Microsoft KB Archive/74788

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Using Near and Far Pointers with C Run-Time Functions

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Q74788

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1

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SUMMARY
In an application designed for the Microsoft Windows graphical environment, many C run-time functions do not work with memory allocated by the GlobalAlloc function when the application is developed in the small or medium memory model.

MS-DOS (non-Windows) applications written in the small or medium memory model assume the presence of only one data segment (DS). Therefore, the C Run-time functions assume that DS will not change.

However, an application can store data in a block of memory allocated with the GlobalAlloc function and locked with the GlobalLock function. The segment returned from GlobalLock will be different from the application's data segment. Specifying the alternate data segment in a C run-time function that assumes a near pointer results in the following C compiler warning:

WARNING: Segment Lost in Conversion

For example, the following code passes far pointers to a Run-time function incorrectly:

  hMem = GlobalAlloc (...); lpMem = GlobalLock (hMem);

strcpy (szBuffer, lpMem);

GlobalUnlock (hMem); This section of incorrect code produces one of two results.


 * 1) If the offset of lpMem extends past the end of application's data segment (DS), the application experiences an unrecoverable application error (UAE).
 * 2) The function copies information from some random portion of the application's DS into the buffer.

If the following line of code is used, the function overwrites data in the application's data segment, which causes the application to crash or run incorrectly:

  strcpy (lpMem, szBuffer); Four ways to work around this situation are:

  For the most common C run-time functions, Windows provides equivalent functions that use far pointers. These functions include:      lstrcat lstrcmp lstrcmpi lstrcpy lstrlen  Use the far pointer versions of these functions (_fstrcat, _fstrcmp, and so on) provided by the Microsoft C Optimizing Compiler versions 6.0 and later. For the less common C Run-time functions, write a far-pointer version as part of the application code. Most of the Microsoft C Run-time library source code is available from Microsoft. Use the large memory model.

NOTE: Using the large memory model in Windows 3.0 had many disadvantages and doing so was not encouraged. These problems were corrected in Windows 3.1 and Microsoft C/C++ compiler. Hence, using the large memory model is not a problem.

Additional query words: 3.00 3.10

Keywords : kb16bitonly

Issue type :

Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310