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Top Forums Programming how to discard instruction from previous signal Post 302341899 by matrixmadhan on Friday 7th of August 2009 12:10:38 AM
Old 08-07-2009
Quote:
Originally Posted by Corona688
Well, you can't cancel signals that way. How about blocking them instead? You can make a signal wait in a queue until you're ready to deal with it with sigprocmask.
I see a potential problem (rare) with this approach, what if, when a file is being processed, signal 2 (2nd instance) is passed, since its blocked it will have to wait, while the first file is still being processed and if there are multiple instances of signals being queued ( due to the reason they are blocked ) ... that could be when there is an application bug that is indefinitely processing a file, without completing the signal handler, or closing a filehandle via possible stale NFS.

All these will have end up in having multiple signals in state " that are yet to be delivered "

---------- Post updated at 09:40 AM ---------- Previous update was at 09:34 AM ----------

Quote:
Originally Posted by nvhoang
Thank you all Corona688 and matrixmadhan for your replies,

Because I have 2 applications and need to make communication via files and signal.
The first application shall write some files then send a signal to the second application telling the second to processes and then delete those files.

The problem is that, when the second is processing those files, the first one may throw new files and signal (the goal is the second gets latest data); therefore the second might delete new files which leads to lost of data or to be consistent.

So I hope that, I could, when a signal comes the second restarts processing those files (and discard instructions from previous signal action). I know this is not nature of signal handling.

(I know that I can make it synchronously by making the second send back a signal to the first to tell that it is ready to receive new signal, for example).
Correct me if am wrong somewhere, please

these are the sequence of actions
  1. first application processing is processing a file
  2. processing done, first application sends a signal to second appln to initiate processing
  3. second application starts processing
  4. before 2nd could complete, first completes processing and makes 2nd file ready for processing, signals 2nd application.

Now, should 2nd application delete the first file and start processing the second file?

If that is the case, there could be a shared memory segment between 1st and 2nd application and make 2nd application to listen for any data contents in the shared memory segment, and based on the filename in the data segment process only that file and discard others.
 

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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() 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 in systems that support POSIX threads. Note that pcap_next() and pcap_next_ex() will, on some platforms, loop reading packets from the OS; that loop will not necessarily be ter- minated 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 posi- tive 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) 5 April 2008 PCAP_BREAKLOOP(3)
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