Multiple arguments to function called by pthread_create() - argument is function pointer - c++

Question

My case is similar to another Question.
I would like to pass a function as argument and an integer value. Testing the case with a simplfied construction:
void print (int x, int y)
{ cout << "x = " << x << ", y = " << y << endl; }
void *interrupt (struct arg_struct args);
struct arg_struct {
void (*func)(int,int); // Argument 1
int a; // Argument 2
int b; // Argument 3
};
int _tmain(int argc, _TCHAR* argv[]){
int a = 1700, b = 1000;
struct arg_struct arguments;
arguments.func = print;
arguments.a = a;
arguments.b = b;
(*interrupt)(arguments);
cin.get(); return 0;
}
void *interrupt (struct arg_struct args) {
void (*func) (int,int) ;
func = args.func;
int x = args.a;
int y = args.b;
(*func)(x,y);
return 0; // Erfordert Rückgabewert
}
So now I want to create a thread to execute this passed function.
void *InThread_func(struct arg_struct *); // Prototype
struct arg_struct {
void (*func)(int); // Argument 1
int IntNumber; // Argument 2
};
int InThread(void (*func)(int), int IntNumber){
struct arg_struct arguments;
arguments.func = func;
arguments.IntNumber = IntNumber;
// Initialize handler
pthread_t InThread_thread;
// Create thread
pthread_create(&InThread_thread,NULL,&InThread_func,(void*) &arguments);
pthread_join(InThread_func,NULL);
return(errno);
}
Using
g++-4.6 -o test test.cpp
the compiler says
invalid conversion from void* (*)(arg_struct*) to void * (*)(void*)
referring to the last argument of pthread_create.
Why is that?

Answer 1

C++ is picky when it comes to casting.
Replace void *InThread_func(struct arg_struct *); by void *InThread_func(void *my_data); and it should solve the problem.
Since this is C++ I'd recommend using std::thread is those are available to you.

Answer 2

"Why is that?" Because you have an invalid conversion from
void* (*)( arg_struct* ) to void* (*)( void* ), perhaps. The third
argument to pthread_create (not the last) must be an
extern "C" void* (*)( void* ). (Some compilers will ignore the
necessity of the extern "C". They are broken in this regard.) So
your InThread_fnc (which I can't find in your code) must be something
like:
extern "C" void*
InThread_fnc( void* from_pthread_create )
{
arg_struct const* p = static_cast< arg_struct const* >( from_pthread_create );
(*p->func)( p->IntNumber );
return nullptr;
}
Of course, this only works if the last argument of pthread_create was
a arg_struct*. Which corresponds to your case, but beware if you
start deriving: passing a new Derived or a &someDerived when the
function you start casts to Base* results in undefined behavior.