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Date: Sat, 16 Jun 2001 14:33:50 -0400
From: Russell Leighton <russell.leighton@247media.com>
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org
Subject: Re: threading question
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Is there a user-space implemenation (library?) for coroutines that would work from C?


Alan Cox wrote:

> > Can you provide any info and/or examples of co-routines? I'm curious to
> > see a good example of co-routines' "betterness."
>
> With co-routines you don't need
>
>         8K of kernel stack
>         Scheduler overhead
>         Fancy locking
>
> You don't get the automatic thread switching stuff though.
>
> So you might get code that reads like this (note that aio_ stuff works rather
> well combined with co-routines as it fixes a lack of asynchronicity in the
> unix disk I/O world)
>
>         select(....)
>
>         if(FD_ISSET(copier_fd))
>                 run_coroutine(&copier_state);
>
>         ...
>
> and the copier might be something like
>
>         while(1)
>         {
>                 // Yes 1 at a time is dumb but this is an example..
>                 // Yes Im ignoring EOF for this
>                 if(read(copier_fd, buf[bufptr], 1)==-1)
>                 {
>                         if(errno==-EWOULDBLOCK)
>                         {
>                                 coroutine_return();
>                                 continue;
>                         }
>                 }
>                 if(bufptr==255  || buf[bufptr]=='\n')
>                 {
>                         run_coroutine(run_command, buf);
>                         bufptr=0;
>                 }
>                 else
>                         bufptr++;
>         }
>
> it lets you express a state machine as a set of multiple such small state
> machines instead.  run_coroutine() will continue a routine where it last
> coroutine_return()'d from. Thus in the above case we are expressing read
> bytes until you see a new line cleanly - not mangled in with keeping state
> in global structures but by using natural C local variables and code flow
>
> Alan
>
> -
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--
---------------------------------------------------
Russell Leighton    russell.leighton@247media.com
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