[Python-Dev] RFC: PEP 475, Retry system calls failing with EINTR

Marko Rauhamaa marko at pacujo.net
Mon Sep 1 00:15:12 CEST 2014

Ethan Furman <ethan at stoneleaf.us>:

> On 08/31/2014 02:19 PM, Marko Rauhamaa wrote:
>> The application will often want the EINTR return (exception) instead
>> of having the function resume on its own.
> Examples?
> As an ignorant person in this area, I do not know why I would ever
> want to have EINTR raised instead just getting the results of, say, my
> read() call.

Say you are writing data into a file and it takes a long time (because
there is a lot of data or the medium is very slow or there is a hardware
problem). You might have designed in a signaling scheme to address just
this possibility. Then, the system call had better come out right away
without trying to complete the full extent of the call.

If a signal is received when read() or write() has completed its task
partially (> 0 bytes), no EINTR is returned but the partial count.
Obviously, Python should take that possibility into account so that
raising an exception in the signal handler (as mandated by the PEP)
doesn't cause the partial result to be lost on os.read() or os.write().


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