# pcap_get_required_select_timeout

## 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) returns **-1** for any descriptor on
           which those calls are being done.
           **pcap_get_required_select_timeout**() should be called for all
           **pcap_t** s 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_t** s 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**(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.
    


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