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  20 .TH PCAP_GET_REQUIRED_SELECT_TIMEOUT 3PCAP "25 July 2018"
  21 .SH NAME
  22 pcap_get_required_select_timeout \- get a file descriptor on which a
  23 select() can be done 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 struct timeval *pcap_get_required_select_timeout(pcap_t *p);
  32 .ft
  33 .fi
  34 .SH DESCRIPTION
  35 .B 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 if
  45 .BR pcap_get_selectable_fd (3PCAP)
  46 returns
  47 .BR PCAP_ERROR .
  48 .PP
  49 The timeout that should be used in those calls must be no larger than
  50 the smallest of all timeouts returned by
  51 .B \%pcap_get_required_select_timeout()
  52 for devices from which packets will be captured.
  53 .PP
  54 The device for which
  55 .B pcap_get_selectable_fd()
  56 returned
  57 .B PCAP_ERROR
  58 must be put in non-blocking mode with
  59 .BR pcap_setnonblock (3PCAP),
  60 and an attempt must always be made to read packets from the device
  61 when the
  62 .BR select() ,
  63 .BR poll() ,
  64 .BR epoll_wait() ,
  65 or
  66 .B kevent()
  67 call returns.
  68 .PP
  69 Note that a device on which a read can be done without blocking may,
  70 on some platforms, not have any packets to read if the packet buffer
  71 timeout has expired.  A call to
  72 .BR pcap_dispatch (3PCAP)
  73 or
  74 .BR pcap_next_ex (3PCAP)
  75 will return 0 in this case, but will not block.
  76 .PP
  77 .B pcap_get_required_select_timeout()
  78 is not available on Windows.
  79 .SH RETURN VALUE
  80 A pointer to a
  81 .B struct timeval
  82 is returned if the timeout is required; otherwise
  83 .B NULL
  84 is returned.
  85 .SH BACKWARD COMPATIBILITY
  86 This function became available in libpcap release 1.9.0.  In previous
  87 releases,
  88 .BR select() ,
  89 .BR poll() ,
  90 .BR epoll_wait() ,
  91 and
  92 .B kevent()
  93 cannot be used on any capture source for which
  94 .B pcap_get_selectable_fd
  95 returns \-1.
  96 .SH SEE ALSO
  97 .BR pcap (3PCAP),
  98 .BR pcap_get_selectable_fd (3PCAP),
  99 .BR select (2),
 100 .BR poll (2),
 101 .BR epoll_wait (2),
 102 .BR kqueue (2)