Updated: 2022/Sep/29

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PCAP_BREAKLOOP(3)          Library Functions Manual          PCAP_BREAKLOOP(3)



NAME
       pcap_breakloop - force a pcap_dispatch() or pcap_loop() call to return

SYNOPSIS
       #include <pcap/pcap.h>

       void pcap_breakloop(pcap_t *);

DESCRIPTION
       pcap_breakloop() sets a flag that will force pcap_dispatch(3) or
       pcap_loop(3) to return rather than looping; they will return the number
       of packets that have been processed so far, or PCAP_ERROR_BREAK if no
       packets have been processed so far.  If the loop is currently blocked
       waiting for packets to arrive, pcap_breakloop() will also, on some
       platforms, wake up the thread that is blocked.  In this version of
       libpcap, the only platforms on which a wakeup is caused by
       pcap_breakloop() are Linux and Windows, and the wakeup will only be
       caused when capturing on network interfaces; it will not be caused on
       other operating systems, and will not be caused on any OS when
       capturing on other types of devices.

       This routine is safe to use inside a signal handler on UNIX or a
       console control handler on Windows, or in a thread other than the one
       in which the loop is running, as it merely sets a flag that is checked
       within the loop and, on some platforms, performs a signal-safe and
       thread-safe API call.

       The flag is checked in loops reading packets from the OS - a signal by
       itself will not necessarily terminate those loops - as well as in loops
       processing a set of packets returned by the OS.  Note that if you are
       catching signals on UNIX systems that support restarting system calls
       after a signal, and calling pcap_breakloop() in the signal handler, you
       must specify, when catching those signals, that system calls should NOT
       be restarted by that signal.  Otherwise, if the signal interrupted a
       call reading packets in a live capture, when your signal handler
       returns after calling pcap_breakloop(), the call will be restarted, and
       the loop will not terminate until more packets arrive and the call
       completes.

       Note also that, in a multi-threaded application, if one thread is
       blocked in pcap_dispatch(), pcap_loop(), pcap_next(3), or
       pcap_next_ex(3), a call to pcap_breakloop() in a different thread will
       only unblock that thread on the platforms and capture devices listed
       above.

       If a non-zero packet buffer timeout is set on the pcap_t, and you are
       capturing on a network interface, the thread will be unblocked with the
       timeout expires.  This is not guaranteed to happen unless at least one
       packet has arrived; the only platforms on which it happens are macOS,
       the BSDs, Solaris 11, AIX, Tru64 UNIX, and Windows.

       If you want to ensure that the loop will eventually be unblocked on any
       other platforms, or unblocked when capturing on a device other than a
       network interface, you will need to use whatever mechanism the OS
       provides for breaking a thread out of blocking calls in order to
       unblock the thread, such as thread cancellation or thread signalling in
       systems that support POSIX threads.

       Note that if pcap_breakloop() unblocks the thread capturing packets,
       and you are running on a platform that supports packet buffering, there
       may be packets in the buffer that arrived before pcap_breakloop() were
       called but that weren't yet provided to libpcap, those packets will not
       have been processed by pcap_dispatch() or pcap_loop().  If
       pcap_breakloop() was called in order to terminate the capture process,
       then, in order to process those packets, you would have to call
       pcap_dispatch() one time in order to process the last batch of packets.
       This may block until the packet buffer timeout expires, so a non-zero
       packet buffer timeout must be used.

       Note that pcap_next() and pcap_next_ex() will, on some platforms, loop
       reading packets from the OS; that loop will not necessarily be
       terminated by a signal, so pcap_breakloop() should be used to terminate
       packet processing even if pcap_next() or pcap_next_ex() is being used.

       pcap_breakloop() does not guarantee that no further packets will be
       processed by pcap_dispatch() or pcap_loop() after it is called; at most
       one more packet might be processed.

       If PCAP_ERROR_BREAK is returned from pcap_dispatch() or pcap_loop(),
       the flag is cleared, so a subsequent call will resume reading packets.
       If a positive number is returned, the flag is not cleared, so a
       subsequent call will return PCAP_ERROR_BREAK and clear the flag.

BACKWARD COMPATIBILITY
       This function became available in libpcap release 0.8.1.

       In releases prior to libpcap 1.10.0, pcap_breakloop() will not wake up
       a blocked thread on any platform.

SEE ALSO
       pcap(3)



                                8 December 2022              PCAP_BREAKLOOP(3)