Message-ID: <4D77C5CB.1040909@tweerlei.de>
Date: Wed, 09 Mar 2011 19:24:11 +0100
From: Robert Wruck <wruck@tweerlei.de>
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To: cygwin@cygwin.com
Subject: Re: 1.7.8: write fails with EAGAIN
References: <4D72992D.4090007@tweerlei.de> <20110307103951.GL6393@calimero.vinschen.de>
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Hi,

> Hang on, you are saying that a *blocking* write(2) to a pipe returns
> with EAGAIN?  Are you sure?  It would be quite a surprise if git would
> actually do that.  EAGAIN is only an expected error for non-blocking
> I/O, so applications which use blocking I/O usually only test for EINTR.

I checked out the winsup sources and built a cygwin1.dll for debugging 
(btw: Could you update the FAQ to state that gcc4 is required instead of 
gcc?).

What I found out:

In fhandler.cc, write_overlapped, there is the call to WriteFile:

	bool res = WriteFile (get_output_handle (), ptr, len, &nbytes,
			      get_overlapped ());

Surprisingly, this returns FALSE / nbytes = 0 / GetLastError = 
ERROR_INVALID_HANDLE on WinXP if the number of bytes exceeds some 
maximum and the handle refers to a pipe.
Since you don't know what kind of handle you're writing to in 
write_overlapped, it seems reasonable to do the write in chunks.

The first comment to WriteFile 
(http://msdn.microsoft.com/en-us/library/aa365747%28v=vs.85%29.aspx) 
suggests to limit the size to less than 32MB per write - although I 
didn't have this problem on X64.

-Robert

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