Continuations In Cee

Warning: code here only implements downward (2nd class) continuations on some 16 bit compilers for MS-DOS.

For a more general example of coroutines in C, see http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html.

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For C/asm folks, here's a C [CeeLanguage] version I just wrote for various nefarious purposes. I'd explain how it works, but that isn't any fun. It should be obvious. If it isn't, I'll explain what's going on.

 /* except.c -- Demonstrates continuations in C.
  * --------
  * Written by Sunir Shah. Placed into the public domain.
  *
  * It only works on x86 systems right now.  
  *
  * USE AT YOUR OWN RISK.
  */

 #include <stdio.h>

 typedef struct {
     void *pin;
 } Continuation;

 int exception( Continuation *continuation, void (*pfn)( Continuation * ) )
 {
     short difference;

     if( pfn ) {
         continuation->pin = &difference;
         pfn(continuation);
         return 0;
     }

     difference = (unsigned char *)continuation->pin - (unsigned char*)(&difference);
     _asm add bp, difference
     return 1;
 }

 void bar( Continuation *continuation, int count )
 {
     int test;
     printf( "bar #%d\n", count );

     if( 5 == count )
         exception( continuation, NULL );

     if( count )
         bar(continuation, --count);
 }

 void foo( Continuation *continuation )
 {
     printf( "foo\n" );
     bar(continuation, 20);
     printf( "end foo\n" );
 }

 main()
 {
     Continuation continuation;
     printf( "Entering call\n" );

     if( exception( &continuation, foo ) ) {
         printf( "Exception when calling foo\n" );
     } else {
         printf( "Safe\n" );
     }

     return 0;
 }

This is like setjmp(), except it doesn't store all the non-volatile register values in the continuation. It may be useful for old compilers that don't have setjmp/longjmp implemented.

Contributors: SunirShah, StephanHouben, ClarkEvans, JonathanTang

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CategoryCee CategoryContinuation