Learn socket programming tips from netcat

Since netcat is honored as “TCP/IP swiss army knife”, I read its source code in OpenBSD to summarize some socket programming tips:

(1) Client connects in non-blocking mode:

......
s = socket(res->ai_family, res->ai_socktype |
            SOCK_NONBLOCK, res->ai_protocol);


......  
if ((ret = connect(s, name, namelen)) != 0 && errno == EINPROGRESS) {
        pfd.fd = s;
        pfd.events = POLLOUT;
        ret = poll(&pfd, 1, timeout));
}
......

Creating socket and set SOCK_NONBLOCK mode for it. Then calling connect() function, if ret is 0, it means connection is established successfully; if errno is EINPROGRESS, we can use timeout to control how long to wait; otherwise the connection can’t be built.

(2) The usage of poll():

......
/* stdin */
pfd[POLL_STDIN].fd = stdin_fd;
pfd[POLL_STDIN].events = POLLIN;

/* network out */
pfd[POLL_NETOUT].fd = net_fd;
pfd[POLL_NETOUT].events = 0;

/* network in */
pfd[POLL_NETIN].fd = net_fd;
pfd[POLL_NETIN].events = POLLIN;

/* stdout */
pfd[POLL_STDOUT].fd = stdout_fd;
pfd[POLL_STDOUT].events = 0;

......
/* poll */
num_fds = poll(pfd, 4, timeout);

/* treat poll errors */
if (num_fds == -1)
    err(1, "polling error");

/* timeout happened */
if (num_fds == 0)
    return;

/* treat socket error conditions */
for (n = 0; n < 4; n++) {
    if (pfd[n].revents & (POLLERR|POLLNVAL)) {
        pfd[n].fd = -1;
    }
}
/* reading is possible after HUP */
if (pfd[POLL_STDIN].events & POLLIN &&
    pfd[POLL_STDIN].revents & POLLHUP &&
    !(pfd[POLL_STDIN].revents & POLLIN))
    pfd[POLL_STDIN].fd = -1;

Usually, we just need to care about file descriptors for reading:

pfd[POLL_STDIN].fd = stdin_fd;
pfd[POLL_STDIN].events = POLLIN;

no need to monitor file descriptors for writing:

/* network out */
pfd[POLL_NETOUT].fd = net_fd;
pfd[POLL_NETOUT].events = 0;

According to poll() manual from OpenBSD, if no need for “high-priority” (maybe out-of-band) data, POLLIN is enough, otherwise the monitor events should be POLLIN|POLLPRI. And this is similar for POLLOUT and POLLWRBAND.

There are 3 values(POLLERR, POLLNVAL and POLLHUP) which are only used in struct pollfd‘s revents. If POLLERR or POLLNVAL is detected, it’s not necessary to poll this file descriptor furthermore:

if (pfd[n].revents & (POLLERR|POLLNVAL)) {
    pfd[n].fd = -1;
}

We should pay more attention to POLLHUP:
(a)

POLLHUP

The device or socket has been disconnected. This event and POLLOUT are mutually-exclusive; a descriptor can never be writable if a hangup has occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not mutually-exclusive. This flag is only valid in the revents bitmask; it is ignored in the events member.

(b)

The second difference is that on EOF there is no guarantee that POLLIN will be set in revents, the caller must also check for POLLHUP.

So it means if POLLHUP and POLLIN are both set in revents, there must be data to read (maybe EOF?), otherwise if only POLLHUP is checked, there is no data to read from.

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