

serg@rodent:~/libtest$ cd static/  
serg@rodent:~/libtest/static$ ls  
static.cpp  static.h

static.h

静态文件

// Header file for the static library

int function_inside_static_lib(int arg1, int arg2);

static.cpp

静态文件

// Source file for the static library

#include <iostream>
using namespace std;

#include "static.h"

int function_inside_static_lib(int arg1, int arg2)
{
    cout << "In function_inside_static_lib()" << endl;

    // Return the sum
    int result = arg1 + arg2;
    return result;
}

Component 2: A shared library that uses the static library and defines a new function.

组件2：使用静态库并定义新函数的共享库。

serg@rodent:~/libtest$ cd shared
serg@rodent:~/libtest/shared$ ls
shared.cpp

serg@rodent:~/libtest$ cd 共享
serg@rodent:~/libtest/shared$ ls
shared.cpp

shared.cpp

共享文件

// The shared library only has one source file.

// The shared library uses the static one.
#include "static.h"

#include <iostream>
using namespace std;

int function_inside_shared_lib(int arg1, int arg2)
{
    cout << "In function_inside_shared_lib()" << endl;

    cout << "Calling function_inside_static_lib()" << endl;
    int result = function_inside_static_lib(arg1, arg2);

    return result;
}

Component 3: An application that uses the shared library.

组件 3：使用共享库的应用程序。

serg@rodent:~/libtest$ cd app
serg@rodent:~/libtest/app$ ls
app.cpp

app.cpp

应用程序

FIXED: Because C++ symbols get mangled, the correct function name to search for is _Z26function_inside_static_libiiinstead of function_inside_static_lib

修正：因为 C++ 符号被破坏，要搜索的正确函数名称是_Z26function_inside_static_libii而不是function_inside_static_lib

// The application loads the shared library at runtime.

#include <dlfcn.h>
#include <iostream>

using namespace std;

int main(int argc, char **argv)
{
    void *handle;
    int (*function_inside_shared_lib)(int, int);
    char *error;

    int arg1 = 3;
    int arg2 = 7;

    cout << "app: loading the shared library." << endl;
    handle = dlopen ("libshared.so", RTLD_LAZY);
    if (!handle) {
        cout << "Error: Failed to open shared library." << endl;
        cout << dlerror() << endl;
        return -1;
    }

    cout << "app: Looking for function_inside_shared_lib" << endl;

    // The next line is now FIXED:
    function_inside_shared_lib = (int (*)(int, int))dlsym(handle, "_Z26function_inside_static_libii");

    if ((error = dlerror()) != NULL)  {
        cout << "Error: Could not find the function." << endl;
        cout << error << endl;
        return -1;
    }

    cout << "app: Calling function_inside_shared_lib(" << arg1 << ", " << arg2 << ")" << endl;
    int result = (*function_inside_shared_lib)(arg1, arg2);

    cout << "app: The result is " << result << endl;

    dlclose(handle);
    return 0;
}

Here are the commands I'm using to build all of these components. Note that I want debugging symbols to be available in the final resulting app. Ideally, I want to be able to do a backtrace inside the app and see symbols from both the shared library and the static library.

这是我用来构建所有这些组件的命令。请注意，我希望调试符号在最终生成的应用程序中可用。理想情况下，我希望能够在应用程序内部进行回溯并查看来自共享库和静态库的符号。

1: Building the static library. I think I'm fine with this step:

1：构建静态库。我觉得这一步没问题：

serg@rodent:~/libtest/static$ g++ -g -ggdb -c static.cpp -o static.o # See the FIXED version just below
serg@rodent:~/libtest/static$ ar rcs libstatic.a static.o
serg@rodent:~/libtest/static$ ls
libstatic.a static.cpp static.h static.o

serg@rodent:~/libtest/static$ g++ -g -ggdb -c static.cpp -o static.o # 查看
serg@rodent下方的 FIXED 版本：~/libtest/static$ ar rcs libstatic.a static.o
serg@rodent:~/libtest/static$ ls
libstatic.a static.cpp static.h static.o

FIXED: The first command above must include -fPIC as well. The correct command is

修正：上面的第一个命令也必须包含 -fPIC。正确的命令是

g++ -g -ggdb -fPIC -c static.cpp -o static.o

2: Building the shared library. I'm pretty sure this is where I'm going wrong.

2：构建共享库。我很确定这是我出错的地方。

serg@rodent:~/libtest/shared$ g++ -g -ggdb -fPIC -rdynamic -I ../static -c shared.cpp -o shared.o
serg@rodent:~/libtest/shared$ g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so # See just below for FIXED version serg@rodent:~/libtest/shared$ ls
libshared.so shared.cpp shared.o

serg@rodent:~/libtest/shared$ g++ -g -ggdb -fPIC -rdynamic -I ../static -c shared.cpp -o shared.o
serg@rodent:~/libtest/shared$ g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so # FIXED 版本见下面 serg@rodent:~/libtest/shared$ ls
libshared.so shared.cpp shared.o

FIXED: The second command above should be:

修正：上面的第二个命令应该是：

g++ -g -ggdb -fPIC -rdynamic -shared -L../static -o libshared.so shared.o -lstatic

At this point, if I run nm to examine the symbols inside libshared.so, I don't see function_inside_shared_lib() anywhere, even with the -a and -D options for nm. (However, I do see it inside shared.o).

此时，如果我运行 nm 来检查 libshared.so 中的符号，我在任何地方都看不到 function_inside_shared_lib()，即使使用 nm 的 -a 和 -D 选项也是如此。（但是，我确实在 shared.o 中看到了它）。

EDIT: With the fix above, the symbol appears as _Z26function_inside_shared_libii.

编辑：通过上述修复，符号显示为_Z26function_inside_shared_libii。

3: Building the app:

3：构建应用程序：

First, copy the shared library into the app folder:

首先，将共享库复制到app文件夹中：

serg@rodent:~/libtest$ cp shared/libshared.so app/
serg@rodent:~/libtest$ cd app
serg@rodent:~/libtest/app$ ls
app.cpp libshared.so

Now compile:

现在编译：

serg@rodent:~/libtest/app$ g++ -g -ggdb -ldl -L. -lshared app.cpp -o app
serg@rodent:~/libtest/app$ ls
app app.cpp libshared.so

serg@rodent:~/libtest/app$ g++ -g -ggdb -ldl -L。-lshared app.cpp -o app
serg@rodent:~/libtest/app$ ls
app app.cpp libshared.so

If I try to run:

如果我尝试运行：

serg@rodent:~/libtest/app$ ./app
app: loading the shared library.
app: Looking for function_inside_shared_lib
Error: Could not find the function.
/home/serg/libtest/app/libshared.so: undefined symbol: function_inside_shared_lib

serg@rodent:~/libtest/app$ ./app
app: 加载共享库。
应用程序：寻找 function_inside_shared_lib
错误：找不到该函数。
/home/serg/libtest/app/libshared.so：未定义符号：function_inside_shared_lib

This makes sense because I could not see function_inside_shared_lib() using nm either, which means I'm probably building the shared library incorrectly in step 2.

这是有道理的，因为我也看不到使用 nm 的 function_inside_shared_lib() ，这意味着我可能在步骤 2 中错误地构建了共享库。

How can I fix my command in the second step so that function_inside_shared_lib gets exported correctly?

如何在第二步中修复我的命令，以便正确导出 function_inside_shared_lib？

Also feel free to give me any other advice if you notice that I'm doing anything odd. I'm still a beginner.

如果您注意到我在做任何奇怪的事情，请随时给我任何其他建议。我还是个初学者。

Answer 1

回答by nos

There's a few errors here:

这里有几个错误：

libshared.so is empty

libshared.so 为空

Your Makefile doesn't actually link in the shared.o , it just creates an empty shared library. Change

您的 Makefile 实际上并没有在 shared.o 中链接，它只是创建了一个空的共享库。改变

g++ -g -ggdb -fPIC -rdynamic -shared -Lstatic -lstatic -o shared/libshared.so

to

到

g++ -g -ggdb -fPIC -rdynamic -shared -Lstatic -o shared/libshared.so shared/shared.o -lstatic

The -lstatic have to come after shared/shared.o as you have to specify static libraries in reverse order of their dependencies.

-lstatic 必须在 shared/shared.o 之后，因为您必须按其依赖项的相反顺序指定静态库。

-fPIC is needed on all object files in a shared library

共享库中的所有目标文件都需要 -fPIC

You create a shared library that links in a static library. That static library also have to be compiled with -fPIC, otherwise you're creating a shared library where some parts of it cannot be relocated. Change

您创建一个链接到静态库的共享库。该静态库还必须使用 -fPIC 进行编译，否则您将创建一个共享库，其中某些部分无法重定位。改变

g++ -g -ggdb -c static/static.cpp -o static/static.o

to

到

g++ -fPIC -g -ggdb -c static/static.cpp -o static/static.o

C++ symbols get mangled

C++ 符号被破坏

As you're creating a shared library from C++ code, function names and similar gets mangeledThis means there is no function name matching the string "function_inside_static_lib" which you try to dynamically load. run nm on the static library, you'll see it's actually named "_Z26function_inside_static_libii" . You can run nm -C to pretty print C++ names.

当您从 C++ 代码创建共享库时，函数名称和类似名称会被破坏这意味着没有与您尝试动态加载的字符串“function_inside_static_lib”匹配的函数名称。在静态库上运行 nm，你会看到它实际上被命名为 "_Z26function_inside_static_libii" 。您可以运行 nm -C 来漂亮地打印 C++ 名称。

This means your code in app.cpp have to be:

这意味着您在 app.cpp 中的代码必须是：

 function_inside_shared_lib = (int (*)(int, int))dlsym(handle, "_Z26function_inside_static_libii");

This is one of the reasons it's often preferrable to export functions from shared objects using C instead of C++ if you want to dynamically (dlopen) fetch something from a shared library. The C++ name mangling have in the past varied from compiler to compiler, though these days they seem to have all agreed to a standard that won't be changing. With C it's simpler, and the symbols in the shared library will be the same as you gave them in your source code.

如果您想从共享库中动态 (dlopen) 获取某些内容，这就是通常更喜欢使用 C 而不是 C++ 从共享对象导出函数的原因之一。过去，C++ 名称修改因编译器而异，尽管现在他们似乎都同意了一个不会改变的标准。使用 C 更简单，共享库中的符号将与您在源代码中提供的相同。

Answer 2

回答by vladmihaisima

So, in step 2 you don't specify shared.o. So, instead of:

因此，在第 2 步中您没有指定 shared.o。所以，而不是：

g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so

You should do:

你应该做：

g++ -g -ggdb -fPIC -rdynamic -shared -L ../static shared.o -lstatic -o libshared.so

Also it is important that shared.o is before -lstatic. Otherwise the linker will not find the function and let it 'undefined'.

同样重要的是 shared.o 在 -lstatic 之前。否则链接器将找不到该函数并让它“未定义”。

C++ 使用 gcc 构建共享库

提问by Sergey K

回答by nos

回答by vladmihaisima

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