CODEWITHSUNDEEP
cwindowsassembly
Henrik
Henrik

Reputation: 342

calling C mainCRTStartup function from assembly (MASM compatible)

I'm writing an assembly program (MASM compatible) for win64 and want to use the C function mainCRTStartup() if possible to set up the environment and pass the command line arguments. Can I do that somehow?

When you run a (console) C program, mainCRTStartup() is the first function to be called and it calls the program's main function. Where does the mainCRTStartup() function come from? Is it added by the compiler or the linker, or is it contained in the MSVCRT.DLL? I'm not using the Microsoft linker, I use JWasm and JWlink.

I know perfectly well how to call ordinary C functions from assembly, but mainCRTStartup() is very special and I'm not even sure it's a C function at all, maybe a windows OS function.

If I can call it somehow I need to pass my own main() along for callback.

Upvotes: 0

Views: 1618

Answers (1)

RbMm
RbMm

Reputation: 33794

int mainCRTStartup() have no arguments. so you can not (or not need) direct pass arguments for it. he take command line or by call GetCommandLine or by calling __[w]getmainargs to MSVCRT.DLL or statically linked CRT code. (very depended from CRT version)

mainCRTStartup is EXE entry point. first code which executed from EXE but of course not first code which executed in process (all statically dll dependences already loaded and initialized (including MSVCRT.DLL if application use it )

mainCRTStartup come from LIB :libcmt.lib or msvcrt.lib or msvcurt.lib. when you link exe - at first you use some LIBs and OBJs as linker input and for EXE you must set /ENTRY:function option. you can set this function explicitly, or if you not do this - default will be used:

By default, the starting address is a function name from the C run-time library. The linker selects it according to the attributes of the program, as shown in the following table.

in your case this will be mainCRTStartup - linker will be search it in all LIBs and OBJs which you pass to him as input. if he not found it - you got error unresolved external symbol. if linker found it - he use it code as part of exe. so linker take code of mainCRTStartup from lib (for example even msvcrt.lib containing mainCRTStartup as code, but not as import thunk) and link it to exe.

mainCRTStartup is not a windows OS function. it just EXE entry point. not more.

I not exactly understand why you try todo. if you want simply use CRT in own assembly code - you need do next:

  1. implement _main or _wmain (for x86) or main or wmain (for x64)
  2. use say msvcrt.lib as linker input lib
  3. set /ENTRY:[w]mainCRTStartup in project settings or in MASM END [w]mainCRTStartup

if you want own custom function, which will be executed before mainCRTStartup - let name it start - you need change point 3:

  1. set /ENTRY:start in project settings or in MASM END start

and code of start

start PROC
; do something here
call [w]mainCRTStartup ; no args, can even jump instead call
start ENDP

and [w]mainCRTStartup alredy call your [w]main callback. you not need to pass own main because [w]mainCRTStartup called [w]main not by address but by name - so all what you need have [w]main implemented in your code. linker already bind call from [w]mainCRTStartup to your [w]main or say about unresolved symbol

Upvotes: 3

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