使用gcc构建一个共享库。

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

解决了。请参阅下面的更正(贴标)。

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

我很难使用gcc创建一个共享库。

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:

我创建了一个小样本项目，它与我正在进行的实际项目的结构密切相关。我已经把它当成焦油了。广州档案:

http://209.59.216.197/libtest.tar.gz

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

修复:我已经在这里提供了固定版本:http://209.59.216.197/libtest_fix.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().

在这个示例项目中，我有一个应用程序(app)，它加载我在运行时编写的共享库(libsharedso)，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，这个共享库使用静态库中定义的函数(libstatic.a): 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”不会被导出。我用“nm”检查了共享库，符号不在那里。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

g++ -g -ggdb -fPIC -rdynamic -I ../静态- c共享。cpp - o共享。o g++ g -ggdb -fPIC -rdynamic -共享-L ../静态-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

g++ -g -ggdb -fPIC -rdynamic -I../静态- c共享。cpp - o共享。o g++ g -ggdb -fPIC -rdynamic -共享-L../静态- o libshared。所以共享。o -lstatic

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

我尝试了这些命令，没有-rdynamic，也没有-fPIC。结果总是一样的。

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

我使用的是ubuntu10.04(64位)和g++版本4.4.3。

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。(或者你可以在我的帖子顶部用链接下载档案)。

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

~/libtest$ ls应用程序共享静态。

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:定义一个名为function_inside_static_lib()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

static.h

static.h

// Header file for the static library

int function_inside_static_lib(int arg1, int arg2);

static.cpp

static.cpp

// 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/共享$ ls。

shared.cpp

shared.cpp

// 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

serg@rodent:~/libtest$ cd应用程序serg@rodent:~/libtest/app$ ls app.cpp。

app.cpp

app.cpp

FIXED: Because C++ symbols get mangled, the correct function name to search for is _Z26function_inside_static_libii instead of function_inside_static_lib

修正:由于c++符号被破坏，正确的函数名是_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.

下面是我用来构建所有这些组件的命令。请注意，我希望在最终的应用程序中可以使用调试符号。理想情况下，我希望能够在应用程序中进行回溯，并从共享库和静态库中查看符号。

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

1:构建静态库。我觉得这一步很好:

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/静态$ g++ g -ggdb -c静态。cpp - o静态的。在serg@rodent下面看到固定版本:~/libtest/静态$ ar rcs libstatic。一个静态的。o serg@rodent:~ / libtest /静态ls libstatic美元。一个静态的。cpp静态的。h static.o

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

固定:上面的第一个命令必须包括-fPIC。正确的命令是

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

g++ -g -ggdb -fPIC -c静态。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/共享$ g++ -g -ggdb -fPIC -rdynamic -I ../静态- c共享。cpp - o共享。@ serg@rodent:~/libtest/共享$ g+ -g -ggdb -fPIC -rdynamic -共享-L ../静态-lstatic - o libshared。因此，请参见下面的固定版本serg@rodent:~/libtest/共享$ ls libshared。所以共享。cpp shared.o

FIXED: The second command above should be:

固定:上面的第二条命令应该是:

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

g++ -g -ggdb -fPIC -rdynamic -共享-L../静态- o libshared。所以共享。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中的符号。因此，我在任何地方都看不到function_inside_shared_lib()，即使是对nm的-a和-D选项。(不过，我确实看到了它的内部。)

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

编辑:在上面的修复中，这个符号显示为_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:

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

serg@rodent:~ / libtest $ cp / libshared共享。所以应用程序/ serg@rodent:~/libtest$ cd应用程序serg@rodent:~/libtest/app$ ls app.cpp libsharedso。

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应用程序serg@rodent:~/libtest/app$ ls app.cpp libsharedso。

If I try to run:

如果我试着跑:

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应用程序:加载共享库。应用程序:查找function_inside_shared_lib错误:无法找到函数。/home/serg/libtest/app/libshared.所以: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.

这很有意义，因为我不能使用nm来查看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?

如何在第二个步骤中修复我的命令，使function_inside_shared_lib能够正确地导出?

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

如果你注意到我做了什么奇怪的事情，你也可以给我任何其他建议。我还是一个初学者。

2 个解决方案

#1

There's a few errors here:

这里有一些错误:

libshared.so is empty

libshared。所以是空的

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

您的Makefile实际上并没有链接到共享。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

来

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必须在共享/共享之后进行。o由于必须以相反的顺序指定静态库。

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

-fPIC在共享库中的所有对象文件上都是必需的。

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

您可以在静态库中创建一个共享库。静态库也必须用-fPIC编译，否则您将创建一个共享库，其中一些部分不能被重新定位。改变

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

to

来

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 mangeled This 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++代码创建一个共享库时，函数名和类似的函数会被修改，这意味着没有函数名匹配字符串“function_inside_static_lib”，您尝试动态加载它。在静态库上运行nm，您会看到它实际上被命名为“_Z26function_inside_static_libii”。你可以运行nm -C到漂亮的打印c++名称。

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

这意味着您的代码在app.cpp中必须是:

 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.

这是一个原因，它通常可以从共享对象中使用C而不是c++来导出函数，如果您想动态地(dlopen)从共享库中获取一些东西。过去，c++的名称已经从编译器变成了编译器，尽管现在他们似乎已经同意了一个不会改变的标准。与C相比，它更简单，而且共享库中的符号将与您在源代码中给出的符号相同。

#2

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

所以，在步骤2中，你没有指定sharo .o。因此,而不是:

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

You should do:

你应该做的是:

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'.

同样重要的是分享。o是-lstatic之前。否则，链接器将无法找到该函数并让它“未定义”。