Attempting to receivce zero-length message with recvmsg

[Gary Jennejohn]

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