Controlling an interactive command-line utility from a Cocoa app - trouble with ptys

What I'm trying to do

My Cocoa app needs to run a bunch of command-line programs. Most of these are non-interactive, so I launch them with some command-line arguments, they do their thing, output something and quit. One of the programs is interactive, so it outputs some text and a prompt to stdout and then expects input on stdin and this keeps going until you send it a quit command.

What works

The non-interactive programs, which just dump a load of data to stdout and then terminate, are comparatively trivial:

Create NSPipes for stdout/stdin/stderr
Launch NSTask with those pipes

Then, either

get the NSFileHandle for the other end of the pipe to read all data until the end of the stream and process it in one go when the task ends

Get the -fileDescriptors from the NSFileHandle of the other end of the output pipes.
Set the file descriptor to use non-blocking mode
Create a GCD dispatch source with each of those file descriptors using dispatch_source_create(DISPATCH_SOURCE_TYPE_READ, ...
Resume the dispatch source and handle the data it throws at you using read()
Keep going until the task ends and the pipe file descriptor reports EOF (read() reports 0 bytes read)

What doesn't work

Either approach completely breaks down for interactive tools. Obviously I can't wait until the program exits because it's sitting at a command prompt and never will exit unless I tell it to. On the other hand, NSPipe buffers the data, so you receive it in buffer-sized chunks, unless the CLI program happens to flush the pipe explicitly, which the one in my case does not. The initial command prompt is much smaller than the buffer size, so I don't receive anything, and it just sits there. So NSPipe is also a no-go.

After some research, I determined that I needed to use a pseudo-terminal (pty) in place of the NSPipe. Unfortunately, I've had nothing but trouble getting it working.

What I've tried

Instead of the stdout pipe, I create a pty like so:

struct termios termp;
bzero(&termp, sizeof(termp));
int res = openpty(&masterFD, &slaveFD, NULL, &termp, NULL);

This gives me two file descriptors; I hand the slaveFD over to an NSFileHandle, which gets passed to the NSTask for either just stdout or both stdout and stdin. Then I try to do the usual asynchronous reading from the master side.

If I run the program I'm controlling in a Terminal window, it starts off by outputting 2 lines of text, one 18 bytes long including the newline, one 22 bytes and with no newline for the command prompt. After those 40 bytes it waits for input.

If I just use the pty for stdout, I receive 18 bytes of output (exactly one line, ending in newline) from the controlled program, and no more. Everything just sits there after the initial 18 bytes, no more events - the GCD event source's handler doesn't get called.

If I also use the pty for stdin, I usually receive 19 bytes of output (the aforementioned line plus one character from the next line) and then the controlled program dies immediately. If I wait a little before attempting to read the data (or scheduling noise causes a small pause), I actually get the whole 40 bytes before the program again dies instantly.

An additional dead end

At one point I was wondering if my async reading code was flawed, so I re-did everything using NSFileHandles and its -readInBackgroundAndNotify method. This behaved the same as when using GCD. (I originally picked GCD over the NSFileHandle API as there doesn't appear to be any async writing support in NSFileHandle)

Questions

Having arrived at this point after well over a day of futile attempts, I could do with some kind of help. Is there some fundamental problem with what I'm trying to do? Why does hooking up stdin to the pty terminate the program? I'm not closing the master end of the pty, so it shouldn't be receiving EOF. Leaving aside stdin, why am I only getting one line's worth of output? Is there a problem with the way I'm performing I/O on the pty's file descriptor? Am I using the master and slave ends correctly - master in the controlling process, slave in the NSTask?

What I haven't tried

I so far have only performed non-blocking (asynchronous) I/O on pipes and ptys. The only thing I can think of is that the pty simply doesn't support that. (if so, why does fcntl(fd, F_SETFL, O_NONBLOCK); succeed though?) I can try doing blocking I/O on background threads instead and send messages to the main thread. I was hoping to avoid having to deal with multithreading, but considering how broken all these APIs seem to be, it can't be any more time consuming than trying yet another permutation of async I/O. Still, I'd love to know what exactly I'm doing wrong.

The problem is likely that the stdio library inside is buffering output. The output will only appear in the read pipe when the command-line program flushes it, either because it writes a "\n" via the stdio library, or fflush()s, or the buffer gets full, or exits (which causes the stdio library to automatically flush any output still buffered), or possibly some other conditions. If those printf strings were "\n"-terminated, then you MIGHT the output quicker. That's because there are three output buffering styles -- unbuffered, line-buffered (\n causes a flush), and block buffered (when the output buffer gets full, it's auto-flushed).

Buffering of stdout is line-buffered by default if the output file descriptor is a tty (or pty); otherwise, block buffered. stderr is by default unbuffered. The setvbuf() function is used to change the buffering mode. These are all standard BSD UNIX (and maybe general UNIX) things I've described here.

NSTask does not do any setting up of ttys/ptys for you. It wouldn't help in this case anyway since the printfs aren't printing out \n.

Now, the problem is that the setvbuf() needs to be executed inside the command-line program. Unless (1) you have the source to the command-line program and can modify it and use that modified program, or (2) the command-line program has a feature that allows you to tell it to not buffer its output [ie, call setvbuf() itself], there's no way to change this, that I know of. The parent simply cannot affect the subprocess in this way, either to force flushing at certain points or change the stdio buffering behavior, unless the command-line utility has those features built into it (which would be rare).

Source: Re: NSTask, NSPipe's and interactive UNIX command