Re: recvmsg() "short receive" after FIONREAD

From: Andriy Gapon <>
Date: Fri, 10 Sep 2021 19:38:21 UTC
On 10/09/2021 22:35, Mark Johnston wrote:
> On Fri, Sep 10, 2021 at 10:15:37PM +0300, Andriy Gapon wrote:
>> On 10/09/2021 21:51, Andriy Gapon wrote:
>>> I observe a problem with the code that can be seen here:
>>> The code uses ioctl(FIONREAD) to check the size of available data in a socket.
>>> Does / should this work?
>>> Then the code calls recvmsg() on the socket with single vector with iov_len
>>> equal to the size obtained earlier.
>>> But the return value from recvmsg() is smaller than the iov_len value.
>>> In my test I see 215 vs expected 263 (so, the difference is 48).
>>> Does this ring a bell to anyone?
>>> I see this on a month old 14.0-CURRENT arm64.
>>   From a quick look at soreceive_dgram() and some dtrace-ing, it seems that each
>> time recvmsg() is called soreceive_dgram() gets an mbuf chain where the first
>> mbuf is MT_SONAME (8), the second one is MT_CONTROL (14) and only the third one
>> is MT_DATA.
>> Could it be that data in the first two mbuf-s (especially the MT_CONTROL one) is
>> reported by FIONREAD?  Or, in other words, accounted in sb_acc?
>> But then it's not actually returned, of course, in recvmsg() ?
> Indeed, I suspect that this is the problem.  Note that for
> kevent(EVFILT_READ) we subtract the number of control message bytes from
> the returned value, see filt_soread().  I wonder if FIONREAD should do
> the same thing.

Thank you for the suggestion.
I think that it is a reasonable expectation that FIONREAD returns a number of 
bytes that can be actually read.
I'll look at filt_soread().

Thank you!

Andriy Gapon