How does __stdcall work?

This is a discussion on How does __stdcall work? within the Windows Programming forums, part of the Platform Specific Boards category; I am curious how the __stdcall keyword works to link a win32 application to the windows operating system. What is ...

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    Question How does __stdcall work?

    I am curious how the __stdcall keyword works to link a win32 application to the windows operating system. What is literally happening inside the computer when this instruction is executed? I read somewhere that __stdcall puts the WndProc() function into another function, how does that work?

    Thank you for any input on my question.

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    int x = *((int *) NULL); Cactus_Hugger's Avatar
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    __stdcall is not an instruction, per se. __stdcall is a calling convention on x86-architectures.

    A calling convention specifies where the arguments to a function are located in memory, how return values are communicated, and who is responsible for cleaning up the arguments. Wikipedia has an article about x86 calling conventions.
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    __stdcall is not an instruction, it is a type modifier which tells the compiler to use the "standard" calling convention for invokations of the function it applies to.

    When a function is called, the parameters are pushed onto the stack. These parameters must be cleaned up after the function call returns. This could be done by the calling code (caller) or the function which is called (callee).

    For __stdcall, the callee cleans the stack. For __cdecl, the caller cleans it. There are other calling conventions as well.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
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    Quote Originally Posted by Cactus_Hugger View Post
    __stdcall is not an instruction, per se. __stdcall is a calling convention on x86-architectures.

    A calling convention specifies where the arguments to a function are located in memory, how return values are communicated, and who is responsible for cleaning up the arguments. Wikipedia has an article about x86 calling conventions.
    Hmmm, how does the calling convention know what arguments should be passed to the function? Where does it go to retrieve the arguments? Also, does that mean any function using __stdcall should always have 4 parameters?
    Last edited by RaisinToe; 04-30-2009 at 09:47 PM.

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    It "knows" what arguments will be passed to the function by the function prototype. With the __stdcall convention the compiler knows that arguments will be passed (pushed) right to left, and when the function pops the return address off the stack, it will also add so much space to the stack pointer to clear the arguments passed by the callee (the local paramaters). No you don't have to have 4 paramaters with a __stdcall. Depending on what paramaters you use in your function the compiler will emit the proper instructions to access the proper offfset into the stack where the argumets should be according to the convention.

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    To... elaborate.

    A calling convention is just that - a convention. It's a way that it's done. There are multiple ways that one could potentially pass arguments to a function at the machine code level, but this particular way is called "stdcall". (Another particular way is called "cdecl", etc.)

    The compiler knows about stdcall & cdecl - the programmers who wrote the compiler wrote the code for these calling conventions. When your C function, marked as stdcall is called, the compiler will generate the assembly instructions that correspond to a function call, doing the function call as "stdcall" says a function call should be done.
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    Question

    Quote Originally Posted by valaris View Post
    It "knows" what arguments will be passed to the function by the function prototype. With the __stdcall convention the compiler knows that arguments will be passed (pushed) right to left,
    The windows operating system calls the __stdcall WndProc() function, right? So how does the program know when to retreive the argument passed by windows? Or am I still not understanding it?

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    Calling conventions work in symmetry: The compiler of the caller code MUST know how to call the callee code, and the callee code must know what to do with it.

    Imagine it like a restaurant, where some staff is responsible for putting plates out, and some other staff serving up the food. But in a different restaurant, the same person carries plates out to the dining table as does the serving. Obviously, if you take the staff out of the one restaurant and simply put them in the other restaurant (without any instructions - and the staff are REALLY stupid - just like computers!), you either end up with food directly on the table or doubled up plates. That wouldn't work.

    Just the same for the compiler - BOTH sides have to know what is done where - which order the arguments are, and "who puts the plate on the table" (stack-clean up). If both sides don't use the same principles, then you get a mess (too many plates, or food directly on the table - in computer terms, probably a crash).

    In this case, Windows DEFINES that WindProc MUST be called with __stdcall convention - and the code in Windows that makes this call is compiled with a __stdcall calling convention.

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    Last edited by matsp; 05-01-2009 at 09:42 AM.
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    OK, so windows places message information on the stack, and then tells WndProc() what has been placed on the stack so it can retrieve it. I think I understand.

    Thank you for that good analogy.

    But now I have to ask; Inside an API application, How is WndProc() called. There is no call to it inside of the application, so what calls it? If I understand correctly now, __stdcall only gives instructions on how to retrieve and send arguments, it does not force a call.
    Last edited by RaisinToe; 05-01-2009 at 11:50 AM. Reason: Change my wording

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    Quote Originally Posted by RaisinToe View Post
    OK, so windows places message information on the stack, and then tells WndProc() what has been placed on the stack so it can retrieve it. I think I understand.

    Thank you for that good analogy.

    But now I have to ask; Inside an API application, How is WndProc() called. There is no call to it inside of the application, so what calls it? If I understand correctly now, __stdcall only gives instructions on how to retrieve and send arguments, it does not force a call.
    You still misunderstanding what stdcall is. It isn't solely used for wndproc.

    Stdcall is short for standard call(ing)(At least, that's my guess). It tells the compiler that the function that is to be converted to machine code(Assembly language) is to put the arguments to the function on the stack in a certain way. In the case of stdcall, arguments are pushed onto the stack in reverse order(Right to left, if you look at a C/++ function declaration). The calling convention also dictates how the memory is set up inside the function, and how it is cleaned up, and how the functions returns data. This here is from wikipedia, and it won't make sense unless you understand assembly language:

    Code:
    calc:
    push eBP            ; save old frame pointer
     mov eBP,eSP         ; get new frame pointer
     sub eSP,localsize   ; reserve place for locals
     .
     .                   ; perform calculations, leave result in AX
     .
     mov eSP,eBP         ; free space for locals
     pop eBP             ; restore old frame pointer
     ret paramsize       ; free parameter space and return
    If you can be a bit more specific on what you are asking, then maybe we can help you out.
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