Question

I want to use one thread to get fields of packets by using tshark utility (using system () command) whos output is then redirected to a file. This same file needs to be read by another thread simultaneously, so that it can make runtime decisions based on the fields observed in the file.

The problem I am having currently now is even though the first thread is writing to the file, the second thread is unable to read it (It reads NULL from the file). I am not sure why its behaving this way. I thought it might be due to simultaneous access to the same file. I thought of using mutex locks but that would block the reading thread, since the first thread will only end when the program terminates.

Any ideas on how to go about it?

Answer 1

If you are using that file for interprocess communication, you could instead use named pipes or message queues instead. They are much easier to use and don’t require synchronization because one thread writes and the other one reads when data is available.

Edit: For inter-thread communication you can simply use shared variables and a conditional variable to signal when some data has been produced (a producer-consumer pattern). Something like:

// thread 1
while(1)
{
   // read packet
   // write packet to global variable
   // signal thread 2
   // wait for confirmation of reading
}

// thread 2
while(1)
{
   // wait for signal from thread 1
   // read from global variable
   // signal thread 2 to continue
}

The signal parts can be implemented with conditional variables: pthread_cond_t.