svn commit: r205165 - head/lib/libc/gen

[Bruce Evans]

On Tue, 16 Mar 2010, Bruce Evans wrote:

> ...
> The unobvious points are:
> - why avoid triggering atexit(), etc. processing in daemon()?
> - why isn't this documented?  Some callers of daemon() need to know that
>  it won't flush open streams, etc., so that they can flush and their
>  open streams, etc., before calling daemon().  Callers should do this
>  anyway before calling any function that can exit (especially exit()
>  itself), so that they can actually check that the output went out,
>  but most are sloppy.  Callers should know if they have any open
>  streams but they would have a hard time telling what else they are
>  missing from not calling exit().  There may be profiling cleanup
>  (profiling won't work right for the child?) and other magic destructors.

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.
- unflushed input in stdin may cause problems, since although stdin
   is redirected, the input may be in stdio's buffers.
- it is unclear if stdio can do something better than crash when its
   fd's for stdinout/err are redirected without telling it.
- fd's above 2 are not touched, so the child inherits any open streams
   on these fd's.  These streams are likely to get flushed on exit() if
   not explicitly.

I think daeomon() should be doing an fflush(NULL) or fcloseall() and
most other cleanups done by atexit processing.  Before the fork() of
course, so that the child doesn't inherit stuff.  fcloseall() would
be too destructive if the fork() fails, but fflush(NULL) is almost
as clean.  At least the fflush(NULL) is safe since it has no effect
except for buggy callers that have unflushed streams.

Bruce