APR threads and signal handling

Question

I am currently trying to implement threads using the Apache Portable Runtime. Everything works fine, except I am not really sure if I am doing it the way it's intended to do due to lack of documentation or examples.

I need two threads and signal handling to catch a CTRL-C on the console to cleanup my server and possibly the threads. This is my current approach:

// Define APR thread pool
apr_pool_t *pool;

// Define server
MyServer *server;

// Define threads
apr_thread_t *a_thread, *b_thread;
apr_status_t status;

static void * APR_THREAD_FUNC func_a(apr_thread_t * thread,
        void *data) {
        // do func_a stuff here
}

static void * APR_THREAD_FUNC func_b(apr_thread_t * thread,
        void *data) {
        // do func_b stuff here
}

// Cleanup before exit
void cleanup(int s) {
    printf("Caught signal %d\n", s);

    // Destroy thread pool
    apr_pool_destroy(pool);

    //apr_thread_exit(a_thread, APR_SUCCESS);
    //apr_thread_exit(b_thread, APR_SUCCESS);

    //apr_terminate();

    // Stop server and cleanup
    server->stopServer();
    delete server;

    exit(EXIT_SUCCESS);
}

int main(void) {
    // Signal handling
    signal(SIGINT, cleanup);

    // Create server
server = MyServerFactory::getServerImpl();

bool success = server->startServer();

// Initialize APR
if (apr_initialize() != APR_SUCCESS) {
    printf("Could not initialize\n");
    return EXIT_FAILURE;
}

// Create thread pool
if (apr_pool_create(&pool, NULL) != APR_SUCCESS) {
    printf("Could not allocate pool\n");
    return EXIT_FAILURE;
}

// Create a_thread thread
if (apr_thread_create(&a_thread, NULL, func_a, NULL,
        pool) != APR_SUCCESS) {
    printf("Could not create a_thread\n");
    return EXIT_FAILURE;
}

//Create b_thread thread
if (apr_thread_create(&b_thread, NULL, func_b, NULL,
        pool) != APR_SUCCESS) {
    printf("Could not create b_thread\n");
    return EXIT_FAILURE;
}

    // Join APR threads
    apr_thread_join(&status, a_thread);
    apr_thread_join(&status, b_thread);

    return EXIT_SUCCESS;
}

This works more or less as expected. The only thing I am not really sure about is if the cleanup works fine.

1. The cleanup-function seems to be called more then one time (String "Caught signal.." appears more than one time on terminal). Is there a way to prevent this? Is this problematic?

2. I found more than one example for cleaning up APR threads after usage. Is my way sufficient or do I need some of the commented stuff? Or am I completely wrong?

Answer 1

APR thread cleanup is explained in decent detail in the threading section of the APR Tutorial. In order, the cleanup steps look like this:

1. Call apr_thread_exit() in the thread itself. While Unix threads terminate automatically, Windows threads do not. Call this function for portability (and to a return a status, if necessary).
2. Call apr_thread_join() in the main thread to wait for the termination of all threads.
3. Call apr_pool_destroy() to free the main memory pool. (Child memory pools are freed with apr_thread_exit().)
4. Call apr_terminate() to release other resources (sockets, etc.). Note that simply calling exit() in another thread without these final steps can cause crashes.

They also have a brief sample program that demonstrates some of this stuff.

As for why your signal handler is firing twice, I don't think that has to do with APR. SIGINT is sent to parent and child processes, so I suspect MyServer is forking a new process and both are invoking the handler.