Date: Wed, 17 Mar 2010 04:55:01 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r205165 - head/lib/libc/gen Message-ID: <20100317041500.V26092@delplex.bde.org> In-Reply-To: <44640.1268676905@critter.freebsd.dk> References: <44640.1268676905@critter.freebsd.dk>
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On Mon, 15 Mar 2010, Poul-Henning Kamp wrote:
> In message <20100316024446.A24853@delplex.bde.org>, Bruce Evans writes:
>> On Tue, 16 Mar 2010, Bruce Evans wrote:
>
>> Due to the way that daemon() works, it is really an error to have any
>> open streams when it is called. This is also undocumented, except
>> implicitly. The errors are:
>> - unflushed output on stdout and stderr won't get flushed normally by
^^^^^^^^
>> the child. stdout and stderr are redirected so it will go there if
>> the child erroneously (?) uses these streams or simply calls exit()
>> which will give the default flush.
>
> The in-core FILE buffers are copied in the child process, so they
> should most certainly not be flushed, but rather, as they correctly
> are, discarded, so that when the child causes the flush, the content
> is only output once.
Nope.
Even if the child causes the flush, the content of stdout and stderr
is not flushed normally since it is redirected to /dev/null. Unflushed
input in stdin is handled more brokenly: although stdin is redirected
to /dev/null, input on it can still be read via stdin's buffer.
Inheriting unflushed input on other streams is a feature (unless these
streams are open on fd's 0-2; then these streams will have the same
corruption as std* streams open on their normal fd's 0-2).
As described in the unquoted part of my reply above, all streams should
be flushed by the parent before the fork(), so that they are flushed
normally.
daemon(3) has a lot to say about problems with open fd's 0-2, but it
says nothing about the extra problems with open streams on these
descriptors. It documents as a feature that fd's above 2 are inherited
normally. It also says too little about the implementation always
using fork() (it cross-references fork() but doesn't say that the
context is always a child context on successful completion).
Bruce
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