Delphi link to windows dll statically or dynamically

StackOverflow https://stackoverflow.com/questions/17726488

Question

I am aware that implicitly linking to libraries at load time can lead to performance increases and as such I was wondering if it was good practice to link in this way at compile time thus increasing executable size (admittedly this is only marginal) compared to linking explicitly at runtime. My question is when linking against Microsoft Windows dll files located in System32, is it 'better' to link at load time as you can be mostly certain that the libraries will be present or follow the explicit approach?

Language used is Delphi (pascal) and the library in question is the WTsAPI32.dll - Terminal Services.

EDIT: As pointed out - my choice of language was incorrect and has been amended. Also, due to having only really every extensively linked to libraries in Unix, my comments about executable size can be omitted, I believed at the time I WAS in fact referring to static linking which bundles the library code into the executable and I now realise this is impossible when using dll files (DUH!). Thanks all.

Was it helpful?

Solution

The two forms of DLL linking are perhaps better named implicit and explicit. Implicit linking is what you refer to as static linking. And explicit linking is what you refer to as runtime linking

For implicit linking the linker writes entries into the import table of the executable file. This import table is metadata that is used by the loader to resolve DLL imports at module load time. A stub function is included for each implicit import that is only a few bytes in size. The executable size implications of implicit linking are negligible.

With explicit linking the imported function's address is resolved by a call to GetProcAddress. This call is made when the programmer chooses. If the DLL or the function cannot be resolved, the programmer can code fall back behaviour. There are size implications to explicit linking that I estimate to be similar to implicit linking. If the function address is evaluated once and remembered between calls then the performance characteristics are similar to implicit linking.

My advice is as follows:

  1. Prefer implicit linking. It is more convenient to code.
  2. If the DLL may not be present, use explicit linking.
  3. If the DLL must be loaded using a full path, use explicit linking.
  4. If you want to unload the DLL during program execution, use explicit linking.

You specifically mention Windows DLLs. You can safely assume that they will be present. Don't try to code to allow your program to run in case user32.dll is missing. Some functions may not be present in older versions of Windows. If you support those older versions you'll need to use explicit linking and provide a fallback. Decide which version you support and use MSDN to be sure that a function is available on your minimum supported platform.

OTHER TIPS

If your only two options are static linking and run-time dynamic linking, then the latter is the best choice for linking with Windows DLLs because it's your only choice. You cannot link statically to a DLL because DLLs are exclusively for dynamic linking; that's what the D stands for. Microsoft does not provide static libraries for the OS modules, so you cannot link to them statically.

But those typically aren't your only two options. There's a third, namely load-time dynamic linking.

In Delphi, you use load-time dynamic linking by marking a function declaration external and specifying the name of the DLL where the function resides. If you use the function, then an entry is created in your module's import table, and when the OS loads your module, it reads the table, loads the referenced DLL, looks up the address of the function, and stores the address in your program's memory image so that your program can call it directly.

You use run-time dyanmic linking by declaring a function pointer, and then using LoadLibrary and GetProcAddress to look up the function's address prior to calling it. In newer Delphi versions, you can also declare a function in the same style that load-time dynamic linking uses, but then mark it with delay. In that case, the Delphi run-time library will call LoadLibrary and GetProcAddress on your behalf the first time you call the function.

The size differences are negligible. Run-time dynamic linking requires your program to contain code to load and link to libraries, but load-time dynamic linking stores more function references in the import table.

Run-time dynamic linking offers more flexibility in the face of uncertain DLL availability. With load-time dynamic linking, if a DLL is missing, or if it doesn't have all the functions mentioned in your import table, then the OS will fail to load your program — none of your code will run. With run-time dynamic linking, however, you have the opportunity to recover from the problem. You can disable certain parts of your program that the missing DLL depends on, or you can search for DLLs in non-standard places, or you can provide alternative implementations of missing functions.

If the functions you're calling are integral to your program's ability to operate, and there's ample reason to expect the functions to be present wherever your program is installed, then you should choose to link at load time. It allows you to write simpler code. You can be confident that you'll have the required functions if they are available on a certain version of windows that you check for in your installer, or if they're provided by DLLs that you distribute with your program.

On the other hand, if the functions you're calling are optional, then you should prefer to link at run time. Use that for loading plug-ins, or for taking advantage of advanced OS features while maintaining backward compatibility. (For example, you might want to take advantage of Windows Vista theme support when it's present, but still allow your program to run on Windows XP.)

Why do you think that compile-time linking to dynamic libraries would increase EXE size ? I believe you are mislead by somewhat poor choice of terms, used in windows programming from far ago. Let us better use relative terms "early binding" and "late binding" instead for the choice who should search for procedure names, compiler/loader or programmer's custom code.

Using early binding (aka static linking against dynamic library) your EXE contains the values (in a special tables):

  • DLL1 Name:
      • procedure "aaaaa" into the variable $1234
      • procedure "bbbbb" into the variable $5678

.

  • DLL2 Name:
      • procedure "ccccc" into the variable $4567

...et cetera.

Now, when you turn this into runtime loading (dynamic linking against dynamic libraries) it would look like

   VarH1 := SafeLoatLibrary(DLL1 Name);
   if Error-Loading-DLL then do-error-handling;

   Var1234 := GetProcAfdress(VarH1, "aaaaa");
   if Error-Searching-For-Function then do-error-handling;

   Var5678 := GetProcAfdress(VarH1, "bbbbb");
   if Error-Searching-For-Function then do-error-handling;

et cetera.

Obviously in the latter case your EXE contains all those values like in the 1st case, but more so - it contains a lot of code to deal with those values, that was just absent before.

So, while EXE size difference is not really large for today memory sizes, it is still in favor of early binding (static compilation against dynamic library).

Then what are the benefits for late binding? For example you can load different DLLs from different paths, determined in runtime by configuration - the flexibility and avoiding of DLL Hell (funny, concept of avoiding DLL Hell is against concept of volume saving). You can make your application work with limited functionality, if DLL load failed while statically binded EXE would just not load - graceful degradation concept. And at least you may give user much better, full of semantics, error messages than Windows could ever do.

And the last word, where you got that concept of EXE size from. I believe you mistaken it from talks about - attention! - static linking against static libraries. That is when OBJ/LIB/DCU files are not the part of distribution, but are just temporary code containers, that ultimately takes its place inside the monolythic EXE. Then yes - then your EXE has all those libraries insideitself and thus grows larger. However this case have nothing about dynamic libraries - DLLs.

The wording chosen once ago overuses static/dynamic terms in two closely related topics: how the library is loaded (compile-time vs runtime) and how functions inside the library are located (or bound. By developer's custom codeing ro by some OS-provided or compiler-provided toolset way before 1st line of your sources started execution).

Due to that ambiguity those close but different concepts start overlapping and sometimes this leads to a total confusion.

Now, what more static linking may give you in modern Windows versions. That is WinSxS folder Novadays Windows tends to keep multiple versions of each system DLL and your program may ask for the specific version of it (while in System32 folder there would be the most recent version that your program may be not get used to. Then you can make a special MANIFEST resource and compile it into EXE asking windows to load not DLLs not be name, but by name+version instead. You can replicaty that functionality with dynamic loading as well, but using Windows-provided toolset it is much easier.

Now you can decide which of those options do or do not have importance for your particular case and make somewhat better informed choice.

HTH.

Licensed under: CC-BY-SA with attribution
Not affiliated with StackOverflow
scroll top