Passing a string to pthread function

Question

I'm a beginner to pthreads so bear with me. I'm trying to pass a string to the thread_routine1 function. It takes a void pointer as an argument. I then want to print it out.

void *thread_routine1(void *arg1){
    cout << arg1 << endl;
    pthread_exit(NULL);
}

int main ()
{
    pthread_t thread1;
    string t1 = "hello", t2 = "world";
    pthread_create(&thread1, NULL,thread_routine1, (void *) t1);
    pthread_join(thread1, NULL);
    return 0;
}

In the pthread_creat function I'm attempting to pass the string but I'm getting a invalid cast type from string to void pointer. I understand I'm probably goin wrong in more than one place so I would appreciate some pointers.

Answer 1

This has nothing to do with pthreads, really. You do this cast (void *) t1, which is essentially taking a sledgehammer onto the type system.

A string in C++ is not a pointer, so casting it to - and treating it as - one is only going to cause undefined behavior.

This is one reason why a C-style cast is to be avoided, because had you done it with a static_cast<void*>(t1) you'd have gotten a proper error, scolding you for doing something you shouldn't be doing.

Now, if you can't use std::thread for all its abstractions, and must contend with the pthreads C API, the way to go is to pass the address of t1. I.e.

pthread_create(&thread1, NULL,thread_routine1, &t1);

And now you don't even need a cast, because you are working with the type system. An object pointer is implicitly convertible to void*. Well-designed code should not be peppered with casts. They should be infrequent and for a good reason only. Furthermore, a cast should never be written just to "make it compile".

Oh, and don't forget to cast back in your thread function. Now you have a good reason, and the code will be correct with a simple static cast:

void *thread_routine1(void *arg1){
    cout << *static_cast<string*>(arg1) << endl;
    pthread_exit(NULL);
    return nullptr; // The function's return type is `void*`, not `void`! Do not omit a return statement.
}