Path: csiph.com!newsfeed.hal-mli.net!feeder3.hal-mli.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!feeder.erje.net!eu.feeder.erje.net!news-1.dfn.de!news.dfn.de!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Newsgroups: comp.os.linux.development.apps
Subject: Linux O_NONBLOCK bug/ quirk
Date: Thu, 27 Mar 2014 15:26:22 +0000
Lines: 38
Message-ID: <878urvu0gx.fsf@sable.mobileactivedefense.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Trace: individual.net 7joNofU8LzNdfnPw5t7elwes6Y9jZtSGBNMqKy9yBfY8kA+Og=
Cancel-Lock: sha1:q0TrmgERHyoeQ3AqkMU0hRv2UoI= sha1:HziHYD+KmOKAswp8nApLcm30Gfg=
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux)
Xref: csiph.com comp.os.linux.development.apps:665

As part of one of the usual 'pleasant exchanges' with the people whose
ability to make a living depends on controlling access to the Linux code
base, it came to light that a receive operation on a socket in
non-blocking mode can actually be blocked forever on Linux, example
code:

---------
#include <fcntl.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>

int main(void)
{
    struct sockaddr_un sun;
    int fd;

    fd = socket(AF_UNIX, SOCK_DGRAM, 0);
    sun.sun_family = AF_UNIX;
    strncpy(sun.sun_path, "/tmp/bla", sizeof(sun.sun_path));
    bind(fd, (struct sockaddr *)&sun, sizeof(sun));

    if (fork() == 0) read(fd, &fd, sizeof(fd));

    sleep(1);

    fcntl(fd, F_SETFL, O_NONBLOCK);
    read(fd, &fd, sizeof(fd));

    return 0;
}
--------

Killing the forked process results in the other aborting the read call
with EAGAIN, as can be determined with strace.

I don't think this is of much practical relevance but it is something
worth knowing about.