Date: Mon, 10 Sep 2018 19:10:53 +0200 From: Tamas Szakaly <sghctoma@gmail.com> To: freebsd-hackers@freebsd.org Subject: Re: Attempting to receivce zero-length message with recvmsg Message-ID: <20180910171053.3jvgw3i7fp6hetxe@pamparam> In-Reply-To: <20180910172832.2c0a1357@ernst.home> References: <20180910085833.d4py4ladlyqchjvo@pamparam> <20180910172832.2c0a1357@ernst.home>
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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@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
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