VB Reading Strings - Lost and Confused

[UAL45]

Anyone who can help !!

I am not a whiz kid at pointers and such. I have been very successful at pulling the data numbers from the offsets. Just when I get to Strings... well it falls apart. Can somebody look at the function below and tell me what I am doing wrong ?? Each time I runs it returns "" or null.

All I am trying to do is pull the ATC Tail Number from offset 313C...

------------------------------------------------------------------------------

Public Function ACID() As String
Dim ReturnVal As Long
Dim RTNString As String

If FSUIPC_Read(&H313C, 12, StrPtr(RTNString), ReturnVal) Then
    If FSUIPC_Process(ReturnVal) Then
       ACID = RTNString
    End If
Else
    ACID = "NA"
End If
End Function 

------------------------------------------------------------------------------

Any help would be appreciated.

Brian[/code]

[vdkeybus]

I'm not familiar with VB, but here are some remarks:

1. After ACID = RTNString, there is an End If, directly followed by Else. Is this syntax correct ?

2. When you assign the RTNString to to function's return value, does VB return the pointer to the string (like in C) or the whole string object ? In the first case, you will probably corrupt the string because it is a local object (you Dim'med it inside the function), as the storage space (stack) is immediately reused when any other function is called. Perhaps try and Dim it outside the function, i.e. at the same level as a function declaration ? This problem doesn't present itself with numbers, which may explain why everything worked fine for you up to now.

Again, I'm not sure. I always program in 'C'.

J.

[Pete Dowson]

ACID = "NA"

I don't know VB, but here are some possible problems for you to investigate:

1. How does the compiler know to provide space for 12 characters? Your "Dim" statements don't appear to state the maximum length needed for the string. The FSUIPC read might be overwriting something important if there aren't at least 12 character's worth of space.

2. Is this "NA" the full ATC Tail Number, or just the first 2 characters? If it isn't the complete tail number, does VB use only the length of the shorter string to do the comparison? Seems odd. In most languages a string "X" will not match a string "XY" unless you deliberately restrict the length of the comparison.

3. Is your VB system using ASCII strings in the same way as Windows and FSUIPC? The string you are reading will contain single (8-bit) bytes, one per chanracter, with a zero byte at the end. If your program is compiled to use double-width characters or Unicode, you will be expecting 2-bytes (16-bits) per character. Additionally, some languages store strings with a length byte at the beginning instead of a zero terminator at the end. All these things should be described someplace in your VB references.

In the last problem, you may have to resort to using an array of byte values instead of a "string" as such, especially if your VB system cannot handle standard ASCIIZ strings ("ASCIIZ" = ASCII 8-bit characters with Zero termination).

You should get some help here from other VB users. Sorry if my hypothesising makes it look rather complex. I'm sure it can't be that bad (though I must say I grew to hate VB a long time ago! :wink: ).

Regards,

Pete

[jd]

as soon as i get back from work, i'll post my routine to read aircraft id

[UAL45]

Thanks All for your input.

vdkeybus: The syntax is good. As far as corruption of the values / pointer . . I dont know... That is what I am hoping somebody will shed some light on. Thanks for the insight and help.

Pete: Yes, the compiler allows for that length and does use ASCIIZ. I am sure that I am just screwed up on how to reference the string pointer. By the way the "NA" is just a default return value if the FSUIPC read fails, nothing fancy.

jd: Thank you !! I look forward to your post !!

Thanks again all..

Brian

[Pete Dowson]

By the way the "NA" is just a default return value if the FSUIPC read fails, nothing fancy.

The Read is less likely to fail than the Process, because it doesn't really do anything other than add the request into a buffer in your own program. To do what you really want to do you need the

Else

ACID = "NA"

within the If/Endif section for the Process.

If VB accepts compound logic without executing both parts, you can of course combine both If's into one, with an "AND". This works okay in C/C++ because the code processes the separate condition from left to right, and will stop when the condition proves false. It may not work in VB though.

None of this helps with your actual problem, though. Sorry. I'm not able to answer that.

Regards,

Pete

[UAL45]

Pete,

Good point. I have found it that if the read fails so does the process. So, I just set the "NA" in the one istance. It would be better for me to have both.

I think the jd will be able to help me later today. Either way, good chance to hear what others think and learn something. Isnt that what it is all about ??

Regards,

Brian

[DocNZ]

This is how I do it in Borland Delphi Pascal.

The translation to VB should be very straigt forward.

var
  dwResult:DWord;
  Aircraft:array[0..256] of char;
  ACTailNum:array[0..12] of char;
begin
      if FSUIPC_Read($3d00, 256, @aircraft, dwResult) then
        begin
          if FSUIPC_Process(dwResult) then Label1.Caption := aircraft else DoCantConnect(dwResult);
        end;
      if FSUIPC_Read($313C, 12, @ACTailNum, dwResult) then
        begin
          if FSUIPC_Process(dwResult) then Label2.Caption :=  ACTailNum else DoCantConnect(dwResult);
        end;
end;

I hope this helps. (PS DoCantConnect is a procedure to handle errors reading from FSUIPC)

Paul T.

[UAL45]

Thanks Paul !!

One question, tho.. In the calls you pass the aircraft as @aircraft. In PASCAL is the @ the same as pointing to the address space for the variable named aircraft ?

Where I am getting lost and confused is between the var pointers and string pointers in VB. Your calls look similar to what I am doing, however, I am still getting an empty string returned to me...

Brian

[DocNZ]

yes @ is the delphi pointer operand.

I dont think Delphi makes the distinction between whether a pointer points to a string or an integer etc. In Delphi a pointer is a pointer. The distenction is made by the variable you declare. If I declared ACTailNum as an integer, then what whould be returned is a 12byte number, and not a null terminated string.

In your code I noticed that you declared RTNString as a string, whereas I decalre ACTailNum as an array of 12 characters. I think I would get an error if I declared it as a string in delphi too, becaue that @ACTailNum points to is not actually a string.

I just dont know, but isnt there an example of this very thing for VB in the FSUIPC SDK?

[Armando Chibante]

Hi Brian,

This is how I read a string in VB6:

Dim acName() As Byte

Dim CraftName As String

Dim dwResult As Long

Dim cnt as Long

If Not FSUIPC_Read(&H3D00, 256, VarPtr(acName(1)), dwResult) Then Exit Sub ' Aircraft name

If FSUIPC_Process(dwResult) Then

cnt = 1

Do While acName(cnt) <> 0

CraftName = CraftName & Chr(acName(cnt))

cnt = cnt + 1

Loop

End If

Hope this helps

Armando Chibante

[UAL45]

Armando,

Thank You. So.. it is a byte by byte move. Thanks !!!

Brian

[UAL45]

Works great now !! Just be sure to:

Dim acName(X) As Byte 

Where x is the length of the string to be read or some nasty compile errors show.

Thanks again everyone for your help !!