1PCAP_GET_SELECTABLE_FD(3PCAP) PCAP_GET_SELECTABLE_FD(3PCAP)
2
6 pcap_get_selectable_fd - get a file descriptor on which a select() can
7 be done for a live capture
8
10 #include <pcap/pcap.h>
11 int pcap_get_selectable_fd(pcap_t *p);
13
15 pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for
16 a file descriptor on which one can do a select() or poll() to wait for
17 it to be possible to read packets without blocking, if such a descrip‐
18 tor exists, or -1, if no such descriptor exists. Some network devices
19 opened with pcap_create() and pcap_activate(), or with
20 pcap_open_live(), do not support select() or poll() (for example, regu‐
21 lar network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so
22 -1 is returned for those devices.
23 Note that in:
25 FreeBSD prior to FreeBSD 4.6;
27 NetBSD prior to NetBSD 3.0;
29 OpenBSD prior to OpenBSD 2.4;
31 Mac OS X prior to Mac OS X 10.7;
33 select() and poll() do not work correctly on BPF devices;
35 pcap_get_selectable_fd() will return a file descriptor on most of those
36 versions (the exceptions being FreeBSD 4.3 and 4.4), but a simple
37 select() or poll() will not indicate that the descriptor is readable
38 until a full buffer's worth of packets is received, even if the read
39 timeout expires before then. To work around this, an application that
40 uses select() or poll() to wait for packets to arrive must put the
41 pcap_t in non-blocking mode, and must arrange that the select() or
42 poll() have a timeout less than or equal to the read timeout, and must
43 try to read packets after that timeout expires, regardless of whether
44 select() or poll() indicated that the file descriptor for the pcap_t is
45 ready to be read or not. (That workaround will not work in FreeBSD 4.3
46 and later; however, in FreeBSD 4.6 and later, select() and poll() work
47 correctly on BPF devices, so the workaround isn't necessary, although
48 it does no harm.)
49 Note also that poll() doesn't work on character special files, includ‐
51 ing BPF devices, in Mac OS X 10.4 and 10.5, so, while select() can be
52 used on the descriptor returned by pcap_get_selectable_fd(), poll()
53 cannot be used on it those versions of Mac OS X. Kqueues also don't
54 work on that descriptor. poll(), but not kqueues, work on that
55 descriptor in Mac OS X releases prior to 10.4; poll() and kqueues work
56 on that descriptor in Mac OS X 10.6 and later.
57 pcap_get_selectable_fd() is not available on Windows.
59
61 A selectable file descriptor is returned if one exists; otherwise, -1
62 is returned.
63
65 pcap(3PCAP), select(2), poll(2)
66 5 April 2008 PCAP_GET_SELECTABLE_FD(3PCAP)