如果从 .dll 导入函数，为什么需要 .lib 文件?

When the linker is combining the compiler-generated object files to make the final executable, it has to figure out what all of the compiler-generated references actually refer to. If it can't match a given reference to some piece of code in your executable, it needs to match it to an external DLL instead. So it needs to know which DLLs to even look at, and how those DLLs export things. A DLL may export a given function/variable by name OR by ordinal number, so the linker needs a way to map the identifiers used by your code references to specific entries in the EXPORTS tables of specific .dll files (especially in the case where things are exported by ordinals). Static-link .lib files provide the linker with that mapping information (ie FunctionA maps to Ordinal 123 in DLL XYZ.dll, FunctionB maps to name _FunctionB@4 in DLL ABC.dll, etc).

链接器然后可以使用有关所需的适当 EXPORTS 条目的信息填充可执行文件的 IMPORTS 表，然后使代码中的 DLL 引用指向正确的IMPORTS 条目(如果链接器无法解析编译器生成的对可执行文件中的一段代码或特定 DLL 导出的引用，它会因未解析的外部"错误而中止).

The linker can then populate the IMPORTS table of your executable with information about the appropriate EXPORTS entries needed, and then make the DLL references in your code point to the correct IMPORTS entries (if the linker can't resolve a compiler-generate reference to a piece of code in your executable, or to a specific DLL export, it aborts with an "unresolved external" error).

然后，当您的可执行文件在运行时加载时，操作系统加载器查看 IMPORTS 表以了解需要哪些 DLL 导出，然后它可以将适当的 DLL 加载到内存中并更新IMPORTS 表中的条目具有基于每个 DLL 的 EXPORTS 表的真实内存地址(如果引用的 DLL 无法加载，或者引用的导出无法找到)，操作系统加载程序中止加载您的可执行文件).这样，当您的代码调用 DLL 函数或访问 DLL 变量时，这些访问会转到正确的位置.

Then, when your executable is loaded at run-time, the OS Loader looks at the IMPORTS table to know which DLL exports are needed, so it can then load the appropriate DLLs into memory and update the entries in the IMPORTS table with real memory addresses that are based on each DLL's EXPORTS table (if a referenced DLL fails to load, or if a referenced export fails to be found, the OS Loader aborts loading your executable). That way, when your code calls DLL functions or accesses DLL variables, those accesses go to the right places.

如果您的源代码在运行时通过显式调用 GetProcAddress() 动态访问 DLL 函数/变量，情况就会大不相同.在这种情况下，这些访问不需要静态链接 .lib 文件，因为您自己的代码正在处理将 DLL 加载到内存中并定位它想要使用的导出.

Things are very different if your source code dynamically accesses DLL functions/variables via explicit calls to GetProcAddress() at run-time. In that case, static-link .lib files are not needed for those accesses, since your own code is handling the loading of DLLs into memory and locating the exports that it wants to use.

然而，有第三个选项将上述场景混合在一起:您可以编写代码以静态访问 DLL 函数/变量，但使用链接器的延迟加载 功能(如果有的话).在这种情况下，对于您访问的每个延迟加载的 DLL，您仍然需要静态链接 .lib 文件，但链接器会在您的可执行文件中填充一个单独的 DELAYLOAD 表，并引用DLL 导出，而不是填充 IMPORTS 表.它将编译器生成的 DLL 引用指向编译器 RTL 中的存根，当在运行时第一次访问存根时，它将用来自 GetProcAddress() 的地址替换引用，从而避免需要用于在加载时由 OS Loader 填充的引用.这允许您的可执行文件正常运行，即使 DLL 导出在加载时不存在，如果它们从未使用过，甚至可能根本不需要加载 DLL(当然，如果您的可执行文件确实尝试访问 DLL 导出)动态加载失败，您的代码可能会崩溃，但这是一个单独的问题).

However, there is a 3rd option that blends the above scenarios together: you can write your code to access the DLL functions/variables statically but use your linker's delay-load feature (if it has one). In that case, you still need static-link .lib files for each delay-loaded DLL you access, but the linker populates a separate DELAYLOAD table in your executable with references to the DLL exports, instead of populating the IMPORTS table. It points the compiler-generated DLL references to stubs in your compiler's RTL that will replace the references with addresses from GetProcAddress() when the stubs are accessed for the first time at run-time, thus avoiding the need for the references to be populated by the OS Loader at load-time. This allows your executable to run normally even if the DLL exports are not present at load-time, and may not even need to load the DLLs at all if they are never used (of course, if your executable does try to access a DLL export dynamically and it fails to load, your code is likely to crash, but that is a separate issue).

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