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Date: Fri, 27 Feb 2015 09:43:11 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: freopen/fread/popen bug
Message-ID: <20150227084311.GF11124@calimero.vinschen.de>
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References: <54EFDEF4 DOT 4060308 AT cornell DOT edu>
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On Feb 26 22:05, Ken Brown wrote:
> I'm not sure exactly where the bug is, but here's what happens (STC at th=
e end):
>=20
> 1. I use freopen to open a file "foo" and associate it with stdin.
>=20
> 2. I use fread to read a byte from foo.
>=20
> 3. I call popen, expecting the child process to have foo as its stdin, wi=
th
> the file-position indicator pointing to the second byte.  But instead the
> child sees an empty stdin.
>=20
> If I omit step 2, the child process does indeed have foo as its stdin.  A=
re
> my expectations wrong, or is this a bug?

Wrong expectations.  Keep in mind that the default read mode using
stdio functions is buffered.  So your fread fills the buffer in f.
The buffer is typically something like 1K or 4K.  If the file is
shorter than that, the file pointer will be set to EOF when calling
popen.  Try this before calling fread:

  setvbuf(f, NULL, _IONBF, 0);


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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