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() or pcap_loop() to return rather than looping; they will return the number of packets that have been processed so far, or -2 if no packets have been processed so far.

This routine is safe to use inside a signal handler on UNIX or a console control handler on Windows, as it merely sets a flag that is checked within the loop.

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(), or pcap_next_ex(), a call to pcap_breakloop() in a different thread will not unblock that thread. 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, or SetEvent() on the result of pcap_getevent() on a pcap_t on which the thread is blocked on Windows. Asynchronous procedure calls will not work on Windows, as a thread blocked on a pcap_t will not be in an alertable state.

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 -2 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 -2 and clear the flag.

# SEE ALSO

pcap(3), pcap_loop(3), pcap_next_ex(3)

8 November 2017 |