__declspec(dllexport) and __declspec(dllimport)

What Exporting Really Means (dll导出的本质)

The only truly interesting thing I introduced in the previous section was the __declspec(dllexport) modifier. When Microsoft's C/C++ compiler sees this modifier before a variable, function prototype, or C++ class, it embeds some additional information in the resulting .obj file. The linker parses this information when all the .obj files for the DLL are linked.

 

当编译器在变量，函数原型，类前面发现有__declspec(dllexport) 时，编译器会嵌入一些额外的信息到生成的obj文件里。

linker在link的时候会解析并处理这些额外的信息。

 

When the DLL is linked, the linker detects this embedded information about the exported variable, function, or class and automatically produces a .lib file. This .lib file contains the list of symbols exported by the DLL.

 

linker发现了编译器插入到这些导出变量的信息后，会根据这些信息生成一个.lib文件，.lib文件大家都很熟悉，这个文件主要存储的是

dll的导出符号表。

 

This .lib file is, of course, required to link any executable module that references this DLL's exported symbols. In addition to creating the .lib file, the linker embeds a table of exported symbols in the resulting DLL file. This export section contains the list (in alphabetical order) of exported variables, functions, and class symbols. The linker also places the relative virtual address (RVA), indicating where each symbol can be found in the DLL module.

 

.lib文件在link的时候是必须的，此外，linker还嵌入了导出符号表 到生成的DLL文件里。这个导出块包含了这个dll所导出的变量，函数，类。linker还将每个符号的相对的虚拟偏移地址放到dll文件里，这样每个符号都可以从这个dll模块被找到。

 

 

Using the Microsoft Visual Studio DumpBin.exe utility (with the -exports switch), you can see what a DLL's export section looks like。

 

使用vs的 dumpbin工具即可查看一个dll的导出块。

 

 

What Importing Really Means （导入的真正含义）

The previous section introduced the __declspec(dllimport) modifier. When you import a symbol, you do not have to use the __declspec(dllimport) keyword—you can simply use the standard C extern keyword.

实际上，当你想导入一个符号时，你不需要用到 __declspec(dllimport). 用extern就可以了。

 

However, the compiler can produce slightly more efficient code if it knows ahead of time that the symbol you are referencing will be imported from a DLL's .lib file. So I highly recommend that you use the __declspec(dllimport) keyword for imported function and data symbols. Microsoft does this for you when you call any of the standard Windows functions.

但是，如果用__declspec(dllimport) 的话，编译器会生成更有效率的代码。

 

When the linker resolves the imported symbols, it embeds a special section called the import section in the resulting executable module. The import section lists the DLL modules required by this module and the symbols referenced from each DLL module.

 

linker解析导入符号时，会嵌入一个导入块到生成的可执行模块里。这个导入块包含了这个模块需要的所有dll和跟这些dll有联关的导入符号

 

 

Using Visual Studio's DumpBin.exe utility (with the -imports switch), you can see what a module's import section looks like.

 

用 DumpBin.exe 工具 (with the -imports 选项), 你可以获得一个模块的导入块