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  22 .TH PCAP_GET_SELECTABLE_FD 3PCAP "5 April 2008"
  23 .SH NAME
  24 pcap_get_selectable_fd \- get a file descriptor on which a select() can
  25 be done for a live capture
  26 .SH SYNOPSIS
  27 .nf
  28 .ft B
  29 #include <pcap/pcap.h>
  30 .ft
  31 .LP
  32 .ft B
  33 int pcap_get_selectable_fd(pcap_t *p);
  34 .ft
  35 .fi
  36 .SH DESCRIPTION
  37 .B pcap_get_selectable_fd()
  38 returns, on UNIX, a file descriptor number for a file descriptor on
  39 which one can
  40 do a
  41 .B select()
  42 or
  43 .B poll()
  44 to wait for it to be possible to read packets without blocking, if such
  45 a descriptor exists, or \-1, if no such descriptor exists.  Some network
  46 devices opened with
  47 .B pcap_create()
  48 and
  49 .BR pcap_activate() ,
  50 or with
  51 .BR pcap_open_live() ,
  52 do not support
  53 .B select()
  54 or
  55 .B poll()
  56 (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace
  57 DAG devices), so \-1 is returned for those devices.
  58 .PP
  59 Note that on most versions of most BSDs (including Mac OS X)
  60 .B select()
  61 and
  62 .B poll()
  63 do not work correctly on BPF devices;
  64 .B pcap_get_selectable_fd()
  65 will return a file descriptor on most of those versions (the exceptions
  66 being FreeBSD 4.3 and 4.4), a simple
  67 .B select()
  68 or
  69 .B poll()
  70 will not return even after the read timeout expires.  To work around
  71 this, an application that uses
  72 .B select()
  73 or
  74 .B poll()
  75 to wait for packets to arrive must put the
  76 .B pcap_t
  77 in non-blocking mode, and must arrange that the
  78 .B select()
  79 or
  80 .B poll()
  81 have a timeout less than or equal to the read timeout,
  82 and must try to read packets after that timeout expires, regardless of
  83 whether
  84 .B select()
  85 or
  86 .B poll()
  87 indicated that the file descriptor for the
  88 .B pcap_t
  89 is ready to be read or not.  (That workaround will not work in FreeBSD
  90 4.3 and later; however, in FreeBSD 4.6 and later,
  91 .B select()
  92 and
  93 .B poll()
  94 work correctly on BPF devices, so the workaround isn't necessary,
  95 although it does no harm.)
  96 .PP
  97 Note also that
  98 .B poll()
  99 doesn't work on character special files, including BPF devices, in Mac
 100 OS X 10.4 and 10.5, so, while
 101 .B select()
 102 can be used on the descriptor returned by
 103 .BR pcap_get_selectable_fd() ,
 104 .B poll()
 105 cannot be used on it those versions of Mac OS X.  Kqueues also don't
 106 work on that descriptor.
 107 .PP
 108 .B pcap_get_selectable_fd()
 109 is not available on Windows.
 110 .SH RETURN VALUE
 111 A selectable file descriptor is returned if one exists; otherwise, \-1
 112 is returned.
 113 .SH SEE ALSO
 114 pcap(3PCAP), select(2), poll(2)