1.5.24: data corruption problem with popen and gzip on a text mounted filesystem

Eric Blake ebb9@byu.net
Mon Jul 23 15:43:00 GMT 2007

Hugh Secker-Walker <hugh <at> merl.com> writes:

> I'm having trouble getting correct behavior on a third-party OpenSource 
> project that I'm building using Cygwin.  The problem involves the 
> writing of corrupt data to a file.  The output file is created and 
> written via popen("gzip > outputfile", "wb").  The data is fine if the 
> filesystem is mounted in binary mode.  The data is corrupted if the 
> filesystem is mounted in text mode.
> My understanding of the documentation is that pipes and shells' stdin 
> and stdout will be in binary mode if the CYGWIN env variable contains 
> the 'binmode' setting, regardless of the binary/text mount flag of the 
> filesystem the file is mounted on.

Indeed - so you are handing the data to gzip in binary mode.  The problem, then,
is likely not your program, but gzip - it seems to me that gzip should be 
forcing binary mode but does not.  cgf is the cygwin gzip maintainer, so 
hopefully he can do something about this.  In the meantime, I'll probably fire 
off a mail to the bug-gzip list with a patch (since the upstream maintainer for 
gzip also maintains coreutils, so he is already familiar with text vs. binary 
issues in other apps that he maintains).

As a workaround, you can force gzip to see stdout in binary mode by using an 
intermediary pipe, as in popen("gzip | cat > outputfile", "wb").

Eric Blake

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