Does FreeBSD have sendmmsg or recvmmsg system calls?

Bruce Evans brde at
Thu Jan 21 13:27:28 UTC 2016

On Thu, 21 Jan 2016, Konstantin Belousov wrote:

> On Wed, Jan 20, 2016 at 10:29:47AM +0200, Boris Astardzhiev wrote:

>> kb>Shouldn't i and rcvd be unsigned as well ?  Shouldn't return value
>> kb>also be unsigned ?
>> I think i and rcvd should be unsigned whereas ret should not - after all
>> if an error occurred we get -1.
> I looked at the real signatures and man pages for the Linux functions,
> finally.  There is indeed the timeout arg, the MSG_WAITFORONE flag for
> recvmmsg(3), and Linux uses ints for what would be naturally size_t.

It knows better than POSIX, so has restored the v7 types :-).  Probably
not intentionally.

FreeBSD-1 used the v7 types for recv* in FreeBSD-1, but this was changed
in 4.4BSD-Lite1.

>> kb>> +     rcvd = 0;
>> kb>> +     for (i = 0; i < vlen; i++) {
>> kb>> +             errno = 0;
>> kb>> +             ret = __sys_recvmsg(s, &msgvec[i].msg_hdr, flags);
>> kb>> +             if (ret < 0 || errno != 0) {
>> kb>I do not see why do you need to clear errno before, and then do this
>> test.
>> kb>Just check ret == -1, in which case errno was set from the immediate
>> syscall.
>> kb>
>> kb>> +                     if (rcvd != 0) {
>> kb>> +                             /* We've received messages. Let caller
>> know. */
>> kb>> +                             errno = 0;
>> kb>This cleaning is not needed as well.  For successfull functions returns,
>> kb>errno value is undefined.
>> Wouldn't I confuse apps if they check errno in the follow case - I want to
>> receive two messages. The first __sys_recvmsg succeeds and then for the
>> second __sys_recvmsg fails. Thus errno will be != 0 and I'm telling the app
>> that I have received one message by returning 1 but errno will be != 0.
>> Is this correct?
> errno value is only defined after the function explicitely returned error.
> Apps which test for errno without testing for error are wrong.

Mostly such tests find wrong implementations.  Only low quality
implmentations set error for non-errors.

I checked some details:

- both C99 and POSIX.1-2001 forbid setting errno to 0 in library functions

- C99 and draft C11 explicitly say that errno may be clobbered for
   non-errors by functions that are not documented to set errno.  It
   intends to also say that errno may not be clobbered for non-errors by
   functions are documented to set errno, but actually says nothing
   about this case (except in a footnote, it says "Thus [the usual
   method of setting errno before the call and checking it after
   ``should'' be used to check for errors]."  Footnotes are not part
   of the standard, so this also says nothing.  But it gives the intent.

- C99 doesn't say anything specific about this for strtol.  But strtol
   needs errno to not be set on non-error for its API.  This gives the
   intent of the documentation for errno in the standard itself.

- POSIX.1-2001 says that applications "should" only be examined when it is
   indicated to be valid by a function's return value.  This is wrong for
   all functions where errno is part of the API (like strtol), and it
   would be inconsistent with C99 and C11 if these standards said what they
   intend to say, for many more functions.  First, all of the C99 functions
   that are documented to set errno.  Then, if POSIX follows the spirit of
   the intent of C99, then just about all POSIX functions must not clobber
   errno for non-errors, since (unlike in C99) just about all functions in
   POSIX are documented to set errno.

- POSIX.1-2001 apparently agrees with me that C99 doesn't say what it
   intends to say.  It explicitly says that strtol shall not change "t
   setting of" errno on success, and marks this as an extension of C99.

- I couldn't find even a buggy generic clause in POSIX.1-2001 saying
   what C99 intends to say even for the C99 parts of POSIX.  If there
   were such a clause, then strtol wouldn't need a special extension.
   This means that some other C99 functions need special clauses, and
   there are likely to be bugs in this since some don't really need
   them except to conform with C99's intentions.

- not many C99 functions are specified to set errno.  The main ones
   are the strtol family, math functions (most can set ERANGE or EDOM),
   and wide character functions (many can set EILSEQ).  POSIX.1-2001
   seems to be missing a special clause for the first wide character
   function that I looked at -- fputwc().  For math functions, it has
   special statements ad nauseum (about 20 lines per function for
   duplicated for 20-50 functions).


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