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From: "Lavrentiev, Anton (NIH/NLM/NCBI) [C] via cygwin" <cygwin AT cygwin DOT com>
Reply-To: "Lavrentiev, Anton (NIH/NLM/NCBI) [C]" <lavr AT ncbi DOT nlm DOT nih DOT gov>
To: "'cygwin AT cygwin DOT com'" <cygwin AT cygwin DOT com>
Subject: SOCK_NONBLOCK not honored
Date: Thu, 1 Nov 2018 20:56:22 +0000
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X-MIME-Autoconverted: from quoted-printable to 8bit by delorie.com id wA1Kue7s003645

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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