Re: [Xen-devel] [PATCH 17/28] libxl: gettimeofday doesn't return an errno on failure

[Matthew Daley <mattjd@xxxxxxxxx>]

On Thu, Sep 26, 2013 at 12:04 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2013-09-18 at 15:37 +1200, Matthew Daley wrote:
>
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html
> agrees with this but the Linux manpages gettimeofday(2) disagrees:
>        gettimeofday() and settimeofday() return 0 for success,  or  -1  for
>        failure (in which case errno is set appropriately).
>
> They may just have confused themselves by lumping get in with set, but
> at least one error code (EFAULT) seems like it could apply to get too.

I think my lack of description and my use of the "MUST" macro is
confusing you here. I agree that they can return values signifying
failure, but indeed they don't return error codes directly, instead
they return -1 and set errno.

>
> Since the spec says it cannot fail I think there's no harm in reporting
> errno if it does fail.
>
> is the CHK_ERRNO macro correct?
> #define CHK_ERRNO( call ) ({                                            \
>         int chk_errno = (call);                                         \
>         if (chk_errno < 0) {                                                \
>             fprintf(stderr,"xl: fatal error: %s:%d: %s: %s\n",          \
>                     __FILE__,__LINE__, strerror(chk_errno), #call);     \
>             exit(-ERROR_FAIL);                                          \
>         }                                                               \
>     })
>
> It seems to report the reutrn code and not errno, so it will always say
> -1 won't it?
>
> I think the actual coverity error "chk_errno" is passed to a parameter
> that cannot be negative." stems from this not the lack of errno, because
> coverity seems to know that strerror cannot take a negative number.

Right, and this is what I was trying to avoid by changing to using the
MUST macro, but I suppose the lack of errno reportage is unacceptable
(and I guess that MUST is meant for internal xl functions that return
a exit()able return code, not just -1. Really, I think another macro
is needed that gets errno not from the call's return value, but errno
directly?

- Matthew

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