On Wed, Mar 10, 2021 at 09:41:01AM -0800, Cole Helbling wrote:
> On Tue Mar 9, 2021 at 7:40 AM PST, Alyssa Ross wrote:
> > stdio can buffer output, so if we close stdout without going through
> > stdio, there might be buffered output that is never written.
> > ---
> > vsockserver.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/vsockserver.c b/vsockserver.c
> > index dd9a74a..196056a 100644
> > --- a/vsockserver.c
> > +++ b/vsockserver.c
> > @@ -90,7 +90,7 @@ int main(int argc, char *argv[])
> >
> > if (notify) {
> > printf("%" PRIu32 "\n", lport);
> > - close(STDOUT_FILENO);
> > + fclose(stdout);
> > }
> >
> > setenvf("VSOCKLOCALCID", 1, "%" PRIu32, lcid);
> > --
> > 2.30.0
>
> I don't write much (if any) C, so I never knew there was both
> STDOUT_FILENO (as an int) _and_ stdout (as a FILE*). I suppose the most
> important part is that fclose() uses fflush(3) under the hood to make
> sure we actually write everything.

Yeah, C on UNIX essentially gives you two different IO interfaces
(open/write/close/etc. from POSIX, and fopen/fprintf/fclose/etc. from
stdio in the C standard library).  The POSIX stuff doesn't know
anything about the buffering that stdio does on top of it.  Mixing them
is probably never a good idea if you're not doing it very deliberately.