Hi,
Looks like CYGWIN defines but does not honor the SOCK_NONBLOCK flag when used
with socket(2).
(It also defines SOCK_CLOEXEC but I haven't checked whether it is honored --
full disclosure.)
Consider the following code:
$ cat bug.c
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <netinet/in.h>
#include <sys/socket.h>
#ifdef BUG
# define SOCK_TYPE (SOCK_STREAM | SOCK_NONBLOCK)
#else
# define SOCK_TYPE SOCK_STREAM
#endif
int main()
{
char buf[80];
ssize_t n;
struct sockaddr_in in;
int s = socket(AF_INET, SOCK_TYPE, 0);
#ifndef BUG
fcntl(s, F_SETFL, O_NONBLOCK);
#endif
memset(&in, 0, sizeof(in));
in.sin_family = AF_INET;
in.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
in.sin_port = htons(6666);
if (connect(s, (struct sockaddr*) &in, sizeof(in)) < 0) {
int err;
socklen_t len = sizeof(err);
struct pollfd pfd;
if (errno != EINPROGRESS) {
printf("connect failed immediately: %m\n");
return 1;
}
memset(&pfd, 0, sizeof(pfd));
pfd.fd = s;
pfd.events = POLLOUT;
if (!poll(&pfd, 1, 1000)) {
printf("poll timed-out\n");
return 2;
}
if (pfd.revents & (POLLERR | POLLHUP)) {
printf("poll failed 0x%04X\n", pfd.revents);
return 3;
}
if (getsockopt(s, SOL_SOCKET, SO_ERROR, (void*) &err, &len) != 0) {
printf("getsockopt failed: %m\n");
return 4;
}
if (err != 0) {
printf("connect failed: %s\n", strerror(err));
return 5;
}
}
errno = 0;
n = recv(s, buf, sizeof(buf), 0);
printf("Got %zd byte(s): %m\n", n);
return 0;
}
When the program is built on Linux with and without the macro BUG defined, the
result is all the same,
the last recv() correctly finishes with -1 and EAGAIN (run "nc -l 6666" as a
server).
$ gcc -Wall -o bug bug.c
$ ./bug
Got -1 byte(s): Resource temporarily unavailable
$ gcc -Wall -DBUG -o bug bug.c
$ ./bug
Got -1 byte(s): Resource temporarily unavailable
However, on CYGWIN when the code is compiled with -DBUG, the last recv()
becomes blocking, and the program never exits:
$ gcc -Wall -o bug bug.c
$ ./bug
Got -1 byte(s): Resource temporarily unavailable
$ gcc -Wall -DBUG -o bug bug.c
$ ./bug
...
strace confirms that the recv() has blocked the execution. Also, connect()
appears to be blocking, too, but it's harder to see.
Thanks,
Anton Lavrentiev
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