Why visual studio needs a static library (.lib) for dynamic linking?

Question

I want to use some dll in my project ( VS2013 - c++ ),

I gave the path of dll and headers using "project->properties->vc++ directories", and after building the project, linker errors (common "unresolved external symbol") appears. the problem can be solved by giving the .lib file to the linker!

But why static library (.lib) is needed for dynamic linking???

That's how dynamic linking works on Windows. Google terms: "import library", "def file", "__declspec(dllimport)" — Angew is no longer proud of SO, May 06 '15 at 07:54
This has already been comprehensively answered here http://stackoverflow.com/questions/3573475/how-does-the-import-library-work-details — sjdowling, May 06 '15 at 08:24

score 5 · Accepted Answer · answered May 06 '15 at 08:00

It is an import library, which contains definitions of exports, that resides inside DLL and name of that DLL (*).

You can use LIB with the /DEF option to create an import library and an export file. LINK uses the export file to build a program that contains exports (usually a dynamic-link library (DLL)), and it uses the import library to resolve references to those exports in other programs.

And also:

In most situations, you do not need to use LIB to create your import library. When you link a program (either an executable file or a DLL) that contains exports, LINK automatically creates an import library that describes the exports. Later, when you link a program that references those exports, you specify the import library.

Dynamic libraries are loaded at runtime (on application startup) - linker does not check where some particular symbol resides in DLL. Header says __dllimport - "this symbol is an extern, it should be imported from somewhere". Lib says "I know where this symbol is - it resides in XXX.dll, so look there after startup".

(*) I saw a lot of people, that were trying to change names of .lib and corresponding .dll and expected, that it will work. Contents of .lib is the reason why it didn't.

So is there any way to use a dll without corresponding lib file in VS ?? (can visual studio extract all info from dll itself?? just like GNU Linker??) — payman, May 06 '15 at 08:39
@payman Better late than never - yes, [there is](https://stackoverflow.com/a/9946390/2170527). — yugr, Nov 20 '19 at 17:37

score 4 · Answer 2 · answered May 06 '15 at 07:58

4

To tell it the name of the DLL and what's in it, and what the entry pioint numbers are if used, and
to isolate the binary interface (the .lib) from its implementation (the DLL). Winsock for example had a standard .lib file which was independent of the actual multi-vendor implementations, back in the day.

answered May 06 '15 at 07:58

user207421

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So what to do when we just have the dll file? I think that all info about the library (name, exports, entry point,...) can be extracted from dll itself?! – payman May 06 '15 at 08:17
1

You can create the .lib file from the DLL. – user207421 May 06 '15 at 08:37

Why visual studio needs a static library (.lib) for dynamic linking?

2 Answers2