Trouble Passing Banks to user32 api functions

This is in fact a bit of a pain. What you need to do is poke the address of the null terminated strings for the caption and text into the parameter block (not the strings themselves). Now you can not just poke a bank address because banks are themselves structures, and although blitz will just pass the bank data when you pass a bank directly to a userlib function, it will not know to do this if you store it in a parameter block. So you will need to poke in the address stored at the bank address + 4 which is where the data is stored. One way of getting that is to use one of those external memory functions. So you could have something like...

Type MSGBOXPARAMS
   Field cbSize
   Field hwndOwner 
   Field hInstance
   Field lpszText$
   Field lpszCaption$ 
   Field dwStyle
   Field lpszIcon$
   Field dwContextHelpId
   Field lpfnMsgBoxCallback
   Field dwLanguageId
End Type

Const IDOK = 1
Const IDCANCEL = 2
Const IDABORT = 3
Const IDRETRY = 4
Const IDCONTINUE = 5
Const IDIGNORE = 5
Const IDYES = 6
Const IDNO = 7
Const MB_APPLMODAL = $0
Const MB_COMPOSITE = $2
Const MB_DEFAULT_DESKTOP_ONLY = $20000
Const MB_DEFBUTTON1 = $0
Const MB_DEFBUTTON2 = $100
Const MB_DEFBUTTON3 = $200
Const MB_DEFMASK = $F00
Const MB_ICONASTERISK = $40
Const MB_ICONEXCLAMATION = $30
Const MB_ICONHAND = $10
Const MB_ICONINFORMATION = MB_ICONASTERISK
Const MB_ICONMASK = $F0
Const MB_ICONQUESTION = $20
Const MB_ICONSTOP = MB_ICONHAND
Const MB_MISCMASK = $C000
Const MB_MODEMASK = $3000
Const MB_NOFOCUS = $8000
Const MB_OK = $0
Const MB_OKCANCEL = $1
Const MB_PRECOMPOSED = $1
Const MB_RETRYCANCEL = $5
Const MB_SETFOREGROUND = $10000
Const MB_SYSTEMMODAL = $1000
Const MB_TASKMODAL = $2000
Const MB_TYPEMASK = $F
Const MB_USEGLYPHCHARS = $4
Const MB_YESNO = $4
Const MB_YESNOCANCEL = $3

 
Graphics3D 800,600,32,2

While Not KeyHit(1)
;val = MessageBox(GetActiveWindow(),"All Your Base Are Belong To Us","Warning",MB_ICONEXCLAMATION+MB_YESNOCANCEL)

 mbx.msgboxparams=New msgboxparams
 mbx\hwndOwner = GetActiveWindow()
 mbx\lpszText$="What foul creature wakes me from mine slumber?"
 mbx\lpszCaption$="Warning"
 mbx\dwStyle=MB_ICONEXCLAMATION+MB_YESNOCANCEL

 val=DoMessageBoxIndirect(mbx)

Print val

WaitKey


Wend

Function doMessageBoxIndirect(mbx.msgboxparams)

	mbx\cbsize=40
	bnk=CreateBank(40)
	PokeInt (bnk,0,mbx\cbSize)
	PokeInt (bnk,4,mbx\hwndOwner)
	PokeInt (bnk,8,mbx\hInstance)
	strbank1=stringtobank(mbx\lpsztext)
	PokeInt (bnk,12,BLZPeekInt(strbank1+4))
	strbank2=stringtobank(mbx\lpszCaption)
	PokeInt (bnk,16,BLZPeekInt(strbank2+4))
	PokeInt(bnk,20,mbx\dwStyle)
	PokeInt (bnk,24,0)
	PokeInt (bnk,28,mbx\dwContextHelpID)
	PokeInt (bnk,32,mbx\lpfnMsgBoxCallback)
	PokeInt (bnk,36,mbx\dwLanguageId)

	val=MessageBoxIndirect(bnk)

	FreeBank bnk
	FreeBank strbank1
	FreeBank strbank2
	
	Return Val
	
End Function

Function Stringtobank(s$,bnk=0)
	size=Len(s$)+1
	If bnk=0 Then
		 bnk=CreateBank(size)
	Else
		If BankSize(bnk)<size Then ResizeBank bnk,size
	EndIf
	For f=0 To size-2
		PokeByte bnk,f,Asc(Mid(s$,f+1,1))
	Next
	PokeByte bnk,f,0
	Return bnk
End Function

In this case I am using the BLZ memory DLL and decls which was posted around here some time since, probably as part of something else. I could email it to you if you do not have it or something similar.

I also came across some constants which you might find useful. I think you may find explanations for them by looking up the MessageboxEx function.