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Subjectsetsockopt() doubles SO_RCVBUF
Date
Hello,

In a project, I'm working on, I need to set the socket receive buffer
size, so I used
setsockopt(fd, SOL_SOCKET, SO_RCVBUF, ...). To make sure, it worked
properly, I read the size back with getsockopt(fd, SOL_SOCKET,
SO_RCVBUF, ...). I was surprised to find, that the value read back was
always twice the size of, what I (tried to) set it to - up to twice the
size of rmem_max.

I found the cause of this behaviour in net/core/sock.c, which contains
the following lines of code in sock_setsockopt:

case SO_RCVBUF:
/* Don't error on this BSD doesn't and if you
think
about it this is right. Otherwise apps have
to
play 'guess the biggest size' games.
RCVBUF/SNDBUF
are treated in BSD as hints */

if (val > sysctl_rmem_max)
val = sysctl_rmem_max;

sk->userlocks |= SOCK_RCVBUF_LOCK;
/* FIXME: is this lower bound the right one? */
if ((val * 2) < SOCK_MIN_RCVBUF)
sk->rcvbuf = SOCK_MIN_RCVBUF;
else
sk->rcvbuf = (val * 2);
break;

Is it intentional, that val is doubled?

Regards,
Anders K. Pedersen
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