[fpc-pascal] Portable coroutines

Mark Morgan Lloyd markMLl.fpc-pascal at telemetry.co.uk
Sun Mar 22 13:42:16 CET 2015

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

var     state: TState;
         once: boolean= false;
         scratch: char;

   function getchar(): char;

   var   sanity: jmp_buf;


     // Check that we're not trying to jump into exception blocks etc.

     if state.line <> 0 then begin
       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)

     if state.line = 0 then begin
       state.line := StrToInt( (*$I %LINE% *) );
     end else
       state.line := 0;


       if state.line = 0 then begin
         state.line := StrToInt( (*$I %LINE% *) );
       end else
         state.line := 0;
       once := true
     until once
   end { getChar } ;

   FillByte(state, SizeOf(state), 0);
   scratch := getChar();
   while scratch <> #$ff do begin
     scratch := getChar()

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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