recvmsg() "short receive" after FIONREAD

From: Andriy Gapon <>
Date: Fri, 10 Sep 2021 18:51:38 UTC

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.

Andriy Gapon