Function pointer syntax

[Parker]

Just thinking out loud here, these are a couple of possible syntaxes I thought of for indirect function calls:

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sub(int x, int y),int MyFunction;
MyFunction = &Something;
z = MyFunction(1,2);
or

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sub *MyFunction(int x, int y),int;
MyFunction = &Something;
z = MyFunction(1,2);

I'm just trying for something that lets us get around the !<> syntax which I never really liked.

[Zen]

i didnt mind it although i would like something that i can use to create dynamic function templates with.

Lewis

[Parker]

Here's a hack you can use right now. Use a project. In one source file put this:

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global __imp_FnPtr;
unsigned int __imp_FnPtr;

global sub SetFnPtr(unsigned int fnptr)
{
    __imp_FnPtr = fnptr;
}

To call the function, do something like this:

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declare import, FirstFunction alias FnPtr(int i);
declare import, SecondFunction alias FnPtr(string s, int x),int;
declare extern SetFnPtr(unsigned int fnptr);

...
SetFnPtr(&Something);
FirstFunction(1);
SetFnPtr(&SomethingElse);
SecondFunction("Hello", 0);

[Zen]

Cool. Ill give it a go. Im waiting on the real thing for my plugin system and maybe do something for another common class for dynamic loading of DLL files.

Lewis

[Ionic Wind Support Team]

The second one was what I was planning on doing.

Paul.

[Zen]

Can either Paul or Parker exaplain what is happening in the second example. I dont quite think i am understanding it. I know the first line is the function declaration, just like it was in IBasic. What is the MyFunction = &Something about? Is that a handle to the function returned by GetProcAddress?

Lewis

[Parker]

It's not GetProcAddress, you can actually use it with anything. It doesn't even have to be a function, although SP2 will kill your program if you try to execute data. But yes, you will be able to do something like this:

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sub *ExitProcess(int dwExitCode);
ExitProcess = GetProcAddress(hKernel32, "ExitProcess");
ExitProcess(0);

Edit - to clear it up a little more, & is the address of operator. It can be used on any variable or subroutine, or anything that has an address really. When you declare a function pointer, you have to give it an address before you call it. Calling 0 would have some strange effect and it would probably be stopped since address 0 is some windows code, I would think.

I've never written a linker so I can't say exactly, but I think that linkers clean up all these labels and resolve them into addresses, which windows does further when it loads your executable. So let's pretend you have a function (MyFunction) that is located at address 0xFFFF. And let's say you have a variable (MyVar) containing the value 0xFFFF. You can call that in two ways. One of which is to directly call the function

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call MyFunction ; gets resolved by the linker into: call 0xFFFF
or to call the address contained in the variable

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call [MyVar] ; MyVar contains the address of the function, 0xFFFF

It's also important to know that whenever you reserve space for a variable in assembly, the label that you have is an address. So while in Aurora you might do MyVar(), in assembly you have to dereference it first. Using sub* as the type instead of UINT might also give you a better feeling that it's being dereferenced instead of just calling a variable.

[Zen]

I guessed as much. I just wanted to make sure.

Lewis

[sapero]

yet another call - version for stdcall:

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//extern int farcall(...);
declare farcall(...),int;

extern MessageBeep as int; // :D
extern Sleep as int;

global sub main
{
farcall(&myFunction1,"blah\n",2);
farcall(&myFunction2,"again ", "blah\n",2);
farcall(&MessageBeep, 16); // speakers on!
farcall(&Sleep, 500);
MessageBox(0, "paused...","");
return;
}


sub myFunction1(string s)
{
openconsole(); writeln("myFunction1 :"+s);
return;
}
sub myFunction2(string s, string d)
{
openconsole(); writeln("myFunction2 :"+s+d);
return;
}



// for stdcall
#asm
global farcall

; extern int farcall(...);
; ret=farcall(&MyFunction, 3, "string", lpPoint ...);

farcall:
enter 0,0
push  esi

mov   esi,[ebp+4] ; get argc [in bytes]
mov   ecx,[esi+2]

sub   ecx,4       ; -1 argument
; * save ecx for cdecl *
jz   .call
lea   esi,[ebp+12-4+ecx]; last parameter
shr   ecx,2       ; [in dwords]
.next
push  dword[esi] ; push all parameters from right to left
sub   esi,4
dec   ecx
jnz  .next
.call
call [ebp+8] ; and call stdcall function
; * add esp,saved ecx for cdecl *
.quit
pop   esi
leave
ret
#endasm

[Zen]

Thanks a lot sapero. This helped a lot. Also leart a little bit too

Lewis

[Ionic Wind Support Team]

Parker will appreciate this.

This works now:

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declare *test(int a),int;

global sub main()
{
test = &MyFunction;
writeln(str$(test(1)) + "\n");
while GetKey() = "";
}

sub MyFunction(int a),int
{
return a;
}


[Parker]

Yay
Will the compiler do type checking on function pointers? They're probably the ones that need typechecking the most, since it could crash if you pass a function with the wrong number of parameters. I don't know how passing parameters would work yet though.

I actually was needing this earlier today and resorted to my "hack" that I posted above. But it'll be nice to have it in the language. I'll have to update the dictionary class to not call with asm now when this update is released.

[Ionic Wind Support Team]

Well it checks the parameters if that is what you meant.  The pointer itself is just an address.