Close the stdin of boost::process child

Question

I'm trying to call a process with a string to its stdin, with Boost-1.64.0. The current code is :

  bp::opstream inStream ;
  bp::ipstream outStream;
  bp::ipstream errStream;

  bp::child child(
    command, // the command line
    bp::shell,
    bp::std_out > outStream,
    bp::std_err > errStream,
    bp::std_in  < inStream);


  // read the outStream/errStream in threads

  child.wait();

The problem is that the child executable is waiting for its stdin EOF. Here child.wait() is hanging indefinitely…

I tried to used asio::buffer, std_in.close(),… But no luck. The only hack I found was to delete() the inStream… And that's not really reliable.

How am I supposed to "notify" the child process and close its stdin with the new boost::process library ?

Thanks !

What do you mean, _"delete() the inStream"_? That doesn't make much sense to me. — sehe, May 29 '17 at 10:15

score 3 · Accepted Answer · answered May 29 '17 at 10:19

I tried to used asio::buffer, std_in.close()

This works. Of course it only works if you pass it to a launch function (bp::child constructor, bp::system, etc).

If you need to pass data, and then close it, simply close the associated filedescriptor. I do something like this:

boost::asio::async_write(input, bp::buffer(_stdin_data), [&input](auto ec, auto bytes_written){
    if (ec) {
        logger.log(LOG_WARNING) << "Standard input rejected: " << ec.message() << " after " << bytes_written << " bytes written";
    }
    may_fail([&] { input.close(); });
});

Where input is

bp::async_pipe input(ios);

Also, check that the process is not actually stuck sending the output! If you fail to consume the output it would be buffering and waiting if the buffer is full.

You saved my week. I suppose may_fail is your custom handler for exceptions, so I call input.close() directly. And you have to call ios.run() too :) — Salamandar, May 29 '17 at 11:02
Yup! Sorry for copying incomplete code this time :( http://paste.ubuntu.com/24701992/ — sehe, May 29 '17 at 11:19

score 2 · Answer 2 · answered May 30 '17 at 07:28

2

Closing the pipe by calling inStream.close(); when you're done writing to it. You can also close it while launching with bp::std_in.close().

The asio solution of course also works and avoids the danger of deadlocks.

answered May 30 '17 at 07:28

Klemens Morgenstern

783
4
18

Can you add to this answer? The OP used a boost::process::opstream object, which does not have a close() function. You link goes to documentation for a basic_pipe object, which does not seem to be a one-to-one replacement for boost::process::opstream. – DCTLib Sep 24 '18 at 11:21
You can access the underlying pipe by calling `opstream::pipe()`. – Klemens Morgenstern Oct 29 '18 at 11:29

Close the stdin of boost::process child

2 Answers2

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