Unix domain socket: can't test for blocking before sendto

I have two processes connected through Unix domain socket. In the
"sender" process I am calling select in order to check if sendto call
will block, because the receiver can't keep pace with the sender. From
the some reason select _always_ indicates that there will be no
blocking, but when I call sendto with MSG_DONTWAIT flag, it fails with
errno equal to EAGAIN.

My question is: why select always marks the socket as ready ?

The receiver code (in the sake of simplicity it never reads from the
socket):

sock = socket(PF_UNIX, SOCK_DGRAM, 0);

memset(&addr, 0, sizeof(addr));
addr.sun_family = PF_UNIX;
sprintf(addr.sun_path, "/tmp/receiver");
unlink("/tmp/receiver");

stat = bind(sock, (struct sockaddr *)&addr, sizeof(struct
sockaddr_un));

while(1)
;

The sender code:

sock = socket(PF_UNIX, SOCK_DGRAM, 0);

memset(&addr, 0, sizeof(addr));
addr.sun_family = PF_UNIX;
sprintf(addr.sun_path, "/tmp/sender.%d", getpid());

stat = bind(sock, (struct sockaddr *)&addr, sizeof(struct
sockaddr_un));

addr.sun_family = PF_UNIX;
sprintf(addr.sun_path, "/tmp/receiver");

while(1)
{
FD_ZERO(&wfds);
FD_SET(sock, &wfds);
tv.tv_sec = TOUT_SEC;
tv.tv_usec = TOUT_USEC;

do
stat = select(sock+1, 0, &wfds, 0, &tv);
while (-1 == stat && EINTR == errno);

/*
FD_ISSET is always true and after a couple of successful
calls
sendto starts to fail with EAGAIN
*/
if (FD_ISSET(sock, &wfds))
stat = sendto(sock, msg, sizeof(msg), MSG_DONTWAIT,
(struct sockaddr *)&addr, sizeof(addr));
}

Any help will be greatly appreciated.

Poluekt <(E-Mail Removed)> wrote:
> I have two processes connected through Unix domain socket. In the
> "sender" process I am calling select in order to check if sendto call
> will block, because the receiver can't keep pace with the sender. From
> the some reason select _always_ indicates that there will be no
> blocking, but when I call sendto with MSG_DONTWAIT flag, it fails with
> errno equal to EAGAIN.
>
> My question is: why select always marks the socket as ready ?

Are you sure this behaivour is in error?

1) Check your return codes for errors.

2) Run your program through strace (or truss on other platforms) to see
what your system calls are doing.

--
Cameron Kerr
(E-Mail Removed) : http://nzgeeks.org/cameron/
Empowered by Perl!

Hi,

I am facing the same problem. Was this issue fixed?

Alternatively, what are the options to deal with the "Resource temporarily unavailable" error returned with Unix domain sockets, besides
1) Increase the receive buffer size (/proc/sys/net/unix/amx_dgram_qlen)
2) Check if send will fail using select (this does not seem to be working)

Thank you for your time,
Vidya