Help with pipes, buffering and pseudoterminals

Daniel Ellis ellisd23 at
Tue Apr 7 06:13:41 CEST 2015

Wow, thank you for the long and detailed reply.

As for the question ptys and C, no, I have no experience in that area.
I've read a bit on the master/slave stuff, but I'm still finding it
confusing.  The manpage for pty states that it creates a "pair" of virtual
devices, one master and one slave.  It doesn't seem like these are simple
file descriptors for stdin, stdout, and stderr, given that there is only
one for the master and one for the slave, so I'm really not understanding
what it means for these devices to be created.

One way to do that is for "your-program" to open a pty and itself invoke
> "command-generating-output" with its output via the pty, which is why so
> many tutorials suppose a "fork" situation.

Yeah, I'm beginning to realize that.  It's a shame, but I case the use-case
in the end isn't that big of a deal.

Thanks again for the explanation.  I think I'm understanding a bit better
now, or at least understanding why my current means of tackling the problem
won't work.  I'd still very much like to figure out the reading/writing to
the master & slave stuff though.

On Sun, Apr 5, 2015 at 5:11 PM, Cameron Simpson <cs at> wrote:

> On 05Apr2015 12:20, Daniel Ellis <ellisd23 at> wrote:
>> I have a small little tool I'd like to make.  It essentially takes piped
>> input, modifies the text in some way, and immediately prints the output.
>> The problem I'm having is that any output I pipe to the program seems to be
>> buffered, removing the desired effect.
> That depends on the upstream program; it does the buffering. The pipe
> itself presents received data downstream immediately.
> However, as you've seen, almost every program buffers its standard output
> if the output is not a tty; this is automatic in the stdio C library and
> results in more fficient use of I/O.
>  From what I understand, I need to somehow have the input be retrieved via
>> a pseudoterminal.
> This is a rather gross hack, though sometimes all you can do. While some
> programs have an option to force unbuffered output, most do not. Attaching
> their output to a pty is one way to encourage them to at least line buffer
> their output.
> However, you should bear in mind that the reason that programs line buffer
> to a terminal is that they presume they are in an interactive situation
> with a person watching. The program _may_ act differently in other ways as
> well, such as asking question it might not otherwise ask in "batch" mode
> (where it might cautiously not ask and presume "no").
> Also, output sent through a pty is subject to the line discipline in the
> terminal; temrinals are funny things with much historical behaviour. At the
> least you pobably want your pty in "raw" mode to avoid all sorts of stuff
> that can be done to your data.
>  The problem that I'm having is that most examples on the internet seem to
>> assume I would like to launch a program in a forked pty process, which
>> doesn't really fit my use case.
> Indeed not, but not to worry. You don't need to fork.
>  I've tried a number of things, but I seem to be unable to get even a
>> basic understanding of how to use the pty module.
> Have you every used a pty from C? Do you know how ptys work? (master side,
> slave side, etc).
>  Here's a piece of code I whipped up just to try to get a feel for what is
>> going on when I use pty.fork, but it doesn't seem to do what I think it
>> should:
>>    import pty
>>    import os
>>    import sys
>>    pid, fd = pty.fork()
>>    print pid, fd
>>    sys.stdout.flush()
>>, 1024)
>> This only seems to print from the parent process.
> The documentation for pty.fork says:
>  Return value is (pid, fd). Note that the child gets pid 0, and the fd is
> invalid.
> So the child cannot used "fd". It further says that the child has its
> stdin and stdout attached to the pty, and that the pty is the child's
> controlling terminal (this means it is affected by things like "typing" ^C
> at the pty, etc).
>  I read that I need to do the call for the fork to happen.  I've
>> also
> tried printing *after* the call.
> Don't try to adapt fork-based tutorials to your needs. Understand ptys
> directly first.
>  I realize this does very little to solve my overall goal, but I figure
>> understanding what is going on is probably a worthwhile first step.
> What you probably want to use is pty.openpty() instead. No fork. You will
> get back file descriptors for the master and slave sides of the pty. Then
> you can use these with the subprocess module to connect your input program.
> Or, guessing from your opening sentence, you can write a wrapper script
> whose whole purpose is to run a program on a pty.
> Regarding terminology: a pseudoterminal (pty) is a device that looks like
> a traditional serial terminal. All terminal emulators like xterm use one,
> and so do other programs presenting a terminal session such as the sshd
> process handling an interactive remote login.
> When you call pty.openpty() you are handed two file descriptors: one for
> the master side of the pty and one for the slave side. The slave side is
> the side that looks like a terminal, and is what a typical use would
> connect a child process to. The master side is the other side of the pty.
> When a program writes to the "slave" side, the output is available for read
> on the master side, much like a pipe. When a program writes to the master
> side, the output is available for read on the slave side, _as_ _if_ _typed_
> at the terminal.
> A pty is not necessarily going to solve your problem unless you can get
> your input via the pty. From the sounds of it you're in this situation:
>  command-generating-output | your-program
> such that your input is attached to a pipe, and because
> "command-generating-output" is attached to a pipe it is block buffering its
> output, hence your problem.
> You can't undo that situation after the fact.
> To solve your problem via a pty you need to contrive to set up
> "command-generating-output" already attached to a pty. One way to do that
> is for "your-program" to open a pty and itself invoke
> "command-generating-output" with its output via the pty, which is why so
> many tutorials suppose a "fork" situation.
> One typical away to do that is to pass the "command-generating-output"
> command name and args to your program, eg:
>  your-program command-generating-output [args...]
> Then your main program can gather that up:
>  import sys
>  command_generating_output = sys.argv[1:]
> Then you can call pty.openpty(), and then use the slave file descriptor
> with subprocess.Popen to invoke command_generating_output. Thus the
> generating command will be talking to you via a pty instead of a pipe.
> Cheers,
> Cameron Simpson <cs at>
> It is interesting to think of the great blaze of heaven that we winnow
> down to animal shapes and kitchen tools.        - Don DeLillo
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Python-list mailing list