Problem returning string from callback

[Charles Dietz]

The code below demonstrates a problem I'm having showing a dialog box's title bar. The title should be blank, but instead, is three extraneous characters. I believe the problem is associated with the myInput routine returning the specified string from its callback function. The problem goes away if I just change the routine from a function to a sub and use a global string for the specified one.

I would like to keep myInput as a function if possible, and to understand why it is incorrect. I sure would appreciate some feedback on this. I have been using this replacement for PowerBasic's input function for some time, not knowing there was a problem with it.

#COMPILE EXE
#DIM ALL
#INCLUDE "WIN32API.INC"

%IDC_myInput = 1000

FUNCTION myInput(OPT BYVAL prompt AS STRING, BYVAL ti AS STRING) AS STRING
   LOCAL hInput, nStyle AS LONG
   LOCAL s AS STRING, ps AS STRING PTR
   nStyle = %WS_CAPTION OR %WS_POPUP OR %WS_SYSMENU OR %DS_MODALFRAME 
   DIALOG NEW 0, ti, , , 130, 85, nStyle TO hInput
   CONTROL ADD LABEL, hInput, -1, prompt, 5, 5, 80, 12
   CONTROL ADD TEXTBOX, hInput, %IDC_myInput, "test", 10, 20, 100, 12
   CONTROL ADD BUTTON, hInput, %IDOK, "Ok", 60, 40, 50, 20 
   ps = STRPTR(s): DIALOG SET USER hInput, 1, ps
   DIALOG SHOW MODAL hInput, CALL myInputProc
   FUNCTION = @ps
END FUNCTION 

CALLBACK FUNCTION myInputProc()
   LOCAL ps AS STRING PTR
   IF CBMSG = %WM_COMMAND AND (CBCTL = %IDOK OR CBCTL = %IDCANCEL) THEN 
      DIALOG GET USER CBHNDL, 1 TO ps
      IF CBCTL = %IDOK THEN CONTROL GET TEXT CBHNDL, %IDC_myInput TO @ps
      DIALOG END CBHNDL
   END IF 
END FUNCTION 

%IDC_Problem =  100
GLOBAL hDlg AS LONG
GLOBAL findStr AS STRING
GLOBAL thisFile AS STRING
DECLARE SUB showProb()

FUNCTION PBMAIN()
   DIALOG NEW 0, "Test Window Title", , , 200, 200, %WS_SYSMENU OR %WS_MINIMIZEBOX TO hDlg
   CONTROL ADD BUTTON,  hDlg, %IDC_Problem, "&Show Problem", 50, 50, 70, 40
   DIALOG SHOW MODAL hDlg, CALL dlgProc
END FUNCTION

CALLBACK FUNCTION dlgProc()
   SELECT CASE AS LONG CBMSG
      CASE %WM_COMMAND
         SELECT CASE CBCTL
            CASE %IDC_Problem
               IF CBCTLMSG = %BN_CLICKED OR CBCTLMSG = 1 THEN
                  findStr = myInput("Search for: ")
                  IF LEN(findStr) THEN thisFile = "": showProb
               END IF 
         END SELECT
   END SELECT   
END FUNCTION

SUB showProb()
   LOCAL nStyle, hProbDlg AS LONG
   DIALOG NEW hDlg, "", 20, 10, 200, 150, %WS_SYSMENU TO hProbDlg
   DIALOG SHOW MODAL hProbDlg, CALL ShowNoteProc
END SUB

CALLBACK FUNCTION showNoteProc()
   SELECT CASE CBMSG
      CASE %WM_INITDIALOG
         DIALOG SET TEXT CBHNDL, thisFile
   END SELECT
END FUNCTION

[Pierre Bellisle]

Hi Charles,
you problem come from mixing asciiz pointer with string pointer

 LOCAL s  AS STRING
 s = "string"
 
 LOCAL ps AS STRING POINTER
 ps = STRPTR(s)
 
 LOCAL pz AS ASCIIZ POINTER
 pz = STRPTR(s)
 
 MSGBOX @ps 'Kaboom, "@" is for ASCIIZ and ps is declared as a STRING POINTER
            'The compiler won't like this.
 
 @ps = "chain" 'Same problem, "@" is for ASCIIZ and ps is declared as a STRING POINTER
 
 MSGBOX @pz 'This will work if there is no "zero" embedded in string, like in multi-sz...
 
 @pz = "longer string" 'If this string is longer than the original one then you will
                       'corrupt memory becose the original string is only 5 characters long
                       'and you try to write past the end

In your code you may use ASCIIZ instead of string by...

replacing LOCAL s AS STRING, ps AS STRING PTR
with      LOCAL s AS ASCIIZ * %MAX_PATH, ps AS ASCIIZ PTR
 
replacing ps = STRPTR(s): DIALOG SET USER hInput, 1, ps
with      ps = VARPTR(s): DIALOG SET USER hInput, 1, ps

[Charles Dietz]

Thanks for your reply, Pierre. There still seems to be something wrong. Insert the following msgbox in the dlgProc() callback function, and you will see that myInput returns a string of length 3 instead of 4.

      findStr = myInput("Search for: ")
      MSGBOX STR$(LEN(findStr)) + ", " + findStr

[Peter Jinks]

Hi Charles,

I've had a quick look through your code and it seems to me that your problem lies in this line in myInput:

ps = STRPTR(s): DIALOG SET USER hInput, 1, ps

because I think it should be this instead:

ps = VARPTR(s): DIALOG SET USER hInput, 1, ps

The reason is, you have ps declared as a STRING PTR, so you need to point it at a STRING. VARPTR(s) is the address of the STRING. STRPTR(s) is the address of the contents of the STRING - effectively an ASCIIZ buffer. So, point your pointer at the STRING and all should be well.



Regards,

Pete.

[Charles Dietz]

Yes, Peter... I think you're exactly right. And thanks for the explanation as well.
I have made this mistake before, getting STRPTR and VARPTR backward. Maybe someday I'll learn. Thanks again.

[Charles Dietz]

And Pierre, your way works also if I replace 'LOCAL ps AS ASCIIZ PTR' in myInputProc() as well. I guess that should have been obvious, but I missed it. Thanks again.