Unicode problems with Delphi 2009 / 2010 and windows API calls

Question

Hi I have been using this function in Delphi 2006, but now with D2010 it throws an error. I think it is related to the switch to Unicode.

  Function TWinUtils.GetTempFile(Const Extension: STRING): STRING;
  Var
     Buffer: ARRAY [0 .. MAX_PATH] OF char;
  Begin
    Repeat
      GetTempPath(SizeOf(Buffer) - 1, Buffer);
      GetTempFileName(Buffer, '~~', 0, Buffer);
      Result := ChangeFileExt(Buffer, Extension);
    Until not FileExists(Result);
  End;

What should I do to make it work?

EDIT

I get an 'access violation' when the ChangeFileExt is called

you should edit your question to include the error message (a good habit - makes sense, right?) — Argalatyr, Sep 14 '09 at 13:02
Please see the following link for both a discussion of the problem (nearly the same code) and a better version that takes into account some more of what the API documentation says regarding maximum buffer size. https://forums.embarcadero.com/thread.jspa?threadID=18246 — Rob Kennedy, Sep 14 '09 at 13:53
This link no longer works. What is the correct way to search by thread id? — user173399, May 04 '20 at 02:32

RRUZ · Accepted Answer · 2009-09-14T11:54:48.983

Windows.Pas

function GetTempFileName(lpPathName, lpPrefixString: PWideChar;
  uUnique: UINT; lpTempFileName: PWideChar): UINT; stdcall;

function GetTempPath(nBufferLength: DWORD; lpBuffer: PWideChar): DWORD; stdcall;

SysUtils.Pas

function ChangeFileExt(const FileName, Extension: string): string;

Try this

  Function TWinUtils.GetTempFile(Const Extension: STRING): STRING;
  Var
     Buffer: ARRAY [0 .. MAX_PATH] OF WideChar;
  Begin
    Repeat
      GetTempPath(Length(Buffer), Buffer);
      GetTempFileName(Buffer, '~~', 0, Buffer);
      Result := ChangeFileExt(Buffer, Extension);
    Until not FileExists(Result);
  End;

or this

  Function GetTempFile(Const Extension: String): String;
  Var
     Buffer: String;
  Begin
      SetLength(Buffer,MAX_PATH);
    Repeat
      GetTempPath( MAX_PATH, PChar( Buffer) );
      GetTempFileName(PChar( Buffer), '~~', 0, PChar( Buffer));
      Result := ChangeFileExt(Buffer, Extension);
    Until not FileExists(Result);
  End;

For Delphi, Char and PChar types are WideChar and PWideChar types, respectively.

If you use any Windows API’s that return data into char buffers , those buffers need to be redeclared as arrays of bytes or an array of AnsiChar.

If you are calling these Windows API’s and sending in buffers, if have been using the sizeof function when telling the API how long your buffer is. Those calls need to be changed to the Length function, as the Windows widechar API’s require the number of characters, not the number of bytes.

Bye.

And the reason the old code was a problem is the SizeOf(Buffer) - 1 in GetTempPath(). SizeOf an array of Chars was the same as the Length for Delphi before 2009, because SizeOf(AnsiChar) = 1. With Delphi 2009 and later, SizeOf(Char) = SizeOf(WideChar) = 2, so the code was asking GetTempPath() to fill the buffer with more characters than it could hold. — jasonpenny, Sep 14 '09 at 11:22
I tried both options in "delphi 2010", and they worked ok. ;) — RRUZ, Sep 14 '09 at 12:18
Char is not Widechar! It is UnicodeChar. There is a slight difference. — Toon Krijthe, Sep 14 '09 at 12:21
Gamecat in the delphi 2010 help ms-help://embarcadero.rs2010/rad/Simple_Types.html "The general character type is Char, which is equivalent to WideChar now that the default string type is UnicodeString". — RRUZ, Sep 14 '09 at 12:31
@Gamecat: `Char` in D2009+ is indeed an alias for `WideChar`. There is no `UnicodeChar` type, and never has been. — Remy Lebeau, Jun 07 '12 at 06:36

score 0 · Answer 2 · answered Jun 07 '12 at 01:52

I'd recommend to always, and I really mean always, to have a look into the docs about the exact expectations of each API function regarding the dwSize parameter and/or the return value.

Unfortunately there are a lot of different cases, so just saying "all string functions need/return number of chars" is not quite correct and may send the reader into a nightmare of sporadic invalid pointer AVs.

Most of the API functions do expect/return number of chars, but some don't.
Some count the terminating null char, and some don't.
Some functions behave differently when a nil pointer is passed vs. when a non-nil pointer is passed.
And there are also some functions that do not offer any way at all to indicate the required buffer size.

All these things can be found in the docs easily, but one should read it. Really. It can save you hours.

score 0 · Answer 3 · edited Sep 14 '09 at 11:25

0

With Delphi 2009, Char is a unicode char. The function probably requires an array of AnsiChar.

Just checked it. Both GetTempFilename and GetTempPath require a PWideString.

What is the error message?

edited Sep 14 '09 at 11:25

Wim ten Brink

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answered Sep 14 '09 at 10:47

Toon Krijthe

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Are you should you don't mean `PWideChar`? `PWideString` is something else entirely. – Rob Kennedy Sep 14 '09 at 13:48

score 0 · Answer 4 · edited Aug 02 '12 at 00:56

0

Use GetTempPathA and GetTempFileNameA, the Ansi versions of GetTempPath and GetTempFileName. They are still available in Delphi 2009, and mentioned in the Delphi 2009 help file, but not advertised.

edited Aug 02 '12 at 00:56

GDP

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answered Mar 27 '10 at 09:19

Reversed Engineer

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On Windows NT OS versions most if not all *A versions of the API functions are just stubs that convert parameters and call the *W versions of same function. So in Delphi 2009+ you would convert internal wide char data to Ansi, call *A function, which converts internally to wide chars and back, and the program would take the result and convert it back to wide chars. Would you say that going this route makes any sense? – mghie Mar 27 '10 at 10:07

Unicode problems with Delphi 2009 / 2010 and windows API calls

4 Answers4

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