[systemd-devel] [PATCH 12/24] sd-dhcp6-client: Add DHCPv6 Solicit message creation and sending

[Lennart Poettering]

On Thu, 19.06.14 14:52, Patrik Flykt (Patrik.Flykt at linux.intel.com) wrote:

> On Wed, 2014-06-18 at 16:25 +0200, Zbigniew Jędrzejewski-Szmek wrote:
> > On Wed, Jun 18, 2014 at 07:05:35AM -0700, Filipe Brandenburger wrote:
> > > On Wed, Jun 18, 2014 at 6:58 AM, Zbigniew Jędrzejewski-Szmek
> > > <zbyszek at in.waw.pl> wrote:
> > > >> +        if (client->fd > 0)
> > > >> +                safe_close(client->fd);
> > > >> +        client->fd = -1;
> > > > client->fd = safe_close(client->fd);
> > > >
> > > > That's what safe_close is for :)
> > > 
> > > And shouldn't the check be for client->fd >= 0? Zero is a valid file descriptor.
> > Yeah... but note that safe_close already has the fd >= 0 check, so the
> > replacement line replaces the if too.
> 
> If I omit the client->fd > 0 check I get the following in the test case:
> 
> Assertion 'close_nointr(fd) != -EBADF' failed at src/shared/util.c:205,
> function safe_close(). Aborting.

safe_close() is a call that will ignore all kinds of errors except one:
EBADF. THe rationale for that is that the kernel has these weird
semantics that close() might fail with errors like EIO or whatever else
if something could't be written to disk or so, but the fd will still be
closed. Hence, you can invoke safe_close() in all those cases where you
just want to get rid of an fd, and don't care about anything else. Now,
it will only trip up on one specific problem which always indicates a
programming error: when close() returns EBADF, since that is the error
that is returned when you invoke close() on an fd that doesn't exist.

Putting this all together: safe_close() is basically your one stop
solution to getting rid of fds, and even updating your variable you
store it in:

fd = safe_close(fd);

safe_close() always returns -1, always gets rid of the fd, will be a NOP
if the fd is < 0. Will never fail. However, it if you invoke it the only
way that is a real programming error which is with an already-closed fd
or a never-opened fd, then it will assert() and die.

Hope this makes sense.

Or long words short: if the code tripped up like yours above, this is no
indication that safe_close() wasn't right. Instead it's an indication
that you are passing it a rubbish fd.

Lennart

-- 
Lennart Poettering, Red Hat