PCAP_GET_REQUIRED_SELECT_TIMEOUT
Section: Misc. Reference Manual Pages (3PCAP)
Updated: 29 January 2020
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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
no-
NULL
value is returned, it must be used regardless of whether
pcap_get_selectable_fd(3PCAP)
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(3PCAP)
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(3PCAP)
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(3PCAP)
should be called for all
pcap_ts
for which a
no-NULL
timeout was returned, regardless of whether it's indicated as having
anything to read from it or not.
All devices with a
no-NULL
timeout must be put in no-blocking mode with
pcap_setnonblock(3PCAP).
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(3PCAP)
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(3PCAP),
pcap_get_selectable_fd(3PCAP),
select(2),
poll(2),
epoll_wait(2),
kqueue(2)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- BACKWARD COMPATIBILITY
-
- SEE ALSO
-
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