Question

I put up many threads running. At a later time, I’d like to check if these threads are still alive (i.e., not finished yet and not terminated unexpectedly).

1. What kind of information should I keep track of regarding the threads in the first place. Thread ID, process ID, etc? How should I get these IDs?

2. When I need to check the liveness of these threads, what functions should I use? Will pthread_kill work here? pthread_kill takes an opaque type pthread_t as parameter, which I believe is typically an unsigned long. Is pthread_t different from a thread ID? I assume a thread ID would pick up an int as its value. In some tutorials on pthread, they assign an integer to a pthread as its ID. Shouldn’t the thread get its ID from the operating system?

Answer 1

1. A thread’s entire identity resides in pthread_t

2. Initializing a thread returns its pthread_t typed ID to its parent

3. Each thread can get it’s own ID with pthread_self()

4. You can compare thread IDs using the function:int pthread_equal (pthread_t, pthread_t)

So: Maintain a common data structure where you can store thread status as STARTED, RUNNING, FINISHED using the pthread_t IDs and pthread_equal comparison function to differentiate between the threads. The parent sets the value to STARTED when it starts the thread, the thread itself sets its own state to RUNNING, does its work, and sets itself to FINISHED when done. Use a mutex to make sure values are not changed while being read.

EDIT:
You can set up a sort of ‘thread destructor’ using pthread_cleanup_push:
http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cleanup_pop.html

i.e. register a routine to be called when the thread exits (either itself, or by cancellation externally). This routine can update the status.