I'm guessing that the prototype for pushfstring is somewhat like this:
void pushfstring(const char *fmt, va_list args);
If it isn't, and is instead:
void pushfstring(const char *fmt, ...);
... then I should have you covered also.
In C, if you have to pass on a call from one variadic function to another, you should use va_list, va_start and va_end, and call the v version of the function. So, if you were implementing printf yourself, you might use vsprintf to format the string - you can't call sprintf directly and pass along the variadic argument list. You need to use va_list and friends.
It's pretty awkward to handle C's va_list from Delphi, and technically it shouldn't be done - the implementation of va_list is specific to the C compiler vendor's runtime.
However, we can try. Suppose we have a little class - though I made it a record for ease of use:
type
TVarArgCaller = record
private
FStack: array of Byte;
FTop: PByte;
procedure LazyInit;
procedure PushData(Loc: Pointer; Size: Integer);
public
procedure PushArg(Value: Pointer); overload;
procedure PushArg(Value: Integer); overload;
procedure PushArg(Value: Double); overload;
procedure PushArgList;
function Invoke(CodeAddress: Pointer): Pointer;
end;
procedure TVarArgCaller.LazyInit;
begin
if FStack = nil then
begin
// Warning: assuming that the target of our call doesn't
// use more than 8K stack
SetLength(FStack, 8192);
FTop := @FStack[Length(FStack)];
end;
end;
procedure TVarArgCaller.PushData(Loc: Pointer; Size: Integer);
function AlignUp(Value: Integer): Integer;
begin
Result := (Value + 3) and not 3;
end;
begin
LazyInit;
// actually you want more headroom than this
Assert(FTop - Size >= PByte(@FStack[0]));
Dec(FTop, AlignUp(Size));
FillChar(FTop^, AlignUp(Size), 0);
Move(Loc^, FTop^, Size);
end;
procedure TVarArgCaller.PushArg(Value: Pointer);
begin
PushData(@Value, SizeOf(Value));
end;
procedure TVarArgCaller.PushArg(Value: Integer);
begin
PushData(@Value, SizeOf(Value));
end;
procedure TVarArgCaller.PushArg(Value: Double);
begin
PushData(@Value, SizeOf(Value));
end;
procedure TVarArgCaller.PushArgList;
var
currTop: PByte;
begin
currTop := FTop;
PushArg(currTop);
end;
function TVarArgCaller.Invoke(CodeAddress: Pointer): Pointer;
asm
PUSH EBP
MOV EBP,ESP
// Going to do something unpleasant now - swap stack out
MOV ESP, EAX.TVarArgCaller.FTop
CALL CodeAddress
// return value is in EAX
MOV ESP,EBP
POP EBP
end;
Using this record, we can manually construct the call frame expected for various C calls. C's calling convention on x86 is to pass arguments from right to left on the stack, with the caller cleaning up. Here's the skeleton of a generic C calling routine:
function CallManually(Code: Pointer; const Args: array of const): Pointer;
var
i: Integer;
caller: TVarArgCaller;
begin
for i := High(Args) downto Low(Args) do
begin
case Args[i].VType of
vtInteger: caller.PushArg(Args[i].VInteger);
vtPChar: caller.PushArg(Args[i].VPChar);
vtExtended: caller.PushArg(Args[i].VExtended^);
vtAnsiString: caller.PushArg(PAnsiChar(Args[i].VAnsiString));
vtWideString: caller.PushArg(PWideChar(Args[i].VWideString));
vtUnicodeString: caller.PushArg(PWideChar(Args[i].VUnicodeString));
// fill as needed
else
raise Exception.Create('Unknown type');
end;
end;
Result := caller.Invoke(Code);
end;
Taking printf as an example:
function printf(fmt: PAnsiChar): Integer; cdecl; varargs;
external 'msvcrt.dll' name 'printf';
const
// necessary as 4.123 is Extended, and %g expects Double
C: Double = 4.123;
begin
// the old-fashioned way
printf('test of printf %s %d %.4g'#10, PAnsiChar('hello'), 42, C);
// the hard way
CallManually(@printf, [AnsiString('test of printf %s %d %.4g'#10),
PAnsiChar('hello'), 42, C]);
end.
Calling the va_list version is slightly more involved, as the va_list argument's location needs to be placed carefully where it is expected:
function CallManually2(Code: Pointer; Fmt: AnsiString;
const Args: array of const): Pointer;
var
i: Integer;
caller: TVarArgCaller;
begin
for i := High(Args) downto Low(Args) do
begin
case Args[i].VType of
vtInteger: caller.PushArg(Args[i].VInteger);
vtPChar: caller.PushArg(Args[i].VPChar);
vtExtended: caller.PushArg(Args[i].VExtended^);
vtAnsiString: caller.PushArg(PAnsiChar(Args[i].VAnsiString));
vtWideString: caller.PushArg(PWideChar(Args[i].VWideString));
vtUnicodeString: caller.PushArg(PWideChar(Args[i].VUnicodeString));
else
raise Exception.Create('Unknown type'); // etc.
end;
end;
caller.PushArgList;
caller.PushArg(PAnsiChar(Fmt));
Result := caller.Invoke(Code);
end;
function vprintf(fmt: PAnsiChar; va_list: Pointer): Integer; cdecl;
external 'msvcrt.dll' name 'vprintf';
begin
// the hard way, va_list
CallManually2(@vprintf, 'test of printf %s %d %.4g'#10,
[PAnsiChar('hello'), 42, C]);
end.
Notes:
The above expects x86 on Windows. Microsoft C, bcc32 (Embarcadero C++) and gcc all pass va_list in the same way (a pointer to the first variadic argument on the stack), according to my experiments, so it should work for you; but as soon as the x86 on Windows assumption is broken, expect this to possibly break too.
The stack is swapped to ease with its construction. This can be avoided with more work, but passing va_list also becomes trickier, as it needs to point at the arguments as if they were passed on the stack. As a consequence, the code needs to make an assumption about how much stack the called routine uses; this example assumes 8K, but this may be too small. Increase if necessary.