# pcap_get_selectable_fd

## NAME



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


## SYNOPSIS



           **#include <pcap/pcap.h>**
    
           **int pcap_get_selectable_fd(pcap_t *p);**
    


## DESCRIPTION



           **pcap_get_selectable_fd**() returns, on UNIX, a file descriptor
           number for a file descriptor on which one can do a select(2),
           poll(2), epoll_wait(2), **kevent**(2), 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**(3PCAP) and
           **pcap_activate**(3PCAP), or with **pcap_open_live**(3PCAP), do not
           support those calls (for example, regular network devices on
           FreeBSD 4.3 and 4.4, and Endace DAG devices), so **-1** is returned
           for those devices.  In that case, those calls must be given a
           timeout less than or equal to the timeout returned by
           **pcap_get_required_select_timeout**(3PCAP) for the device for which
           **pcap_get_selectable_fd**() returned **-1** , the device must be put in
           non-blocking mode with a call to **pcap_setnonblock**(3PCAP), and an
           attempt must always be made to read packets from the device when
           the call returns.  If **pcap_get_required_select_timeout**() returns
           **NULL** , it is not possible to wait for packets to arrive on the
           device in an event loop.
    
           Note that a device on which a read can be done without blocking
           may, on some platforms, not have any packets to read if the packet
           buffer timeout has expired.  A call to **pcap_dispatch**(3PCAP) or
           **pcap_next_ex**(3PCAP) 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**(), **poll**(), and **kevent**() 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**(), **poll**(), or **kevent**() call will not indicate
           that the descriptor is readable until a full buffer's worth of
           packets is received, even if the packet timeout expires before
           then.  To work around this, code that uses those calls to wait for
           packets to arrive must put the **pcap_t** in non-blocking mode, and
           must arrange that the call have a timeout less than or equal to
           the packet buffer timeout, and must try to read packets after that
           timeout expires, regardless of whether the call 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, those calls work correctly on BPF devices,
           so the workaround isn't necessary, although it does no harm.)
    
           Note also that **poll**() and **kevent**() 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**() and **kevent**() cannot be used on it
           those versions of Mac OS X.  **poll**(), but not **kevent**(), works on
           that descriptor in Mac OS X releases prior to 10.4; **poll**() and
           **kevent**() work on that descriptor in Mac OS X 10.6 and later.
    
           **pcap_get_selectable_fd**() is not available on Windows.
    


## RETURN VALUE



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


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