The 4.3 BSD awrite() solution

[Larry McVoy]

In article <6068:00:23:14 at stealth.acf.nyu.edu> brnstnd at stealth.acf.nyu.edu (Dan Bernstein) writes:
>Larry, why don't you at least test the code before asserting that it
>doesn't work? I've now run my async library through a rather thorough
>series of tests. It works perfectly. aread() and awrite() don't block.
>
>Your theoretical error is the assertion that a system call can't be
>interrupted by a signal (except in an obscure situation, where you
>consider the behavior to be in error). But a blocked system call can
>be interrupted. That's why siginterrupt() exists.
>
>Try compiling this code and feeding it into various situations. Try it
>with the interrupt flag changed from 1 to 0. Try it with write() instead
>of awrite(). Try similar tests with reading. You'll become a believer too.
>
>#include <signal.h>
>#include <stdio.h>
>#include "async.h"
>
>main()
>{
> char buf[100000];
>
> siginterrupt(SIGALRM,1); /* this should be in the async library, I guess */
> fprintf(stderr,"%d\n",awrite(1,buf,sizeof(buf)));
>}
>
>Here are a few tests I tried on a Sun with the above program:
>
>kramden% ./astest > /dev/null
>100000
>kramden% ./astest | cat | wc
>4096
>       0       0    4096

First of all, you're right, I was thinking about restarting the system
call which only happens if they haven't moved any data.  As you pointed
out, partially complete system calls (basically read and write) will allow
themselves to be interrupted part way through (if you remember the analogy
in the last message, this is like allowing "blockmove R0,R1,R3" to be
interrupted, whereas you wouldn't allow "inc R0" to be interrupted.)

So let's consider your stuff again.  You claim that you've implemented
awrite().  So what's awrite()?  Well, in my mind, awrite(fd, buf, n)
should start the I/O and return "n" (it would be nice if you could get
status, but you can't with writes either...).  The I/O should all go out
unless there is some sort of error condition (no mem, bad fd, whatever).

That is not at all what you have.  You've got a very perverse way to do 
non-blocking writes.  If that's what you want why not implement it like
so:

awrite(fd, buf, n)
    void *buf;
    unsigned n;
{
    fcntl(fd, F_SETFL, O_NONBLOCK);
    return (write(fd, buf, n));
}

Well, I suppose that you could argue that this won't work on disk files
(and it won't, at least not in the SunOS code I looked at).  But for disk
files you obviously don't care - they are async anyway, so you're only
interested in pipes/sockets/etc.  And those sorts of things support
non-blocking I/O in all sorts of ways.

So... My point is basically the same.  This is *not* an implementation of
awrite() by any reasonable definition - it fails to send all the data
through.  I'm not saying that this has no use, but I am saying (a) this is
not an asynchronous write, and (b) you can get the same behavior in a much
cleaner way by using O_NONBLOCK.
---
What I say is my opinion.  I am not paid to speak for Sun, I'm paid to hack.
    Besides, I frequently read news when I'm drjhgunghc, err, um, drunk.
Larry McVoy, Sun Microsystems     (415) 336-7627       ...!sun!lm or lm at sun.com