对于具有相同文件的两个调用，dlopen是否会产生相同的句柄? [英] Will dlopen yield the same handle for two calls with the same file?

问题描述

如果在同一应用程序运行中两次在同一个lib/文件上使用dlopen，在两种情况下它会产生相同的句柄吗?是否有任何保证(简短的实验表明至少在我的盒子上可以做到)?

If I use dlopen on the same lib/file two times in the same application-run, will it yield the same handle in both cases? Is there any guarantee for this (a short experiment showed that it at least does on my box)?

我目前正在玩一个小的插件系统(出于好奇)，如果对此观察到的行为有某种保证，我可以将此地址用作插件的键，以防止重复加载. /p>

I'm currently playing around with a little plugin-system (out of curiosity) and if there would be some kind of guarantee for this observed behaviour I could use this address as a key for a plugin to prevent duplicated loads.

推荐答案

是. dlopen(3) linux手册页说:

Yes. The dlopen(3) linux man page says:

   If the same library is loaded again with dlopen(), the same file 
   handle is returned. The dl library maintains reference counts for 
   library handles, so a dynamic library is not deallocated until 
   dlclose() has been called on it as many times as dlopen() 
   has succeeded on it.

顺便说一句，在Linux系统上，您可以dlopen很多(成千上万)共享库，例如我的例子

BTW, on Linux systems, you can dlopen a lot (many dozens of thousands) of shared libraries, as my example manydl.c demonstrates. The main limitation is address space. So practically, not bothering about dlclose-ing stuff is possible.

^{(除非您的dlopen版本共享库具有怪异的或消耗资源的构造函数或析构函数)}

^{2017年12月添加:}

请注意，相关的是传递给dlopen的确切路径字符串.因此，如果使用"./foo.so"和"././foo.so"(或"../foosymlink.so"，其中foosymlink.so是与foo.so的符号链接)，则dlopen句柄是不同的，并且在某些情况下，该共享库的两个实例的行为异常可能发生.

Notice that what is relevant is the exact path string passed to dlopen. So if you use "./foo.so" and "././foo.so" (or "../foosymlink.so" where foosymlink.so is a symlink to foo.so) the dlopen-ed handles are different, and in some cases weird behavior of the two instances of that shared library might happen.

^{于2019年6月添加:}

另请阅读Drepper的 如何编写共享库 纸(它也很好地解释了如何使用它们！).

Read also Drepper's How to write shared libraries paper (it explains also well how to use them!).

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