Matthew Dempsky <matt...@dempsky.org> writes:
> On Wed, May 14, 2014 at 4:58 AM, Jérémie Courrèges-Anglas
> <j...@wxcvbn.org> wrote:
>> if (cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
>> cmsg->cmsg_level == SOL_SOCKET &&
>> cmsg->cmsg_type == SCM_RIGHTS) {
>> passed_fd = *(int *)CMSG_DATA(cmsg);
>
> In your test program you only ever send one FD, so this is fine; but
> in general multiple FDs can be sent at a time, and you'll receive a
> single SOL_SOCKET/SCM_RIGHTS control message with an array of N ints.
>
> E.g., see
> https://code.google.com/p/chromium/codesearch#chromium/src/base/posix/unix_domain_socket_linux.cc&l=130.
Yup, I was trying to emulate the behavior of a a father process which
sends its child one socket to consume, from time to time.
>> So the two issues show up when you want to call sendmsg and recvmsg
>> with no data at all, but only control messages. sendmsg allocates an
>> empty mbuf for data (m_len == 0), this mbuf is not discarded and freed
>> by soreceive for SOCK_STREAM sockets, and stays in the socket buffer.
>>
>> I understand that this API was probably not designed to pass only
>> control messages, and no data at all. But it is easily fixable, and the
>> manpage could provide portability information.
>
> Blah, I'm not really a fan of allowing sending zero-length data
> messages. It makes it tricky for receivers to tell if they actually
> received an EOF or not.
For SOCK_DGRAM / SOCK_SEQPACKET, a zero-length message is fine.
For SOCK_STREAM, it sounds weird to wake up the listening process when
it's going to receive 0 as a return value, indeed.
> But if we're going to allow it (which we do currently it seems), then
> your diff seems like a reasonable fix to me. mbufs aren't my
> specialty though. =/
I'm not saying it's reasonable. If people have a clearer view of the
situation than me, I'd be glad to hear it. I lack available brain cells
these days... and after all it's not really an issue, just a "funny"
corner case.
--
jca | PGP : 0x1524E7EE / 5135 92C1 AD36 5293 2BDF DDCC 0DFA 74AE 1524 E7EE
