Question

I have a program that is trying to use create and cancel through an implemented pool.

The creation is as follows:

while (created<threadsNum){
    pthread_t newThread;
    pthread_struct *st; //Open the thread that handle the deleting of the sessions timeout.
    st = (pthread_struct*)malloc(sizeof(pthread_struct));
    st->id = created;
    st->t = &newThread;
    pthread_mutex_lock( &mutex_threadsPool );
    readingThreadsPool[created] = st;
    pthread_mutex_unlock( &mutex_threadsPool );
        if((threadRes1 = pthread_create( &newThread, NULL, pcapReadingThread, (void*)created)))
        {
        syslog(LOG_CRIT, "Creating Pcap-Reading Thread %d  failed.",created); 
                printf( "Creating Pcap-Reading Thread %d  failed.\n",created);
                exit(1);
        }
    syslog(LOG_INFO, "Created Pcap-Reading Thread %d Successfully.",created); 
    created++;
}

Later I try to cancel them and restart them :

pthread_t* t;
pthread_struct* tstr;
int i;
pthread_mutex_unlock( &mutex_threadsPool );
//first go on array and kill all threads
for(i = 0; i<threadsNum ; i++ ){
    tstr = readingThreadsPool[i];
    if (tstr!=NULL){
        t = tstr->t;
                    //Reaches here :-)
        if (pthread_cancel(*t)!=0){
            perror("ERROR : Could not kill thread");
        }
        else{
            printf("Killed Thread %d \n",i);
        }
                    //doesnt reach here

    }
}

I checked the addresses in the memory of the created thread in part one and the address of the about to be cancelled thread in the second part..they match..
I read about the thread manager that can’t work if one calls killall().

But I don’t..

Anyone have any idea?

Thanks

Answer 1

while (created<threadsNum){
    pthread_t newThread;
    pthread_struct *st;
    /* ... */
    st->t = &newThread;
    /* ... */
}

You’ve got st->t pointing to a local variable newThread. newThread is only in scope during the current loop iteration. After this iteration st->t will contain an invalid address.

newThread is on the stack, so after it goes out of scope that stack space will be used for other variables. That could be different pthread_ts on successive iterations, or once the loop is over then that stack space will be used for completely different types of values.

To fix this I’d probably change pthread_struct.t to be a pthread_t instead of a pthread_t *, and then change the pthread_create call to:

pthread_create(&st->t, /*...*/)

Also, you should be careful about adding st to the thread pool before you’ve called pthread_create. It should probably be added after. As it stands, there’s a small window where st->t is on the thread pool but has not been initialized.