stdcall and cdecl

Question

There are  among others  two types of calling conventions - stdcall and cdecl  I have few questions on them    When a cdecl function is called  how does a caller know if it should free up the stack   At the call site  does the caller know if the function being called is a cdecl or a stdcall function   How does it work   How does the caller know if it should free up the stack or not   Or is it the linkers responsibility   If a function which is declared as stdcall calls a function which has a calling convention as cdecl   or the other way round  would this be inappropriate   In general  can we say that which call will be faster - cdecl or stdcall

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It s specified in the function type  When you have a function pointer  it s assumed to be cdecl if not explicitly stdcall  This means that if you get a stdcall pointer and a cdecl pointer  you can t exchange them  The two function types can call each other without issues  it s just getting one type when you expect the other  As for speed  they both perform the same roles  just in a very slightly different place  it s really irrelevant

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I noticed a posting that say that it does not matter if you call a   stdcall from a   cdecl or visa versa  It does   The reason  with    cdecl the arguments that are passed to the called functions are removed form the stack by the calling function  in   stdcall  the arguments are removed from the stack by the called function  If you call a   cdecl function with a   stdcall  the stack is not cleaned up at all  so eventually when the   cdecl uses a stacked based reference for arguments or return address will use the old data at the current stack pointer  If you call a   stdcall function from a   cdecl  the   stdcall function cleans up the arguments on the stack  and then the   cdecl function does it again  possibly removing the calling functions return information    The Microsoft convention for C tries to circumvent this by mangling the names  A   cdecl function is prefixed with an underscore  A   stdcall function prefixes with an underscore and suffixed with an at sign         and the number of bytes to be removed  Eg   cdecl f x  is linked as  f    stdcall f int x  is linked as  f 4 where sizeof int  is 4 bytes   If you manage to get past the linker  enjoy the debugging mess

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I want to improve on  adf88 s answer  I feel that pseudocode for the STDCALL does not reflect the way of how it happens in reality   a    b   and  c  aren t popped from the stack in the function body  Instead they are popped by the ret instruction  ret 12 would be used in this case  that in one swoop jumps back to the caller and at the same time pops  a    b   and  c  from the stack   Here is my version corrected according to my understanding   STDCALL     1  calling STDCALL in pseudo-assembler  similar to what the compiler outputs     push on the stack a copy of  z   then copy of  y   then copy of  x  call move contents of register A to  i  variable     2  STDCALL  Function  body in pseaudo-assembler    copy  a   from stack  to register A copy  b   from stack  to register B add A and B  store result in A copy  c   from stack  to register B add A and B  store result in A jump back to caller code and at the same time pop  a    b  and  c  off the stack  a  b and      c are removed from the stack in this step  result in register A

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In CDECL arguments are pushed onto the stack in revers order  the caller clears the stack and result is returned via processor registry  later I will call it  register A    In STDCALL there is one difference  the caller doeasn t clear the stack  the calle do   You are asking which one is faster  No one  You should use native calling convention as long as you can  Change convention only if there is no way out  when using external libraries that requires certain convention to be used   Besides  there are other conventions that compiler may choose as default one i e  Visual C   compiler uses FASTCALL which is theoretically faster because of more extensive usage of processor registers   Usually you must give a proper calling convention signature to callback functions passed to some external library i e  callback to qsort from C library must be CDECL  if the compiler by default uses other convention then we must mark the callback as CDECL  or various WinAPI callbacks must be STDCALL  whole WinAPI is STDCALL    Other usual case may be when you are storing pointers to some external functions i e  to create a pointer to WinAPI function its type definition must be marked with STDCALL   And below is an example showing how does the compiler do it      1  calling function in C      i   Function x  y  z       2  function body in C      int Function int a  int b  int c    return a   b   c     CDECL      1  calling CDECL  Function  in pseudo-assembler  similar to what the compiler outputs     push on the stack a copy of  z   then a copy of  y   then a copy of  x  call  jump to function body  after function is finished it will jump back here  the address where to jump back is in registers  move contents of register A to  i  variable pop all from the stack that we have pushed  copy of x  y and z      2  CDECL  Function  body in pseudo-assembler       Now copies of  a    b  and  c  variables are pushed onto the stack    copy  a   from stack  to register A copy  b   from stack  to register B add A and B  store result in A copy  c   from stack  to register B add A and B  store result in A jump back to caller code  a  b and c still on the stack  the result is in register A   STDCALL      1  calling STDCALL in pseudo-assembler  similar to what the compiler outputs     push on the stack a copy of  z   then a copy of  y   then a copy of  x  call move contents of register A to  i  variable     2  STDCALL  Function  body in pseaudo-assembler    pop  a  from stack to register A pop  b  from stack to register B add A and B  store result in A pop  c  from stack to register B add A and B  store result in A jump back to caller code  a  b and c are no more on the stack  result in register A

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Raymond Chen gives a nice overview of what   stdcall and   cdecl does    1  The caller  knows  to clean up the stack after calling a function because the compiler knows the calling convention of that function and generates the necessary code   void   stdcall StdcallFunc       void   cdecl CdeclFunc            The compiler knows that StdcallFunc   uses the   stdcall        convention at this point  so it generates the proper binary        for stack cleanup      StdcallFunc        It is possible to mismatch the calling convention  like this   LRESULT MyWndProc HWND hwnd  UINT msg      WPARAM wParam  LPARAM lParam             Compiler usually complains but there s this cast here    windowClass lpfnWndProc   reinterpret cast lt WNDPROC gt   amp MyWndProc     So many code samples get this wrong it s not even funny  It s supposed to be like this      CALLBACK is  define d as   stdcall LRESULT CALLBACK MyWndProc HWND hwnd  UINT msg     WPARAM wParam  LPARAM lParam          windowClass lpfnWndProc    amp MyWndProc    However  assuming the programmer doesn t ignore compiler errors  the compiler will generate the code needed to clean up the stack properly since it ll know the calling conventions of the functions involved    2  Both ways should work  In fact  this happens quite frequently at least in code that interacts with the Windows API  because   cdecl is the default for C and C   programs according to the Visual C   compiler and the WinAPI functions use the   stdcall convention    3  There should be no real performance difference between the two

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Calling conventions have nothing to do with the C C   programming languages and are rather specifics on how a compiler implements the given language  If you consistently use the same compiler  you never need to worry about calling conventions   However  sometimes we want binary code compiled by different compilers to inter-operate correctly  When we do so we need to define something called the Application Binary Interface  ABI   The ABI defines how the compiler converts the C C   source into machine-code  This will include calling conventions  name mangling  and v-table layout  cdelc and stdcall are two different calling conventions commonly used on x86 platforms    By placing the information on the calling convention into the source header  the compiler will know what code needs to be generated to inter-operate correctly with the given executable

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Those things are Compiler- and Platform-specific  Neither the C nor the C   standard say anything about calling conventions except for extern  C  in C        how does a caller know if it should free up the stack     The caller knows the calling convention of the function and handles the call accordingly      At the call site  does the caller know if the function being called is a cdecl or a stdcall function     Yes      How does it work     It is part of the function declaration      How does the caller know if it should free up the stack or not     The caller knows the calling conventions and can act accordingly      Or is it the linkers responsibility     No  the calling convention is part of a function s declaration so the compiler knows everything it needs to know      If a function which is declared as stdcall calls a function which has a calling convention as cdecl   or the other way round  would this be inappropriate      No  Why should it      In general  can we say that which call will be faster - cdecl or stdcall     I don t know  Test it

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a  When a cdecl function is called by the caller  how does a caller know if it should free up the stack    The cdecl modifier is part of the function prototype  or function pointer type etc   so the caller get the info from there and acts accordingly      b  If a function which is declared as stdcall calls a function which has a calling convention as cdecl   or the other way round  would this be inappropriate    No  it s fine      c  In general  can we say that which call will be faster - cdecl or stdcall    In general  I would refrain from any such statements  The distinction matters eg  when you want to use va arg functions  In theory  it could be that stdcall is faster and generates smaller code because it allows to combine popping the arguments with popping the locals  but OTOH with cdecl  you can do the same thing  too  if you re clever   The calling conventions that aim to be faster usually do some register-passing

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The caller and the callee need to use the same convention at the point of invokation - that s the only way it could reliably work  Both the caller and the callee follow a predefined protocol - for example  who needs to clean up the stack  If conventions mismatch your program runs into undefined behavior - likely just crashes spectacularly   This is only required per invokation site - the calling code itself can be a function with any calling convention   You shouldn t notice any real difference in performance between those conventions  If that becomes a problem you usually need to make less calls - for example  change the algorithm

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