Re: recvmsg() "short receive" after FIONREAD

On 10/09/2021 21:51, Andriy Gapon wrote:
> 
> 
> I observe a problem with the code that can be seen here:
> https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/blob/master/src/modules/rtp/sap.c#L142 
> 
> 
> 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() ?

-- 
Andriy Gapon