Updated: 2022/Sep/29

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PCAP_GET_REQUIRED_SELECT_TIMEOUT(3)                   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)