How to properly finalize capture in a Wireshark extcap plugin?

asked 2020-11-23 09:07:05 +0000

I am writing a extcap plugin for Wireshark (Windows version). The documentation on how Wireshark stops a extcap capture is a bit sketchy, but it seems it simply terminates the extcap plugin.

If I run the extcap binary standalone, and stops it with Ctrl+C, everything works as expected. The written pcapng file contains all blocks. But when Wireshark runs the extcap binary, the last block, the "interface statistics block", never shows up in the Wireshark capture.

Is this a bug in Wireshark? Does Wireshark ignore any additional blocks in the pcapng fifo after it has sent the signal to kill the extcap binary?

The essential parts of the extcap plugin looks like this:

static volatile int keepRunning = 1;
void intHandler(int dummy) {
    keepRunning = 0;

int main(int argc, char *argv[])
   ... Parse arguments ...

   fp = fopen (pcOutputFilename, "wb");
   fwrite( &sSHB, sizeof(sSHB), 1, fp ); // write section header block to pcapng file.
   fwrite( &sIDB, sizeof(sIDB), 1, fp ); // write interface description block to pcapng file.

   signal(SIGINT, intHandler);
   signal(SIGTERM, intHandler);

      ... Capture frames and write to fp ...
   while( keepRunning );

   fwrite( &sISB, sizeof(sISB), 1, fp ); // write interface statistics block to pcapng file.

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You might get a better response if you post this question to the Wireshark dev list listed here.

grahamb gravatar imagegrahamb ( 2020-11-23 09:24:36 +0000 )edit