Re: Problem with socketpair , AF_UNIX and select call - can anybody through any light on this!

Howard Wilkinson wrote:

I wanted to add a socketpair connection from the master process to the servers which implement the actual functionality. I created the socketpair, set both ends non-blocking and added it to the select set of fd's. I get an immediate return saying that the socket is ready to read but when I issue a recv I get  -1 (EAGAIN). Nothing I have tried sets the required behaviour of only returning when there is 'real' data to read.

You're probably better off asking on a Posix programming list than here.  Wherever you ask, though, you should post the code in question, or code that demonstrates the problem in a state that can be compiled and verified by others.  While you've described what you think you've done, you haven't given enough information for us to do anything but guess at the problem.

int fd[2] = { -1, -1 };
fd_set rfd;
int rv;
char buf[1024];
struct timeval timeout = { 10, 0 };

rv = socketpair(PF_UNIX, SOCK_DGRAM, 0, fd);
if (rv < 0) Err("socketpair");

fcntl(fd[0], F_SETFL, fcntl(fd[0], F_GETFL, 0) | O_NONBLOCK);

FD_ZERO(rfd);
FD_SET(fd[0], rfd);

rv = select(fd[0]+1, &rfd, NULL, NULL, &timeout);

if (FD_ISSET(fd[0], rfd)) {
	printf("Socket is ready to read\n");
	rv = recv(fd[0], buf, sizeof(buf), MSG_DONTWAIT);
	if (rv < 0) Err("recv - %s(%d)", strerror(errno), errno);
	printf("Socket returned %d bytes\n", rv);
} else {
	printf("Socket is not ready to read\n");
}
exit (0);