pcap_get_required_select_timeout.3pcap

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.TH PCAP_GET_REQUIRED_SELECT_TIMEOUT 3PCAP "29 January 2020"
.SH NAME
pcap_get_required_select_timeout \- get a timeout to be used when doing
select() for a live capture
.SH SYNOPSIS
.nf
.ft B
#include <pcap/pcap.h>
.ft
.LP
.ft B
const struct timeval *pcap_get_required_select_timeout(pcap_t *p);
.ft
.fi
.SH DESCRIPTION
.BR pcap_get_required_select_timeout ()
returns, on UNIX, a pointer to a
.B struct timeval
containing a value that must be used as the minimum timeout in
.BR select (2),
.BR poll (2),
.BR epoll_wait (2),
and
.BR kevent (2)
calls, or
.B NULL
if there is no such timeout.
If a
.RB non- NULL
value is returned, it must be used regardless of whether
.BR pcap_get_selectable_fd (3PCAP)
returns
.B \-1
for any descriptor on which those calls are being done.
.BR pcap_get_required_select_timeout ()
should be called for all
.BR pcap_t s
before a call to
.BR select (),
.BR poll (),
.BR epoll_wait (),
or
.BR kevent (),
and any timeouts used for those calls should be updated as appropriate
given the new value of the timeout.
.PP
For
.BR kevent (),
one
.B EVFILT_TIMER
filter per selectable descriptor can be used, rather than using the
timeout argument to
.BR kevent ();
if the
.B EVFILT_TIMER
event for a particular selectable descriptor signals an event,
.BR pcap_dispatch (3PCAP)
should be called for the corresponding
.BR pcap_t .
.PP
On Linux systems with
.BR timerfd_create (2),
one timer object created by
.BR timerfd_create ()
per selectable descriptor can be used, rather than using the timeout
argument to
.BR epoll_wait ();
if the
timer object for a particular selectable descriptor signals an event,
.BR pcap_dispatch (3PCAP)
should be called for the corresponding
.BR pcap_t .
.PP
Otherwise, a timeout value no larger than
the smallest of all timeouts returned by
.BR \%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,
.BR pcap_dispatch (3PCAP)
should be called for all
.BR pcap_t s
for which a
.RB non- NULL
timeout was returned, regardless of whether it's indicated as having
anything to read from it or not.
.PP
All devices with a
.RB non- NULL
timeout must be put in non-blocking mode with
.BR pcap_setnonblock (3PCAP).
.PP
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
.BR pcap_dispatch ()
or
.BR pcap_next_ex (3PCAP)
will return
.B 0
in this case, but will not block.
.PP
.BR pcap_get_required_select_timeout ()
is not available on Windows.
.SH RETURN VALUE
A pointer to a
.B struct timeval
is returned if the timeout is required; otherwise
.B NULL
is returned.
.SH BACKWARD COMPATIBILITY
This function became available in libpcap release 1.9.0.  In previous
releases,
.BR select (),
.BR poll (),
.BR epoll_wait (),
and
.BR kevent ()
could not be used for devices that don't provide a selectable file
descriptor (in other words, on any capture source for that
.BR pcap_get_selectable_fd ()
returns
.BR \-1 ).
.PP
In libpcap release 1.10.0 and later, the timeout value can change from
call to call, so
.BR pcap_get_required_select_timeout ()
must be called before each call to
.BR select (),
.BR poll (),
.BR epoll_wait (),
or
.BR 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
.BR pcap_get_required_select_timeout ()
should be changed to call it for each call to
.BR select (),
.BR poll (),
.BR epoll_wait (),
or
.BR kevent ()
even if the code must also work with libpcap 1.9.x.
.SH SEE ALSO
.BR pcap (3PCAP),
.BR pcap_get_selectable_fd (3PCAP),
.BR select (2),
.BR poll (2),
.BR epoll_wait (2),
.BR kqueue (2)