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  22 .TH PCAP_GET_SELECTABLE_FD 3PCAP "4 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.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_open_live()
  48 do not support
  49 .B select()
  50 or
  51 .B poll()
  52 (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace
  53 DAG devices), so \-1 is returned for those devices.
  54 .PP
  55 Note that on most versions of most BSDs (including Mac OS X)
  56 .B select()
  57 and
  58 .B poll()
  59 do not work correctly on BPF devices;
  60 .B pcap_get_selectable_fd()
  61 will return a file descriptor on most of those versions (the exceptions
  62 being FreeBSD 4.3 and 4.4), a simple
  63 .B select()
  64 or
  65 .B poll()
  66 will not return even after a timeout specified in
  67 .B pcap_open_live()
  68 expires.  To work around this, an application that uses
  69 .B select()
  70 or
  71 .B poll()
  72 to wait for packets to arrive must put the
  73 .B pcap_t
  74 in non-blocking mode, and must arrange that the
  75 .B select()
  76 or
  77 .B poll()
  78 have a timeout less than or equal to the timeout specified in
  79 .BR pcap_open_live() ,
  80 and must try to read packets after that timeout expires, regardless of
  81 whether
  82 .B select()
  83 or
  84 .B poll()
  85 indicated that the file descriptor for the
  86 .B pcap_t
  87 is ready to be read or not.  (That workaround will not work in FreeBSD
  88 4.3 and later; however, in FreeBSD 4.6 and later,
  89 .B select()
  90 and
  91 .B poll()
  92 work correctly on BPF devices, so the workaround isn't necessary,
  93 although it does no harm.)
  94 .PP
  95 Note also that
  96 .B poll()
  97 doesn't work on character special files, including BPF devices, in Mac
  98 OS X 10.4 and 10.5, so, while
  99 .B select()
 100 can be used on the descriptor returned by
 101 .BR pcap_get_selectable_fd() ,
 102 .B poll()
 103 cannot be used on it those versions of Mac OS X.  Kqueues also don't
 104 work on that descriptor.
 105 .PP
 106 .B pcap_get_selectable_fd()
 107 is not available on Windows.
 108 .SH RETURN VALUE
 109 A selectable file descriptor is returned if one exists; otherwise, \-1
 110 is returned.
 111 .SH SEE ALSO
 112 select(2), poll(2)