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You are here: manpages


Section: Misc. Reference Manual Pages (3PCAP)
Updated: 22 July 2011
Index Return to Main Contents


pcap_get_selectable_fd - get a file descriptor on which a select() can be done for a live capture  


#include <pcap/pcap.h>

int pcap_get_selectable_fd(pcap_t *p);


pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for a file descriptor on which one can do a select(), poll(), or other such call to wait for it to be possible to read packets without blocking, if such a descriptor exists, or -1, if no such descriptor exists. Some network devices opened with pcap_create() and pcap_activate(), or with pcap_open_live(), do not support select() or poll() (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so -1 is returned for those devices.

Note that a descriptor on which a read can be done without blocking may, on some platforms, not have any packets to read if the read timeout has expired. A call to pcap_dispatch() will return 0 in this case, but will not block.

Note that in:

FreeBSD prior to FreeBSD 4.6;
NetBSD prior to NetBSD 3.0;
OpenBSD prior to OpenBSD 2.4;
Mac OS X prior to Mac OS X 10.7;

select() and poll() do not work correctly on BPF devices; pcap_get_selectable_fd() will return a file descriptor on most of those versions (the exceptions being FreeBSD 4.3 and 4.4), but a simple select() or poll() will not indicate that the descriptor is readable until a full buffer's worth of packets is received, even if the read timeout expires before then. To work around this, an application that uses select() or poll() to wait for packets to arrive must put the pcap_t in non-blocking mode, and must arrange that the select() or poll() have a timeout less than or equal to the read timeout, and must try to read packets after that timeout expires, regardless of whether select() or poll() indicated that the file descriptor for the pcap_t is ready to be read or not. (That workaround will not work in FreeBSD 4.3 and later; however, in FreeBSD 4.6 and later, select() and poll() work correctly on BPF devices, so the workaround isn't necessary, although it does no harm.)

Note also that poll() doesn't work on character special files, including BPF devices, in Mac OS X 10.4 and 10.5, so, while select() can be used on the descriptor returned by pcap_get_selectable_fd(), poll() cannot be used on it those versions of Mac OS X. Kqueues also don't work on that descriptor. poll(), but not kqueues, work on that descriptor in Mac OS X releases prior to 10.4; poll() and kqueues work on that descriptor in Mac OS X 10.6 and later.

pcap_get_selectable_fd() is not available on Windows.  


A selectable file descriptor is returned if one exists; otherwise, -1 is returned.  


pcap(3PCAP), select(2), poll(2)




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