I have been looking at the following working code for executing code as a pthread in c++:
void * PrintHello(void * blank) {
cout << "Hello World" << endl
}
...
pthread_create(&mpthread, NULL, PrintHello, NULL);
I would like to know why I need to use a void * method instead of void method and also the same with the parameter. Why do they need to be pointers and what is the difference in the case of this void method and void arguement.
You need to use a method that takes void* because it's called by pthread library, and pthread library passes your method a void* - the same pointer that you pass pthread_create as the last parameter.
Here is an example of how you can pass arbitrary parameters to a thread using a single void*:
struct my_args {
char *name;
int times;
};
struct my_ret {
int len;
int count;
};
void * PrintHello(void *arg) {
my_args *a = (my_args*)arg;
for (int i = 0 ; i != a->times ; i++) {
cout << "Hello " << a->name << endl;
}
my_ret *ret = new my_ret;
ret->len = strlen(a->name);
ret->count = strlen(a->name) * a->times;
// If the start_routine returns, the effect is as if there was
// an implicit call to pthread_exit() using the return value
// of start_routine as the exit status:
return ret;
}
...
my_args a = {"Peter", 5};
pthread_create(&mpthread, NULL, PrintHello, &a);
...
void *res;
pthread_join(mpthread, &res);
my_ret *ret = (my_ret*)(*res);
cout << ret->len << " " << ret->count << endl;
delete ret;
Even though your function does not want to take any arguments or return anything, since pthread library passes it parameters and collects its return value, your function needs to have the appropriate signature. Passing a pointer to a void function that takes no parameters in place of a void* function that takes one parameter would be undefined behavior.
