Calling conventions influence two things:
- how parameters are passed to a function/procedure (=routines)
- how the call stack is cleaned up when the call returns
Delphi routines can use the calling conventions pascal (the
Delphi 1 default), register (the default for Delphi 2-5), cdecl
(the default used by C/C++ compilers), stdcall (the default used
by the Windows API). There is a fifth one: safecall, which
is only used in the context of interface methods. A good
explanation for what it entails can be found in issue 51
(Nov. 99) of The Delphi Magazine, i will not go into it
further here. Lets go through the first four in detail, using a
couple of test functions with the same parameter list but
different calling conventions. For clearity we compile with
stack frames on, so each routine will start with the prologue
push ebp
mov ebp, esp
The stack layouts given below are for the mov line. Each test
function is called with the same parameter values so one can
use the CPU windows stack pane to study the resulting stack
layout.
1. Pascal calling convention
Function Test1( i: Integer; b: Boolean; d: Double ): Integer;
Pascal;
Pascal calling convention passes parameters on the stack and
pushes them from left to right in the parameter list. Each
parameter occupies a multiple of 4 bytes. The resulting stack
layout is
ebp + 20 value of i, 4 bytes
ebp + 16 value of b, 4 bytes, only lowbyte significant
ebp + 08 value of d, 8 bytes
ebp + 04 return address, 4 bytes
ebp + 00 old ebp value
The parameters are cleared off the stack by the called function
using a
ret $10
instruction ($10 = 16 is the total size of the parameters on
stack).
2. Register calling convention
Function Test2( i: Integer; b: Boolean; d: Double ): Integer;
Register;
Register calling convention passes parameters in registers
eax, edx, ecx and on the stack and processes them from left to
right in the parameter list. There are rules to decide what
goes into registers and what goes on the stack, as detailed
in the Object Pascal Language guide. The resulting stack layout
is
ebp + 08 value of d, 8 bytes
ebp + 04 return address, 4 bytes
ebp + 00 old ebp value
The value of i is passed in eax, the value of b in edx.
The parameters are cleared off the stack by the called function
using a
ret $8
instruction ($8 = 8 is the total size of the parameters on
stack).
3. cdecl calling convention
Function Test3( i: Integer; b: Boolean; d: Double ): Integer;
cdecl;
Cdecl calling convention passes parameters on the stack and
pushes them from right to left in the parameter list. Each
parameter occupies a multiple of 4 bytes. The resulting stack
layout is
ebp + 16 value of d, 8 bytes
ebp + 12 value of b, 4 bytes, only lowbyte significant
ebp + 08 value of i, 4 bytes
ebp + 04 return address, 4 bytes
ebp + 00 old ebp value
The parameters are cleared off the stack by the calling
function, so the function ends with a
ret 0
and after the call instruction we find a
add esp, $10
instruction ($10 = 16 is the total size of the parameters on
stack).
4. Stdcall calling convention
Function Test4( i: Integer; b: Boolean; d: Double ): Integer;
stdcall;
Sdtcall calling convention passes parameters on the stack and
pushes them from right to left in the parameter list. Each
parameter occupies a multiple of 4 bytes. The resulting stack
layout is
ebp + 16 value of d, 8 bytes
ebp + 12 value of b, 4 bytes, only lowbyte significant
ebp + 08 value of i, 4 bytes
ebp + 04 return address, 4 bytes
ebp + 00 old ebp value
The parameters are cleared off the stack by the called function
using a
ret $10
instruction ($10 = 16 is the total size of the parameters on
stack).
When writing DLLs that are only be meant to be used from Delphi
programs you will usually use the register calling convention,
since it is the most efficient one. But this really ties the
DLL to Delphi, no program compiled in another language (with
the exception of BC++ Builder perhaps) will be able to use the
DLL unless it uses assembler to call the functions, since the
Register calling convention (like MS VC _fastcall) is
compiler-specific.
When writing DLLs that should be usable by other programs
regardless of language you use the stdcall calling convention
for exported routines. Any language that can call Windows API
routines will be able to call routines from such a DLL, as long
as you stay away from Delphi-specific data types, like String,
Boolean, objects, real48.
Pascal calling convention is Win16 heritage, it was the default
for the Win16 API but is no longer used on Win32.
A topic loosely tied to calling conventions is name decoration
for exported names in DLLs. Delphi (5 at least) does not
decorate names, regardless of calling convention used. The name
appears in the exported names table exactly as you cite it in
the exports clause of the DLL, case and all. Case is
significant for exported functions in Win32!
Other compilers may decorate names. Unless told to do otherwise
a C compiler will prefix all cdecl functions with an underbar
and will decorate stdcall functions in the format _name@x,
where x is the total parameter size, e.g. _Test3@16. C++ is
even worse, unless functions are declared as extern "C" it will
export names in a decorated format that encodes parameter size
and type, in a compiler-specific fashion. For routines exported
with Pascal calling convention the names may be all uppercase,
but as said above you will not usually encouter this convention
on Win32.
Due to these naming issues it is often appropriate to sic TDUMP
on an unknown DLL you want to interface to, to figure out the
actual names of the exported functions. These can then be given
in a name clause for the external statement if they are
decorated.
Demo DLL:
library DemoDLL;
uses
Windows;
function Test1(i: Integer; b: Boolean; d: Double): Integer; pascal;
begin
Result := Round(i * Ord(b) * d);
end;
function Test2(i: Integer; b: Boolean; d: Double): Integer; register;
begin
Result := Round(i * Ord(b) * d);
end;
function Test3(i: Integer; b: Boolean; d: Double): Integer; cdecl;
begin
Result := Round(i * Ord(b) * d);
end;
function Test4(i: Integer; b: Boolean; d: Double): Integer; stdcall;
begin
Result := Round(i * Ord(b) * d);
end;
exports
Test1 index 1,
Test2 index 2,
Test3 index 3,
Test4 index 4;
begin
end.
// Example call from test project:
implementation
{$R *.DFM}
function Test1(i: Integer; b: Boolean; d: Double): Integer;
pascal; external 'DEMODLL.DLL' Index 1;
function Test2(i: Integer; b: Boolean; d: Double): Integer;
register; external 'DEMODLL.DLL' Index 2;
function Test3(i: Integer; b: Boolean; d: Double): Integer;
cdecl; external 'DEMODLL.DLL' Index 3;
function Test4(i: Integer; b: Boolean; d: Double): Integer;
stdcall; external 'DEMODLL.DLL' Index 4;
procedure TForm1.Button1Click(Sender: TObject);
var
i: Integer;
begin
i := Test1(16, True, 1.0);
i := Test2(16, True, 1.0);
i := Test3(16, True, 1.0);
i := Test4(16, True, 1.0);
end;
Set breakpoints on the lines and step into the routines with the
CPU window open to see the stack layout.
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