On Mon, 15 Mar 2010, Poul-Henning Kamp wrote:
In message <20100316024446.a24...@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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