Reasons for using the "!!" operator in C

Question

I was reading through the QEMU source code when I encountered a piece of code like below :-

CPUState *cpu_by_arch_id(int64_t id)
{
    CPUState *cpu;

    CPU_FOREACH(cpu) {
        CPUClass *cc = CPU_GET_CLASS(cpu);

        if (cc->get_arch_id(cpu) == id) {
            return cpu;
        }
    }
    return NULL;
}

bool cpu_exists(int64_t id)
{
    return !!cpu_by_arch_id(id);
}

The function cpu_exists is a boolean function that returns the value returned by the function cpu_by_arch_id(after applying the operator !!). The cpu_by_arch_id returns a pointer to a specific QEMU struct as you can see. I am finding it quite hard to understand how using the !! operator to the return value converts it into a boolean value.

What is the specific purpose of using the "!!"- double negation operator here ?