Re: Two way pipe

[Ludovic Courtès]

Hi Neil,

Neil Jerram <address@hidden> writes:

> Is there any technical reason for why (ice-9 popen) only provides one
> way pipes?  In other words, you can either write to the subprocess, or
> read from it, but not both.  Are there extra technical difficulties
> involved with a two way pipe?

Doesn't `OPEN_BOTH' do what you want?  However, the manual reads:

   Care should be taken with `OPEN_BOTH', a deadlock will occur if
   both parent and child are writing, and waiting until the write
   completes before doing any reading.  Each direction has `PIPE_BUF'
   bytes of buffering (*note Ports and File Descriptors::), which
   will be enough for small writes, but not for say putting a big
   file through a filter.

A socket pair might be suitable, too.

> I have a current requirement to create a subprocess (mplayer in slave
> mode) and then both write to and read from it.  And it seems to me
> that this kind of requirement probably isn't uncommon.
>
> Telford Tendys once posted a possible implementation of this at
> http://www.red-bean.com/guile/guile/old/3038.html.  Can anyone see any
> problems with that implementation?

No, it looks nice.  :-)

It could potentially suffer from the deadlock issue as well, but since
communication with a terminal (via SSH) follows a simple pattern (write
command, read result), I guess deadlocks are unlikely.

Thanks,
Ludovic.