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Date: Sun, 16 Sep 2001 01:39:32 -0400
From: Christopher Faylor <cgf AT redhat DOT com>
To: cygwin-developers AT cygwin DOT com
Subject: Re: possible explanation for make hang
Message-ID: <20010916013932.A30789@redhat.com>
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References: <20010915234433 DOT A30407 AT redhat DOT com> <009601c13e6d$c4466670$0200a8c0 AT lifelesswks>
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On Sun, Sep 16, 2001 at 03:09:29PM +1000, Robert Collins wrote:
>----- Original Message -----
>From: "Christopher Faylor" <cgf AT redhat DOT com>
>To: <cygwin-developers AT cygwin DOT com>
>Sent: Sunday, September 16, 2001 1:44 PM
>Subject: possible explanation for make hang
>
>
>> I don't know precisely what make is doing but it is possible that
>> if more than one program is attempting to read from the same pipe
>> there will be a race that will cause cygwin to block in an
>uninterruptible
>> read.  So, if something like a SIGCHLD comes in while cygwin is
>reading
>> a pipe it won't be possible to deal with it.  I'm not aware of any
>> way to interrupt a blocking read on a pipe so cygwin will hang until
>> something shows up on the pipe.
>
>Can we wait on the pipe and a signal event?
>Ie
>==
>HANDLE handles[2];
>handles[0] = signalevent;
>handles[1] = pipe;
>dword rc = WaitForMultipleObjects (2, handles, FALSE, INFINITE);
>if ((rc - WAIT_OBJECT_0) == 1)
>  ReadFile (pipe);
>else
>  handle_signal;
>  /* the pipe may also have data, but can check again once the signal is
>handled*/
>==

You can't normally use WFMO for file handles.  I actually tried this an
hour or so ago to see if maybe it actually worked on Windows 95.  It
doesn't.

>> I thought I had a promising lead a few weeks ago but the Windows API
>> stood staunchly in my path, as usual.
>
>What was that? Perhaps a brain storm could find something?

Someone on the cygwin mailing list suggested just closing the pipe
handle in another thread as a way of forcing the read to return.
Unfortunately, the CloseHandle blocks when you do this.

I had another brainstorm that maybe if I deallocated the memory for
the buffer being used in the ReadFile, it would return.

Nope.

cgf