How to Correctly Create Threads for C Class Member Functions?

Creating Threads on Class Member Functions

When working with C classes, a common challenge is creating threads on member functions. Consider the following class:

class c { 
    // ...
    void *print(void *){ cout << "Hello"; }
}

Let's say we have a vector of c objects and want to create a thread on the c.print() function. However, the following code leads to an error:

pthread_create(&t1, NULL, &c[0].print, NULL);

The error message indicates that the function pointer does not match the expected type for the third argument of pthread_create().

Solution:

The error occurs because C class member functions have a hidden this parameter passed implicitly. pthread_create() doesn't know which instance of c to use for the this parameter. To resolve this, we need to use a static class method (which doesn't have a this parameter) or a plain function to bootstrap the class.

Static Class Method Approach:

class C
{
public:
    static void *hello_helper(void *context)
    {
        return ((C *)context)->hello();
    }
};

// ...

C c;
pthread_t t;
pthread_create(&t, NULL, &C::hello_helper, &c);

This approach defines a static class method hello_helper(), which has no this parameter and wraps the call to the hello() member function.

Plain Function Approach:

void hello(void *context)
{
    C *c = (C *)context;
    c->hello();
}

// ...

C c;
pthread_t t;
pthread_create(&t, NULL, &hello, &c);

Here, we define a plain function hello() that takes a void pointer passing to it and casts it to an instance of c. We then call the hello() member function on the c instance.

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