Hello Wladimir,
On 3/7/19 1:32 PM, Wladimir Mutel wrote:
> My Linux is Ubuntu 18.04.2 with all latest updates, and its glibc is
> 2.27-3ubuntu1
> manpages are 4.15-1 , also I looked into man7.org after you announced 5.0
> I studied sources of zdkimfilter and found a place where they consult
> h_errno value after res_query
> returns -1.
> I turned to man res_query and did not find anything about h_errno
> surprised, I contacted zdkimfilter author and he told me this has
> always be this way as per BSD specification.
> also checked glibc sources and found that h_errno is really filled
> when underlying calls fail
Your original tweeted comment was:
"man 3 resolver still fails to mention whether these functions
should update h_errno or not. had to refer to FreeBSD man pages
to actually learn this"
resolver(3) documents 15 functions. I asked you about *which*
functions set h_errno, but you've told me about only one.
For the rest, you seem to guess rather than look more deeply.
>From a bit of a reading of the glibc code... The following appear
*not* to set h_errno:
res_init, resninit
res_nclose
res_mkquery, res_nmkquery (differs from FreeBSD man page)
res_send, res_nsend (but do set 'errno')
dn_comp, dn_expand
And the following appear to set h_errno:
res_query, res_nquery
res_search, res_nsearch
res_querydomain, res_nquerydomain
> what is left, is to specify this behavior in Linux man pages as
> explicitly as it is done in BSD
You have much more faith in the FreeBSD documentation than me. The
FreeBSD manual page [1] does not even document several functions
that the implementation appears to provide (res_nquery, res_nsearch,
res_nquerydomain, etc.) and says:
The res_mkquery(), res_search(), and res_query() functions return the
size of the response on success, or -1 if an error occurs. The integer
h_errno may be checked to determine the reason for error. See
gethostbyname(3) for more information.
But in my reading of the FreeBSD code, res_mkquery() does not
set h_errno (which is consistent with what I find in glibc).
I'm sure the there is more detail that could be added, but
by way of improving things, I've applied this patch:
diff --git a/man3/resolver.3 b/man3/resolver.3
index b50aa3611..54c5d2f7e 100644
--- a/man3/resolver.3
+++ b/man3/resolver.3
@@ -461,6 +461,20 @@ and
.BR dn_expand ()
functions return the length
of the compressed name, or \-1 if an error occurs.
+.PP
+In the case of an error return from
+.BR res_nquery (),
+.BR res_query (),
+.BR res_nsearch (),
+.BR res_search (),
+.BR res_nquerydomain (),
+or
+.BR res_querydomain (),
+the global variable
+.I h_errno
+(see
+.BR gethostbyname (3))
+can be consulted to determine the cause of the error.
.SH FILES
.TP
.I /etc/resolv.conf
Have a good weekend,
Michael
[1] https://www.freebsd.org/cgi/man.cgi?query=resolver&apropos=0&sektion=0&manpath=FreeBSD+12.0-RELEASE+and+Ports&arch=default&format=html
>
> чт, 7 мар. 2019 г. в 12:52, Michael Kerrisk (man-pages)
> <mtk.manpages@xxxxxxxxx>:
>>
>> Hello Wladimir,
>>
>> On Thu, 7 Mar 2019 at 10:57, Wladimir Mutel <muwlgr@xxxxxxxxx> wrote:
>>>
>>> in https://www.freebsd.org/cgi/man.cgi?query=resolver we read :
>>>
>>> The res_mkquery(), res_search(), and res_query() functions return the
>>> size of the response on success, or -1 if an error occurs. The integer
>>> h_errno may be checked to determine the reason for error. See
>>> gethostbyname(3) for more information.
>>>
>>> in http://man7.org/linux/man-pages/man3/resolver.3.html , h_errno is
>>> not mentioned
>>> but in the end of this page we read
>>>
>>> CONFORMING TO
>>> 4.3BSD.
>>>
>>> the fact that h_errno is really modified as specified by BSD, could be
>>> confirmed experimentally, but would not it be more convenient if this
>>> was mentioned documentally ?
>>
>> Have you verified it experimentally on Linux/glibc (and for which of
>> the functions)?
>>
>> Thanks,
>>
>> Michael
>>
>>
>>
>>
>> --
>> Michael Kerrisk
>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>> Linux/UNIX System Programming Training: http://man7.org/training/
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
