How can I pass the index of a for loop as the argument for pthread_create

This is happening because once you pass a pointer to i you now have multiple threads using the same value. This causes a data race because the first thread is modifying i and your second thread is expecting it to never change. You can always allocate a temporary int and pass it to the thread function.

pthread_create(&p[i], NULL, &somefunc, new int(i));

This will allocate an integer in dynamic storage (heap) and initialize it with the value of i. A pointer to the newly allocated integer will then be passed to the thread function.

Then in the thread function you can take the value passed as you already do and then delete the int object.

void *somefunc (void* ptr){
    int id = *(int*)ptr;
    delete (int*)ptr;
}

[Suggestion: Avoid C style casts.]