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  20 .TH PCAP_GET_REQUIRED_SELECT_TIMEOUT 3PCAP "29 January 2020"
  21 .SH NAME
  22 pcap_get_required_select_timeout \- get a timeout to be used when doing
  23 select() for a live capture
  24 .SH SYNOPSIS
  25 .nf
  26 .ft B
  27 #include <pcap/pcap.h>
  28 .ft
  29 .LP
  30 .ft B
  31 const struct timeval *pcap_get_required_select_timeout(pcap_t *p);
  32 .ft
  33 .fi
  34 .SH DESCRIPTION
  35 .BR pcap_get_required_select_timeout ()
  36 returns, on UNIX, a pointer to a
  37 .B struct timeval
  38 containing a value that must be used as the minimum timeout in
  39 .BR select (2),
  40 .BR poll (2),
  41 .BR epoll_wait (2),
  42 and
  43 .BR kevent (2)
  44 calls, or
  45 .B NULL
  46 if there is no such timeout.
  47 If a
  48 .RB non- NULL
  49 value is returned, it must be used regardless of whether
  50 .BR pcap_get_selectable_fd (3PCAP)
  51 returns
  52 .B \-1
  53 for any descriptor on which those calls are being done.
  54 .BR pcap_get_required_select_timeout ()
  55 should be called for all
  56 .BR pcap_t s
  57 before a call to
  58 .BR select (),
  59 .BR poll (),
  60 .BR epoll_wait (),
  61 or
  62 .BR kevent (),
  63 and any timeouts used for those calls should be updated as appropriate
  64 given the new value of the timeout.
  65 .PP
  66 For
  67 .BR kevent (),
  68 one
  69 .B EVFILT_TIMER
  70 filter per selectable descriptor can be used, rather than using the
  71 timeout argument to
  72 .BR kevent ();
  73 if the
  74 .B EVFILT_TIMER
  75 event for a particular selectable descriptor signals an event,
  76 .BR pcap_dispatch (2)
  77 should be called for the corresponding
  78 .BR pcap_t .
  79 .PP
  80 On Linux systems with
  81 .BR timerfd_create (2),
  82 one timer object created by
  83 .BR timerfd_create ()
  84 per selectable descriptor can be used, rather than using the timeout
  85 argument to
  86 .BR epoll_wait ();
  87 if the
  88 timer object for a particular selectable descriptor signals an event,
  89 .BR pcap_dispatch (2)
  90 should be called for the corresponding
  91 .BR pcap_t .
  92 .PP
  93 Otherwise, a timeout value no larger than
  94 the smallest of all timeouts returned by
  95 .BR \%pcap_get_required_select_timeout ()
  96 for devices from which packets will be captured and any other timeouts
  97 to be used in the call should be used as the timeout for the call, and,
  98 when the call returns,
  99 .BR pcap_dispatch (2)
 100 should be called for all
 101 .BR pcap_t s
 102 for which a
 103 .RB non- NULL
 104 timeout was returned, regardless of whether it's indicated as having
 105 anything to read from it or not.
 106 .PP
 107 All devices with a
 108 .RB non-NULL
 109 timeout must be put in non-blocking mode with
 110 .BR pcap_setnonblock (3PCAP).
 111 .PP
 112 Note that a device on which a read can be done without blocking may,
 113 on some platforms, not have any packets to read if the packet buffer
 114 timeout has expired.  A call to
 115 .BR pcap_dispatch ()
 116 or
 117 .BR pcap_next_ex (3PCAP)
 118 will return 0 in this case, but will not block.
 119 .PP
 120 .BR pcap_get_required_select_timeout ()
 121 is not available on Windows.
 122 .SH RETURN VALUE
 123 A pointer to a
 124 .B struct timeval
 125 is returned if the timeout is required; otherwise
 126 .B NULL
 127 is returned.
 128 .SH BACKWARD COMPATIBILITY
 129 This function became available in libpcap release 1.9.0.  In previous
 130 releases,
 131 .BR select (),
 132 .BR poll (),
 133 .BR epoll_wait (),
 134 and
 135 .BR kevent ()
 136 cannot be used on any capture source for which
 137 .BR pcap_get_selectable_fd ()
 138 returns \-1.
 139 .PP
 140 In libpcap release 1.10.0 and later, the timeout value can change from
 141 call to call, so
 142 .BR pcap_get_required_select_timeout ()
 143 must be called before each call to
 144 .BR select (),
 145 .BR poll (),
 146 .BR epoll_wait (),
 147 or
 148 .BR kevent (),
 149 and the new value must be used to calculate timeouts for the call.  Code
 150 that does that will also work with libpcap 1.9.x releases, so code
 151 using
 152 .BR pcap_get_required_select_timeout ()
 153 should be changed to call it for each call to
 154 .BR select (),
 155 .BR poll (),
 156 .BR epoll_wait (),
 157 or
 158 .BR kevent ()
 159 even if the code must also work with libpcap 1.9.x.
 160 .SH SEE ALSO
 161 .BR pcap (3PCAP),
 162 .BR pcap_get_selectable_fd (3PCAP),
 163 .BR select (2),
 164 .BR poll (2),
 165 .BR epoll_wait (2),
 166 .BR kqueue (2)