Q75439: INFO: Tips for Writing DBCS-Compatible Applications

Article: Q75439
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kb16bitonly kbSDKPlatform
Last Modified: 10-JUN-1999

3.00 3.10
WINDOWS
kbprg

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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SUMMARY
=======

String operations in systems that use a Double-Byte Character Set (DBCS) are
slightly different from a single-byte character system. This article provides
guidelines to reduce the work necessary to port an application written for a
single-byte system to a DBCS system.

MORE INFORMATION
================

In a double-byte character set, some characters require two bytes, while some
require only one byte. The language driver can distinguish between these two
types of characters by designating some characters as "lead bytes." A lead byte
will be followed by another byte (a "tail byte") to create a Double-Byte
Character (DBC). The set of lead bytes is different for each language. Lead
bytes are always guaranteed to be extended characters; no 7-bit ASCII characters
can be lead bytes. The tail byte may be any byte except a NULL byte. The end of
a string is always defined as the first NULL byte in the string. Lead bytes are
legal tail bytes; the only way to tell if a byte is acting as a lead byte is
from the context.

The Windows Software Development Kit (SDK) version 3.0 includes two functions for
moving through strings that may contain DBCs: AnsiNext() and AnsiPrev(). The
AnsiPrev() function is a time expensive call because it must run through the
string from the beginning to determine where the previous character begins. It
is best to search for characters from the beginning rather than the end of a
string.

The Windows SDK version 3.1 includes the IsDBCSLeadByte() function, which returns
TRUE if and only if the byte CAN BE a lead byte. Because this function takes a
char parameter, it cannot report if the byte IS a lead byte (to do so would
require context).

To make non-DBCS code run as quickly as possible, a source file may use "#ifdef
DBCS" around code that is only for DBCS, and compile two versions of the object
(OBJ) file. For example:

     #ifdef DBCS
       for (pszTemp = szString; *pszTemp; pszTemp = AnsiNext(pszTemp))
     #else
       for (pszTemp = szString; *pszTemp; ++pszTemp)
     #endif

     ...

To make the code easier to read, an application could define macros for the
AnsiNext() and AnsiPrev() functions if DBCS is not defined:

     #ifndef DBCS

     #define AnsiNext(x) ((x)+1)
     #define AnsiPrev(y, x) ((x)-1)

     #ifdef WIN31

     #define IsDBCSLeadByte(x) (FALSE)

     #endif

     #endif

With these definitions in place, all of the code can be written for DBCS. Note
that the AnsiNext() function will not go past the end of a string and the
AnsiPrev() function will not go past the beginning of a string, while the macros
will. In addition, because the "y" parameter in the AnsiPrev() macro is ignored,
some code will give different results when compiled with and without DBCS
defined. The following code is an example of this phenomenon:

     pszEnd = AnsiPrev(++pszStart, pszEnd);

The following code demonstrates how to find the offset of the filename in a full
path name:

     LPSTR GetFilePtr(LPSTR lpszFullPath)
     {

      LPSTR lpszFileName;

      for (lpszFileName = lpszFullPath; *lpszFullPath;
                 lpszFullPath = AnsiNext(lpszFullPath))
          if (*lpszFullPath == ':' || *lpszFullPath == '\\')
              lpszFileName = lpszFullPath + 1;

      return lpszFileName;

     }

Note that ':' and '\\' are guaranteed not to be lead bytes. The search started
from the beginning of the string rather than the end to avoid using the
AnsiPrev() function.

The following code demonstrates a string copy into a limited size buffer. Note
that it ensures that the string does not end with a lead byte.

     int StrCpyN(LPSTR lpszDst, LPSTR lpszSrc, unsigned int wLen)

     {

      LPSTR lpEnd;
      char cTemp;

      // account for the terminating NULL
      --wLen;

      for (lpEnd = lpszSrc; *lpEnd && (lpEnd - lpszSrc) < wLen;
                 lpEnd = AnsiNext(lpEnd))
          ;  // scan to the end of string, or wLen bytes

      // The following can happen only if lpszSrc[wLen-1] is a lead
      // byte, in which case do not include the previous DBC in the copy.
      if (lpEnd - lpszSrc > wLen)
          lpEnd -= 2;

      // Terminate the source string and call lstrcpy.
      cTemp = *lpEnd;
      *lpEnd = '\0';
      lstrcpy(lpszDst, lpszSrc);
      *lpEnd = cTemp;

     }

Additional query words: 3.00 3.10

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Keywords          : kb16bitonly kbSDKPlatform 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1
Issue type        : kbinfo

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