pcap_get_selectable_fd(3pcap) — Linux manual page



NAME         top

       pcap_get_selectable_fd - get a file descriptor on which a
       select() can be done for a live capture

SYNOPSIS         top

       #include <pcap/pcap.h>

       int pcap_get_selectable_fd(pcap_t *p);

DESCRIPTION         top

       pcap_get_selectable_fd() returns, on UNIX, a file descriptor
       number for a file descriptor on which one can do a select(2),
       poll(2), epoll_wait(2), kevent(2), or other such call 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(3PCAP) and
       pcap_activate(3PCAP), or with pcap_open_live(3PCAP), do not
       support those calls (for example, regular network devices on
       FreeBSD 4.3 and 4.4, and Endace DAG devices), so -1 is returned
       for those devices.  In that case, those calls must be given a
       timeout less than or equal to the timeout returned by
       pcap_get_required_select_timeout(3PCAP) for the device for which
       pcap_get_selectable_fd() returned -1, the device must be put in
       non-blocking mode with a call to pcap_setnonblock(3PCAP), and an
       attempt must always be made to read packets from the device when
       the call returns.  If pcap_get_required_select_timeout() returns
       NULL, it is not possible to wait for packets to arrive on the
       device in an event loop.

       Note that a device on which a read can be done without blocking
       may, on some platforms, not have any packets to read if the
       packet buffer timeout has expired.  A call to
       pcap_dispatch(3PCAP) or pcap_next_ex(3PCAP) will return 0 in this
       case, but will not block.

       Note that in:

              FreeBSD prior to FreeBSD 4.6;

              NetBSD prior to NetBSD 3.0;

              OpenBSD prior to OpenBSD 2.4;

              Mac OS X prior to Mac OS X 10.7;

       select(), poll(), and kevent() 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), but a simple select(), poll(), or kevent() call will not
       indicate that the descriptor is readable until a full buffer's
       worth of packets is received, even if the packet timeout expires
       before then.  To work around this, code that uses those calls to
       wait for packets to arrive must put the pcap_t in non-blocking
       mode, and must arrange that the call have a timeout less than or
       equal to the packet buffer timeout, and must try to read packets
       after that timeout expires, regardless of whether the call
       indicated 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; however, in FreeBSD 4.6 and later, those calls work
       correctly on BPF devices, so the workaround isn't necessary,
       although it does no harm.)

       Note also that poll() and kevent() 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() and kevent() cannot be used on
       it those versions of Mac OS X.  poll(), but not kevent(), works
       on that descriptor in Mac OS X releases prior to 10.4; poll() and
       kevent() work on that descriptor in Mac OS X 10.6 and later.

       pcap_get_selectable_fd() is not available on Windows.

RETURN VALUE         top

       A selectable file descriptor is returned if one exists;
       otherwise, -1 is returned.

SEE ALSO         top

       pcap(3PCAP), kqueue(2)

COLOPHON         top

       This page is part of the libpcap (packet capture library)
       project.  Information about the project can be found at 
       ⟨http://www.tcpdump.org/⟩.  If you have a bug report for this
       manual page, see ⟨http://www.tcpdump.org/#patches⟩.  This page
       was obtained from the project's upstream Git repository
       ⟨https://github.com/the-tcpdump-group/libpcap.git⟩ on 2023-12-22.
       (At that time, the date of the most recent commit that was found
       in the repository was 2023-12-13.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

                             29 January 2020PCAP_GET_SELECTABLE_FD(3PCAP)