Question

EDIT: Just try with something like this:

__declspec(dllexport) int foo(int param)
    {return param*param;}

and compile. It turns out that functions from CRT and kernel32 is still imported. I have found that these come from the runtime. The question is why this runtime is assumed to be needed? I have read something about relocations. Is it a clue?

I have created a dll that provides a Win32 GUI widget. If I look at the import table for the dll generated by mingw-gcc it looks like the following (functions marked with (*) is those I called myself):

The Import Tables (interpreted .idata section contents)

vma:            Hint    Time      Forward  DLL       First
                Table   Stamp     Chain    Name      Thunk
00006000    00006050 00000000 00000000 00006354 000060d4

DLL Name: KERNEL32.dll
vma:  Hint/Ord Member-Name Bound-To
6158      207  DeleteCriticalSection
6170      236  EnterCriticalSection
6188      352  FreeLibrary
6196      510  GetLastError
61a6      529  GetModuleHandleA  (x)
61ba      577  GetProcAddress
61cc      734  InitializeCriticalSection
61e8      814  LeaveCriticalSection
6200      817  LoadLibraryA
6210     1173  TlsGetValue
621e     1213  VirtualProtect
6230     1215  VirtualQuery

00006014    00006084 00000000 00000000 00006394 00006108

DLL Name: msvcrt.dll
vma:  Hint/Ord Member-Name Bound-To
6240       52  __dllonexit
624e      182  _errno
6258      266  _iob
6260      538  _winmajor
626c      583  abort
6274      595  calloc
627e      610  fflush
6288      625  free    (x)
6290      633  fwrite
629a      676  malloc  (x)
62a4      682  memcpy  (x)
62ae      748  vfprintf

00006028    000060b8 00000000 00000000 000063b8 0000613c

DLL Name: USER32.dll
vma:  Hint/Ord Member-Name Bound-To
62ba      134  DefWindowProcA    (x)
62cc      342  GetWindowLongA    (x)
62de      405  LoadCursorA       (x)
62ec      480  RegisterClassExA  (x)
6300      508  SendMessageA      (x)
6310      569  SetWindowLongA    (x)

It seems that there are much to do before I actually can use the DLL. For example, I never used fwrite in my code, yet it shows up in the import table. This indicates that it is used in some initialization routine that the compiler adds when linking. Why? Shouldn’t the (x)-marked functions be enough?

Answer 1

C code runs startup code. For a primer on what that means, read for example the first link on Google about “C startup code”

In short: it sets up the heap, stack and initializes static variables, along with things like the “environment”, the memory for the parameters passed to the program, internal locale, etc… What exactly depends on the OS, compiler and respective versions.

The dependencies are indeed linked in because of startup code which initializes thread-local storage and whatnot. You can prevent this (at least partially) by adding -nodefaultlibs when you link with gcc, but beware: Dragons Ahead!