C ++ Linux:dlopen找不到.so库 [英] C++ linux: dlopen can't find .so library

问题描述

有疑问的问题(尽管已经解决了):

使用dlopen(3)在Linux上加载共享对象库时遇到了麻烦.该库是由我构建的库系统的一部分，这些库均在运行时由中央可执行文件加载.所有这些都在Code :: Blocks中组织到一个工作区中，每个项目在名为Source的目录中都有自己的文件夹，该目录随程序一起提供.可执行文件的构建目录是从其自身源代码向后的两个目录，以便exectuable和Source文件夹位于同一目录中.这些库也与可执行文件构建在同一目录中，因此我自然地传递了库名我正在尝试按如下所示打开:

int main(int argc, char** argv) {
    void* hLibrary = dlopen("libLibrary.so", RTLD_NOW | RTLD_GLOBAL);
    if(hLibrary == NULL) {
        fprintf(stderr, "%s\n", dlerror());
        return 1;
    }

    return 0;
}

这曾经在构建目录与源代码相同的时候起作用，直到我将源代码的目录更改为上述布局.此时的问题是，即使文件明显存在且与可执行文件位于同一目录中，dlerror()也会返回无法打开libLibrary.so:没有这样的文件或目录".然后，我尝试传递"/libLibrary.so"，因为根据dlopen(3)的手册页，添加/表示相对目录.这返回了相同的错误.

解决方案是需要一个"./"-.".表示可执行文件的工作目录-需要在Code :: Blocks中将工作目录更改为要生成可执行文件的位置.以下内容非常适用:

void* hLibrary = dlopen("./libLibrary.so", RTLD_NOW | RTLD_GLOBAL);

这并没有真正显示完整的解决方案，但是以下内容基本上与我正在做的事情等效:

void* hLibrary = dlopen("./../../libLibrary.so", RTLD_NOW | RTLD_GLOBAL);

希望这可以更好地解释这种情况.

解决方案

阅读

   o   (ELF only) If the executable file for the calling program
       contains a DT_RPATH tag, and does not contain a DT_RUNPATH tag,
       then the directories listed in the DT_RPATH tag are searched.

   o   If, at the time that the program was started, the environment
       variable LD_LIBRARY_PATH was defined to contain a colon-separated
       list of directories, then these are searched.  (As a security
       measure this variable is ignored for set-user-ID and set-group-ID
       programs.)

   o   (ELF only) If the executable file for the calling program
       contains a DT_RUNPATH tag, then the directories listed in that
       tag are searched.

   o   The cache file /etc/ld.so.cache (maintained by ldconfig(8)) is
       checked to see whether it contains an entry for filename.

   o   The directories /lib and /usr/lib are searched (in that order).

因此，您需要调用dlopen("./libLibraryName.so", RTLD_NOW)-不仅是dlopen("libLibraryName.so", RTLD_NOW)，它希望您的插件位于$LD_LIBRARY_PATH中的$LD_LIBRARY_PATH中，等等...--或将.添加到您的LD_LIBRARY_PATH(出于安全原因，我不建议这样做.

作为 Jhonnash回答，当dlopen(或dlsym )失败:

  void* dlh = dlopen("./libLibraryName.so", RTLD_NOW);
  if (!dlh) 
    { fprintf(stderr, "dlopen failed: %s\n", dlerror()); 
      exit(EXIT_FAILURE); };

您可能需要阅读高级Linux编程之类的书籍，以大致了解Linux系统编程. /p>

Reworded Question (although it's been solved already):

I've been having trouble using dlopen(3) to load a shared object library on linux. The library is part of a system of libraries built by me that are all loaded at runtime by a central executable. All of this is organized into a single workspace in Code::Blocks, where each project is given its own folder within a directory called Source, which is to be shipped with the program. The build directory of the executable is two directories backward from its own source code so that the exectuable and the Source folder are in the same directory, The libraries also build to the same directory as the executable, so naturally I pass the name of the library I'm trying to open as shown:

int main(int argc, char** argv) {
    void* hLibrary = dlopen("libLibrary.so", RTLD_NOW | RTLD_GLOBAL);
    if(hLibrary == NULL) {
        fprintf(stderr, "%s\n", dlerror());
        return 1;
    }

    return 0;
}

This was working at one point when the build directory was the same as the source code, until I changed the directories of the source code around to the arrangement described above. The problem at this point is that dlerror() returns "Cannot open libLibrary.so: no such file or directory," even though the file clearly exists and is in the same directory as the executable. I then tried passing in "/libLibrary.so" instead, because according to the man page on dlopen(3), adding a / indicates a relative directory. This returned the same error.

The solution to this was that a "./" was needed - where "." represents the working directory of the executable - and the working directory needed to be changed in Code::Blocks to where the executable was to be built. The following works perfectly:

void* hLibrary = dlopen("./libLibrary.so", RTLD_NOW | RTLD_GLOBAL);

This doesn't really show the full solution, but the following is basically the equivalent of what I'm doing:

void* hLibrary = dlopen("./../../libLibrary.so", RTLD_NOW | RTLD_GLOBAL);

Hopefully this explains the situation a little better.

解决方案

Read the dlopen(3) man page (e.g. by typing man dlopen in a terminal on your machine):

If filename contains a slash ("/"), then it is interpreted as a (relative or absolute) pathname. Otherwise, the dynamic linker searches for the library as follows (see ld.so(8) for further details):

   o   (ELF only) If the executable file for the calling program
       contains a DT_RPATH tag, and does not contain a DT_RUNPATH tag,
       then the directories listed in the DT_RPATH tag are searched.

   o   If, at the time that the program was started, the environment
       variable LD_LIBRARY_PATH was defined to contain a colon-separated
       list of directories, then these are searched.  (As a security
       measure this variable is ignored for set-user-ID and set-group-ID
       programs.)

   o   (ELF only) If the executable file for the calling program
       contains a DT_RUNPATH tag, then the directories listed in that
       tag are searched.

   o   The cache file /etc/ld.so.cache (maintained by ldconfig(8)) is
       checked to see whether it contains an entry for filename.

   o   The directories /lib and /usr/lib are searched (in that order).

So you need to call dlopen("./libLibraryName.so", RTLD_NOW) -not just dlopen("libLibraryName.so", RTLD_NOW) which wants your plugin to be in your $LD_LIBRARY_PATH on in /usr/lib/ etc .... - or add . to your LD_LIBRARY_PATH (which I don't recommend for security reasons).

As Jhonnash answered you should use and display the result of dlerror when dlopen (or dlsym) fails:

  void* dlh = dlopen("./libLibraryName.so", RTLD_NOW);
  if (!dlh) 
    { fprintf(stderr, "dlopen failed: %s\n", dlerror()); 
      exit(EXIT_FAILURE); };

You might want to read some books like Advanced Linux Programming to get some knowledge about Linux system programming in general.

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