PCAP_GET_REQUIRED_SELECT_TIMEOUT(3)           FreeBSD Library Functions Manual

NAME

     pcap_get_required_select_timeout - get a timeout to be used when doing
     select() for a live capture

SYNOPSIS

     #include <pcap/pcap.h>

     const struct timeval *pcap_get_required_select_timeout(pcap_t *p);

DESCRIPTION

     pcap_get_required_select_timeout() returns, on UNIX, a pointer to a
     struct timeval containing a value that must be used as the minimum
     timeout in select(2), poll(2), epoll_wait(2), and kevent(2) calls, or
     NULL if there is no such timeout.  If a non-NULL value is returned, it
     must be used regardless of whether pcap_get_selectable_fd(3) returns -1
     for any descriptor on which those calls are being done.
     pcap_get_required_select_timeout() should be called for all pcap_ts
     before a call to select(), poll(), epoll_wait(), or kevent(), and any
     timeouts used for those calls should be updated as appropriate given the
     new value of the timeout.

     For kevent(), one EVFILT_TIMER filter per selectable descriptor can be
     used, rather than using the timeout argument to kevent(); if the
     EVFILT_TIMER event for a particular selectable descriptor signals an
     event, pcap_dispatch(3) should be called for the corresponding pcap_t.

     On Linux systems with timerfd_create(2), one timer object created by
     timerfd_create() per selectable descriptor can be used, rather than using
     the timeout argument to epoll_wait(); if the timer object for a
     particular selectable descriptor signals an event, pcap_dispatch(3)
     should be called for the corresponding pcap_t.

     Otherwise, a timeout value no larger than the smallest of all timeouts
     returned by pcap_get_required_select_timeout() for devices from which
     packets will be captured and any other timeouts to be used in the call
     should be used as the timeout for the call, and, when the call returns,
     pcap_dispatch(3) should be called for all pcap_ts for which a non-NULL
     timeout was returned, regardless of whether it's indicated as having
     anything to read from it or not.

     All devices with a non-NULL timeout must be put in non-blocking mode with
     pcap_setnonblock(3).

     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() or pcap_next_ex(3) will return 0
     in this case, but will not block.

     pcap_get_required_select_timeout() is not available on Windows.

RETURN VALUE

     A pointer to a struct timeval is returned if the timeout is required;
     otherwise NULL is returned.

BACKWARD COMPATIBILITY

     This function became available in libpcap release 1.9.0.  In previous
     releases, select(), poll(), epoll_wait(), and kevent() could not be used
     for devices that don't provide a selectable file descriptor (in other
     words, on any capture source for that pcap_get_selectable_fd() returns
     -1).

     In libpcap release 1.10.0 and later, the timeout value can change from
     call to call, so pcap_get_required_select_timeout() must be called before
     each call to select(), poll(), epoll_wait(), or kevent(), and the new
     value must be used to calculate timeouts for the call.  Code that does
     that will also work with libpcap 1.9.x releases, so code using
     pcap_get_required_select_timeout() should be changed to call it for each
     call to select(), poll(), epoll_wait(), or kevent() even if the code must
     also work with libpcap 1.9.x.

SEE ALSO

     pcap(3), pcap_get_selectable_fd(3), select(2), poll(2), epoll_wait(2),
     kqueue(2)

                                29 January 2020
                                           PCAP_GET_REQUIRED_SELECT_TIMEOUT(3)