On Thu, Feb 16, 2017 at 11:43:59AM -0500, Jeff King wrote:
> On Thu, Feb 16, 2017 at 11:10:18AM +0100, Andreas Schwab wrote:
>
> > >> int xfclose(FILE *fp)
> > >> {
> > >> return ferror(fp) | fclose(fp);
> > >> }
> > >
> > > Yes, that's exactly what I had in mind (might be worth calling out the
> > > bitwise-OR, though, just to make it clear it's not a typo).
> >
> > Since the order of evaluation is unspecified, it would be better to
> > force sequencing ferror before fclose.
>
> Good point. Arguably the call in tempfile.c is buggy.
Here's a fix.
I think close_tempfile() suffers from the same errno problem discussed
earlier in this thread (i.e., that after calling it, you may get an
error return with a random, unrelated errno value if ferror() failed but
fclose() did not).
-- >8 --
Subject: [PATCH] tempfile: avoid "ferror | fclose" trick
The current code wants to record an error condition from
either ferror() or fclose(), but makes sure that we always
call both functions. So it can't use logical-OR "||", which
would short-circuit when ferror() is true. Instead, it uses
bitwise-OR "|" to evaluate both functions and set one or
more bits in the "err" flag if they reported a failure.
Unlike logical-OR, though, bitwise-OR does not introduce a
sequence point, and the order of evaluation for its operands
is unspecified. So a compiler would be free to generate code
which calls fclose() first, and then ferror() on the
now-freed filehandle.
There's no indication that this has happened in practice,
but let's write it out in a way that follows the standard.
Noticed-by: Andreas Schwab <schwab@xxxxxxxxxxxxxx>
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
tempfile.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/tempfile.c b/tempfile.c
index 2990c9242..ffcc27237 100644
--- a/tempfile.c
+++ b/tempfile.c
@@ -247,12 +247,8 @@ int close_tempfile(struct tempfile *tempfile)
tempfile->fd = -1;
if (fp) {
tempfile->fp = NULL;
-
- /*
- * Note: no short-circuiting here; we want to fclose()
- * in any case!
- */
- err = ferror(fp) | fclose(fp);
+ err = ferror(fp);
+ err |= fclose(fp);
} else {
err = close(fd);
}
--
2.12.0.rc1.559.gd292418bf