Marco van de Voort wrote:
In our previous episode, Mark Morgan Lloyd said:
Efficient implementation of coroutines requires CPU-specific code in the
RTL and possibly the compiler. However
http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html suggests a
way that coroutines can be implemented in a portable fashion in C, how
can this be done in Object Pascal?
Seems more an oddity than a system, but it relies heavily on preprocessor
and fallthrough case.
The preprocessor part can be done, just use whatever preprocessor
(maybe even cpp) and then haul the resulting code through fpc.
But there is no fallthrough case (and personally I think that is a good
thing)
Generally agreed. I'm not particularly bothered about the syntax,
although obviously having it fairly compact would be an advantage. I
don't think it's possible to use a case/goto arrangement since this
wouldn't be happy jumping into repeat or while loops, but it does look
as though it's possible to use LongJmp() (insomnia has its advantages):
program coroutines;
uses SysUtils;
type TState= record
env: jmp_buf;
line: cardinal
end;
var state: TState;
once: boolean= false;
scratch: char;
function getchar(): char;
var sanity: jmp_buf;
begin
// Check that we're not trying to jump into exception blocks etc.
if state.line <> 0 then begin
SetJmp(sanity);
Assert(PtrUInt(state.env.sp) + PtrUInt(state.env.bp) =
PtrUInt(sanity.sp) + PtrUInt(sanity.bp),
'Bad SP or BP at xfer to line ' + IntToStr(state.line));
LongJmp(state.env, 1)
end;
SetJmp(state.env);
if state.line = 0 then begin
state.line := StrToInt( (*$I %LINE% *) );
exit('A')
end else
state.line := 0;
..
repeat
SetJmp(state.env);
if state.line = 0 then begin
state.line := StrToInt( (*$I %LINE% *) );
exit(#$ff)
end else
state.line := 0;
once := true
until once
end { getChar } ;
begin
FillByte(state, SizeOf(state), 0);
scratch := getChar();
while scratch <> #$ff do begin
WriteLn(scratch);
scratch := getChar()
end
end.
The sanity check at the start is obviously processor-specific, but it
doesn't use anything hidden and can at a pinch be omitted. It's
obviously excessively verbose, but it is comparatively regular and does
appear to handle at least some loop types properly.
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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