07-31-2014
Somewhere in fctl() is O_NONBLOCK, which changes your sockets or whatever fd so it does not block.
https://www.unix.com/man-page/opensolaris/0/fcntl/ Blocking is the default. You can avoid blocking issues by using thread per fd/direction, poll()/select() to detect what is ready, aio (async IO) or nonblocking fd's.
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LEARN ABOUT NETBSD
pcap_get_selectable_fd
PCAP_GET_SELECTABLE_FD(3) Library Functions Manual PCAP_GET_SELECTABLE_FD(3)
NAME
pcap_get_selectable_fd - get a file descriptor on which a select() can be done for a live capture
SYNOPSIS
#include <pcap/pcap.h>
int pcap_get_selectable_fd(pcap_t *p);
DESCRIPTION
pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for a file descriptor on which one can do a select() or poll() to wait
for it to be possible to read packets without blocking, if such a descriptor exists, or -1, if no such descriptor exists. Some network
devices opened with pcap_create() and pcap_activate(), or with pcap_open_live(), do not support select() or poll() (for example, regular
network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so -1 is returned for those devices.
Note that on most versions of most BSDs (including Mac OS X) select() and poll() do not work correctly on BPF devices;
pcap_get_selectable_fd() will return a file descriptor on most of those versions (the exceptions being FreeBSD 4.3 and 4.4), a simple
select() or poll() will not return even after the read timeout expires. To work around this, an application that uses select() or poll()
to wait for packets to arrive must put the pcap_t in non-blocking mode, and must arrange that the select() or poll() have a timeout less
than or equal to the read timeout, and must try to read packets after that timeout expires, regardless of whether select() or poll() indi-
cated that the file descriptor for the pcap_t is ready to be read or not. (That workaround will not work in FreeBSD 4.3 and later; how-
ever, in FreeBSD 4.6 and later, select() and poll() work correctly on BPF devices, so the workaround isn't necessary, although it does no
harm.)
Note also that poll() doesn't work on character special files, including BPF devices, in Mac OS X 10.4 and 10.5, so, while select() can be
used on the descriptor returned by pcap_get_selectable_fd(), poll() cannot be used on it those versions of Mac OS X. Kqueues also don't
work on that descriptor. poll(), but not kqueues, work on that descriptor in Mac OS X releases prior to 10.4; poll() and kqueues work on
that descriptor in Mac OS X 10.6 and later.
pcap_get_selectable_fd() is not available on Windows.
RETURN VALUE
A selectable file descriptor is returned if one exists; otherwise, -1 is returned.
SEE ALSO
pcap(3), select(2), poll(2)
5 April 2008 PCAP_GET_SELECTABLE_FD(3)