I am sorry if this is a dumb question, but I was trying to understand the
FreeBSD TCP stack,
and In particular I was trying to understand the use of the TF_NEEDSYN flag.
This flag
is referenced a number of times in tcp_input.c and tcp_output.c, but I don't
think that
it can ever be set.
In particular grepping through the "../src/sys/netinet", one discovers that the
only code
that can set this flag is lines 1018 and 1020 of tcp_input.c. But, it appears
to me that
none of the lines in tcp_input.c from 999 thru 1023 are even reachable! The
reason they
are not reachable is because just ahead of them are the following lines:
if (!syncache_add(&inc, &to, th, &so, m))
goto drop;
if (so == NULL) {
... // uninteresting lines, but no gotos
return;
}
... /unreachable code here
Studying syncache_add (in file tcp_syncache.c), reveals three return
statements.
One of the returns, returns the value 0, which will cause the "goto drop" to
be executed.
The other two returns, return both the value 1 AND set "*sop = NULL", which
should cause
the following "if (so == NULL)" to execute the subsequent return statement.
Is this intentional? (i.e. dead code awaiting future development?), or a bug?
Or I am going blind to something obvious?
Thanx Barry Spinney.
(p.s. I doubt it matters which version of code, but to be precise this is from
the
/pub/FreeBSD/development/tarballs named "src_stable_6.tar.gz" dated "4/21/2013
01:15 AM",
gotten from ftp1.us.freebsd.org<ftp://ftp1.us.freebsd.org>)
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