SEHException
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Thread: SEHException

  1. #1
    Join Date
    Feb 2009
    Posts
    17

    SEHException

    SEHException : External component has thrown an exception.

    I'm getting the above exception in the following part of the code when I run the program.

    Code:
                IntPtr fcode = IntPtr.Zero;
                bpf_program bpf = new bpf_program();
                fcode = Marshal.AllocHGlobal(Marshal.SizeOf(bpf));
                Marshal.StructureToPtr(bpf,fcode,false);
                string fter = "ip and tcp";
                if (wpcap_wrap.pcap_compile(ref adhandle, ref fcode, fter, 1, nmask) < 0)
                {
                    Console.WriteLine("Unable to compile filter");
                }
                if (wpcap_wrap.pcap_setfilter(ref adhandle, ref fcode) < 0)
                    Console.WriteLine("Error setting filter");
                Marshal.FreeHGlobal(fcode);
    The exception occurs at this line
    Code:
     if (wpcap_wrap.pcap_setfilter(ref adhandle, ref fcode) < 0)
    I declared the method as follows
    Code:
    [DllImport("wpcap.dll")]
    public static extern int pcap_setfilter(ref IntPtr p,ref IntPtr fp);
    I'm trying to write an app that filters packets.
    Any help would be greatly appreciated.

  2. #2
    Join Date
    May 2007
    Posts
    1,546

    Re: SEHException

    Isn't the signature for that method:

    Code:
    int pcap_setfilter(pcap_t *, struct bpf_program *);
    If so, you're P/Invoke is incorrect. It should be:

    Code:
    public static extern int pcap_setfilter(IntPtr p,IntPtr fp);
    Alternatively, it may be possible to use something like this:

    Code:
    public struct pcap_t
    {
        // contents should mirror the native struct
    }
    
    public struct bpf_program
    {
        // this should match the native version too
    }
    
    // Your P/Invoke can look like this now
    public static extern int pcap_setfilter(ref pcap_t p, ref bpf_program fp);
    Though not everything can be rewritten 'nicely' like this. It depends on the method signature you're trying to wrap. Also if you write the code like this, you *cannot* store the pointer in native code. If you want to store a copy of the data in native code, you'll have to use the IntPtr method like you originally did.
    www.monotorrent.com For all your .NET bittorrent needs

    NOTE: My code snippets are just snippets. They demonstrate an idea which can be adapted by you to solve your problem. They are not 100% complete and fully functional solutions equipped with error handling.

  3. #3
    Join Date
    Feb 2009
    Posts
    17

    Re: SEHException

    The reason I put ref before the parameters is because otherwise I get an AccessViolationException.
    I managed to get rid of the SEHException by changing the method declaration as

    Code:
    public static extern int pcap_setfilter(IntPtr p,ref IntPtr fp);
    If I remove the ref for both, I get AccessViolation and when I put for both I get the SEH one.

    The problem I now have is that the packets are not getting filtered. Nothing is getting captured.
    However, when I give an empty string at the place of the filtering string, the program captures all the packets.

    Any thoughts on what could be the problem?
    Last edited by padfoot; February 14th, 2009 at 11:19 AM.

  4. #4
    Join Date
    Feb 2009
    Posts
    17

    Re: SEHException

    One more thing:

    When I pass a random string as argument, the method returns an error.

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