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EM_GETLINE Problem


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#1 PhilHalf   Members   -  Reputation: 148

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Posted 29 March 2003 - 04:23 AM

I have made a dialog based program that adds a string that is typed into an edit box into a list box. Previously this was working with no problems, however recently I have formatted my computer (after backing up all of the files for my application) and now that I have reinstalled everything, my program isn''t working properly. The code to add the string is as follows:
  
case IDC_ADDSTRING:
    // Add the string in IDC_STRING to the string list box IDC_LISTSTRINGS) and reset the string edit control

    iLineLength = SendMessage(GetDlgItem(hwnd, IDC_STRING), EM_LINELENGTH, 0, 0);
    szString = new TCHAR[iLineLength];
    iLineLength = SendMessage(GetDlgItem(hwnd, IDC_STRING), EM_GETLINE, NULL, (LPARAM)&szString);
    SendMessage(GetDlgItem(hwnd, IDC_LISTSTRINGS), LB_ADDSTRING, 0, (LPARAM)&szString);
    SetWindowText(GetDlgItem(hwnd, IDC_STRING), "");
    delete [ ] szString;
    break;
  
As I said, this worked before reinstalling everything, but now it is just adding garbage to the list box. Having done some debugging, it is reading zero characters in the EM_GETLINE call which, according to the MSDN, means that "the line number specified by the line parameter is greater than the number of lines in the edit control". However, the edit control that I''m using is set to single line and so the WPARAM parameter should be ignored. Any ideas as to what the problem is and why it might have just started happening? Thanks in advance for any help or advice, PhilHalf

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#2 S1CA   Members   -  Reputation: 1398

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Posted 29 March 2003 - 06:38 AM

I think you were very lucky that the code was ever working!, as it stands, it trashes random memory.


iLineLength = SendMessage(GetDlgItem(hwnd, IDC_STRING), EM_GETLINE, NULL, (LPARAM)&szString);

1) To quote MSDN regarding the buffer you pass in lParam: Before sending the message, set the first word of this buffer to the size, in TCHARs, of the buffer. For ANSI text, this is the number of bytes; for Unicode text, this is the number of characters.

2) You allocate the string with a "new", so szString is a **pointer** to (i.e. the address of) the memory for that string. So your extra "&" is making that a pointer to a pointer to the buffer - remove that &.

3) NULL != 0 (sometimes), wParam should be 0, not NULL.


SendMessage(GetDlgItem(hwnd, IDC_LISTSTRINGS), LB_ADDSTRING, 0, (LPARAM)&szString);

1) MSDN: "Pointer to the null-terminated string that is to be added". Is the string in the buffer null terminated?... Personally I''d use the length value you''ve retrieved to extend the size of the buffer by an extra character and explicitly put that \0 in.

2) Same issue as the previous line with another inccorrect "&" taking a pointer to the pointer rather than just passing a single pointer.

--
Simon O''Connor
Creative Asylum Ltd
www.creative-asylum.com

#3 PhilHalf   Members   -  Reputation: 148

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Posted 29 March 2003 - 09:52 AM

Thanks for the reply. I really should ask go back and learn all this properly.
Anyway, having looked through the edit box sample in the MSDN I have managed to get it working with the line

*((LPWORD)szString) = iLineLength;

Could someone explain what this is doing? I''m not sure I understand it properly. The way I think it''s working is that it''s casting szString to a pointer to a WORD then dereferencing it (to access the variable rather than the address) and assigning the value of iLineLength to it. Is that right? If so, wouldn''t that have lost the memory that was allocated previously if the type is changed to a WORD? I''m just not clear on exactly how casts work. Or pointers for that matter, not completely anyway.

Thanks again for any help.

PhilHalf

#4 S1CA   Members   -  Reputation: 1398

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Posted 29 March 2003 - 02:28 PM

1) Make sure you go back and fix the other things I mentioned - they''re causes of future random instability bugs.


2) As for what that line does:

a. "szString" is a pointer to (the address of) an array of TCHARs for your string.

b. "(LPWORD)" is really "(WORD *)" - which is casting from a TCHAR* to a WORD*, i.e. turn the pointer from a pointer to TCHARs to a pointer to WORDs. The value in the pointer (the address of the array) remains the same. This cast is only to tell the compiler to reinterpret the type of what the pointer points to (i.e. telling the type safety of the compiler "don''t worry, I know best, this memory can also be interpreted as an array of WORDs").

c. "*(blah)" - accesses the memory whose address is specified inside the brackets. In the use you posted, it writes to that memory. The "type" that the pointer contained in the brackets determines the size that gets written.

d. You could rewrite that code as follows:
WORD* wptr;
wptr = (WORD *)szString;
*wptr = iLineLength;

e. Or you could rewrite as:
WORD* wptr;
wptr = (WORD *)szString;
*(wptr+0) = iLineLength;

f. Or as:
WORD* wptr;
wptr = (WORD *)szString;
wptr[0] = iLineLength;




--
Simon O''Connor
Creative Asylum Ltd
www.creative-asylum.com




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