timeout with read operations

G. S. Hayes sjdevnull at yahoo.com
Thu Jul 22 16:50:12 CEST 2004


jim_8421 at hotmail.com (Jim) wrote in message news:<c5694ea3.0407211519.62a56f1a at posting.google.com>...
> Hi,
> 
> I am trying to figure out a way to implement a timeout along with a
> read() call on an open file.  It only has to work on linux,
> for now I am trying:
> 
> ret = select.select( [fileno],[],[], timeout )
> if ret[0] == [fileno]
>     # do read
> 
> but .. what if between the select and read call something happens and
> the read blocks indefinitly?

What's the application?  It's easier to make rational suggestions if
we know what you're trying to do.

Disk reads are uninterruptable--things like setting O_NONBLOCK don't
have any effect on files AFAICS (they work fine on sockets/pipes).

In general (for non-disk system calls), investigate alarm() and
setitimer().  Python wraps the former in signal.alarm (check the docu
for the signal module), there is no standard Python wrapper for the
latter that I know of.

Possible solutions for disk reads include:
1. Use async I/O (I don't know of a Python API for this)
2. Use some sort of parallel processing, either threads or processes;
I'd prefer processes for this particular application (unless you're on
a platform like Windows where there is a significant performance
penalty to processes).
3. Read files into memory before entering the time-sensitive portion
of the code

If 3 is infeasible, consider 2; set up a pipe with os.pipe, fork with
os.fork, and have the child process read off disk and write to the
parent.  The parent can then select on the pipe and read--the pipe
doesn't select as readable until data is already in the pipe buffer,
so you're guaranteed not to block on THAT (you may get an error
returned immediately).  If you're lazy, popen("/bin/cat filename")
will work, though you may more control and efficiency if you do the
fork, pipe, and read calls on your own (popen invokes an unnecessary
shell)

Threads won't be any easier to integrate with a select loop, but if
there's a major performance issue you can avoid a memory copy by
having a thread do the read and using a pipe, socket, or signal to
trigger the select loop.  Almost certainly not worth the
implementation pain.

> Is there a way I can have a functions like file.read( timeout ), where
> timeout out is a gauranteed lower bound?  I know that there is for
> sockets, with the settimeout function, but I don't know how they
> implemented it.

AFAIK, SO_RCVTIMEO and SO_SNDTIMEO are not writeable under Linux, so
I'd guess that settimeout plays games with alarm() or setitimer().

Sumner



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