nonblocking read()

[Donn Cave]

In article <cnbgd0$ls4$1 at news.apple.com>,
 Peter Ammon <peter_ammon at rocketmail.com> wrote:

> I would like to read from a pipe in which data may arrive slowly.  From 
> experimenting, it looks like os.read() will block until it returns the 
> maximum amount of data you asked for, in the second parameter to read(), 
> or until it hits EOF.  I cannot find a way to return only the data that 
> the file object has immediately available.
> 
> If no data is available, blocking is OK.
> 
> The only workaround I can think of is to call select() in a loop, 
> reading and storing one byte each time, and then returning them when 
> select() indicates that the pipe is not ready for reading.

I don't think that will work either!

But os.read (a.k.a. posix.read) does just what you want.
Use it with a file descriptor -

   fp = os.popen(cmd, 'r')
   fd = fp.fileno()
   while 1:
       data = os.read(fd, 4096)
       if not data:
           break
       dispose_of(data)

Don't use the file object read or readline methods at all,
because they could leave data in their own input buffer,
where posix.read can't see it (nor select, which is why
I doubt your work-around would work.)

   Donn Cave, donn at u.washington.edu