C - Keeping track of numbering for multiple pThreads

Question

I am trying to write a C program that makes use of pThreads. This particular program creates multiple threads (1 to maxNumber ) and does something with them (in the method threadMethod ). I pass a struct Arguments to the pThread creator that contains necessary information about the threads, and threadNumber is included in that struct. I am trying to make the program print which thread is doing work at that moment, but when I run the program, it always prints maxNumber instead of threadNumber . For example, if I want to make 3 threads, I should have an output like:

I am thread 1
I am thread 2
I am thread 3

But instead of the above, I get an output like:

I am thread 3
I am thread 3
I am thread 3

What is going wrong with my program? I have a feeling it has something to do with the struct, but I am not certain. Below is the relevant code.

Arguments *arg = malloc(sizeof(Arguments));

int i;
for (i = 0; i < maxNumber; i++) {
    arg->threadNumber = (i + 1);  /* eg. first thread is Thread 1, not Thread 0 */

    if (pthread_create(&threads[i], NULL, threadMethod, (void *)arg)) {
        printf("Error while creating thread\n");
        return 1;
    }
}

--------------------

void *threadMethod(void *arg) {
    Arguments *argument;
    int threadNumber;

    argument = (Arguments*)arg;
    threadNumber = argument->threadNumber;

    printf("I am thread %d\n", threadNumber);
    /* do stuff */
    return NULL;
}

Answer 1

All your threads are sharing a single arg object, because you are only allocating it once. Move the malloc() inside the loop.