pcap_get_required_select_timeout(3)
NAME
pcap_get_required_select_timeout - get a timeout to be used when doing
select() for a live capture
SYNOPSIS
#include <pcap/pcap.h>
pcap_get_required_select_timeout(3) *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 a
null pointer if there is no such timeout. If a non-null timeout
pointer 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 pointer was returned, regardless of whether it's indicated
as having anything to read from it or not.
All devices with a non-null timeout pointer 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 a null pointer 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)
6 September 2026
pcap_get_required_select_timeout(3)
libpcap 1.11.0 - Generated Tue Sep 15 08:25:29 CDT 2026