Question

I want to use a class method in the signal method

signal(SIGALRM, syncro);

syncro is the name of another class method.

void states::syncro( int parameter )
{

}

I try with syncro, &syncro, states::syncro, and &states::syncro but all of them throw error messages when compiling.

error: argument of type ‘void (states::)(int)’ does not match ‘void (*)(int)’

Answer 1

You are able to call only free function or static class function in this case

signal‘s signature is

typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);

That means that it accepts only pointer to free function or static function.

Note, that you should always have an object to call member function, say, if you have

struct S
{
    void method();
}
S obj;

you can only call obj.method(), but never S::method() or method(), and pointer to S::method has type void (S::*)().

But if your S::method() was declared static you could call it with S::method().

That’s why pointers to free functions are incompatible with pointers to member functions.