On Wed, 10 Aug 2005, Dirk GOUDERS wrote:
> This is intentational. We try to avoid having headers bring in more
> then absolutly required when included. I'm not sure what your second
> question means.
With my second question I wanted to ask if this intention is only for
kernel level code or a general one. I am asking this, because somewhen
in a project that I was not actually participating in I heard or read a
rule that roughly said: "all include files have to include all files
they depend on and compile cleanly", but that project was on a user
space program.
In general, in the role the operating system vendor, it's important to
minimize "header pollution" as much as possible. Unlike C++, C doesn't
have a notion of structured use of the name space, and if things are
massively nested included, that dramatically increases the chance of a
conflict of use between "the system" and a user application. You'll
notice that increasingly, FreeBSD-specific defines are prefixed with '_',
as that indicates use of reserved "you're the system" symbol space. For
example, the "#ifdef KERNEL"'s all over the place became "#ifdef _KERNEL",
as there's no reason an application shouldn't use a define named KERNEL.
The rules are a bit different if you're the application, although it's in
your interest to include as a few unnecessary headers as possible, to
reduce the chances of getting definitions that conflict with your
application.
Robert N M Watson
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