Attempting to receivce zero-length message with recvmsg

[Tamas Szakaly]

On Mon, Sep 10, 2018 at 05:28:32PM +0200, Gary Jennejohn wrote:
> On Mon, 10 Sep 2018 10:58:33 +0200
> Tamas Szakaly <sghctoma at gmail.com> wrote:
> 
> > Hi,
> > 
> > I have a question about the recvmsg syscall. According to POSIX, unless
> > O_NONBLOCK is set on the socket fd, recvmsg [1] should block until a message
> > arrives. However, recvmsg returns immediately with 0, if we are trying to
> > receive a 0-byte message from a SOCK_SEQPACKET AF_UNIX socket. Consider the
> > following code:
> > 
> > #include <stdio.h>
> > #include <sys/socket.h>
> > 
> > int main(int argc, char** argv) {
> > 	int sock[2];
> > 	socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sock);
> > 
> > 	struct msghdr msghdr = {0};
> > 	int ret = recvmsg(sock[1], &msghdr, 0);
> > 
> > 	printf("ret=%d, msghdr.msg_flags=0x%08x\n\n", ret, msghdr.msg_flags);
> > }
> > 
> > Running this yields this output:
> > 
> > [0x00 socketstuff]$ cc socketpair.c -o socketpair && ./socketpair
> > ret=0, msghdr.msg_flags=0x00000000
> > 
> > You can see that recvmsg returns with 0, even though there were no messages
> > sent, and neither of the sockets are closed, so it should block indefinitely.
> > 
> > Is this behavior intentional to match the semantics of read [2] (i.e.
> > attempting to read zero bytes should be a no-op)?
> > 
> > 
> > [1] recvmsg: http://pubs.opengroup.org/onlinepubs/9699919799/functions/recvmsg.html
> > [2] read: http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
> > 
> 
> You have to initalize msghdr.msg_iov and msghde.iov_len, otherwise
> the kernel notices there is no place to put a received message and
> simply returns 0.
> 
> I did that and recvmsg() did not return.
> 
> -- 
> Gary Jennejohn
> 

That's okay, but I want to receive exactly zero bytes. I can send such messages
with sendmsg, and I would like to receive them. Of course I can introduce some
dummy variable to initialize msg_iov and iov_len with, but it will never be
used, and it just feels ugly.

-- 
Tamas Szakaly
@sghctoma