Reputation: 2043

Can someone explain the parameter void(pfn)(void)?

I've come across this piece of code

inline pthread_t CreateThread(void(*pfn)(void*), void* parg, bool fWantHandle=false)

I don't understand this part

void(*pfn)(void*)

Can someone tell me what it means/is?

This is btw not listed in books for beginners so if you want to mention to read books, it's not there.

Afaik, void is datatype of a function meaning it will not return anything, however that part there...void is used on a pointer?

Upvotes: 6

Answers (8)

Johann Gerell

Reputation: 25591

The declaration of CreateThread says that the first parameter pfn is a pointer to a callback function that will be used by CreateThread. The callback pfn is your own function that CreateThread will call so that you can execute your code in a new thread.

You define a function

void MyThreadCallback(void* data)
{
    MyData myData = reinterpret_cast<MyData*>(data);
    ...
}

and pass it to CreateThread as

MyData* myData = new MyData();
CreateThread(MyThreadCallback, myData, ... );

so that CreateThread can call it in the context of the new thread.

Upvotes: 2

Preet Sangha

Reputation: 65516

This is a function pointer (or a pointer to a function).

void(*pfn)(void*)

This is broken down as such:

*pfn (the name of the pointer i.e. pointer to a function)

(void *) (these are the parameters to the function ie. a simple pointer to anything)

void (this is return from the function)

So if you have a function like this:

void DoSomeThing(void *data) {
  ... does something....
}

then you can pass it into the CreateThread function like so...

int i  = 99;
void * arg = (void*)&i;
pthread_t thread = CreateThread(DoSomeThing, arg, ... other parameters ...);

So somewhere in CreateThread it will make a call:

pfn(parg);

and your function DoSomeThing will be called and void * data you get will be the arg you passed in.

More info:

Remember that code is just a sequence of bytes in memory. It's just how the cpu interprets them that makes them different from the thing we call data.

So at any point in a program we can refer to another part of the code by it's memory address. Since the code is broken down into functions with in C, this is a useful unit of reuse that C understands and allows us to treat the address of the function as just another pointer to some data.

In the above example the CreateThread function needs the address of a function so it can execute that function in a new thread. So we pass it a pointer to that function. Hence we pass it a function pointer.

Upvotes: 7

Ted Hopp

Reputation: 234847

These things are easiest read inside out:

(*pfn) => "pfn is a pointer"
void x(void*) => "x is a function accepting a void * argument and returning void"

Put them together and you have:

"pfn is a pointer to a function accepting a void * argument and returning void"

Note that the parens around (*pfn) are necessary because of precedence. void *pfn(void*) would be interpreted as if it were written (void *)pfn(void *): "pfn is a function that accepts a void * and returns a void *".

Upvotes: 8

Kate Gregory

Reputation: 18964

It is a function pointer. The name pfn is a clue. Specifically it is a pointer to a function that takes a "void pointer" and returns nothing. A "void pointer" is a fake type that can be cast to some other kind of pointer.

Most C++ books cover both void pointers and function pointers in some detail. Probably more detail than necessary, since you can write perfectly good C++ code without ever using either. If your books are somehow all super modern and don't cover them, look for a C reference.

Upvotes: 1