PCAP_GET_REQUIRED_SELECT_TIMEOUT(3PCAP)PCAP_GET_REQUIRED_SELECT_TIMEOUT(3PCAP)
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(3PCAP) re-
turns -1 for any descriptor on which those calls are being done.
pcap_get_required_select_timeout() should be called for all pcap_ts be-
fore 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 EV-
FILT_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 us-
ing 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 non-
NULL timeout was returned, regardless of whether it's indicated as hav-
ing anything to read from it or not.
All devices with a non-NULL timeout must be put in non-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 time-
out 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() cannot be used
on any capture source for which 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 be-
fore 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.
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.
SEE ALSO
pcap(3PCAP), pcap_get_selectable_fd(3PCAP), select(2), poll(2),
epoll_wait(2), kqueue(2)
29 JanuPCAP2GET_REQUIRED_SELECT_TIMEOUT(3PCAP)
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