Microsoft KB Archive/91149

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Using volatile to Prevent Optimization of try/except

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Q91149

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The information in this article applies to:


 * Microsoft Win32 Application Programming Interface (API), included with:
 * the operating system: Microsoft Windows NT, versions 3.1, 3.5, 3.51
 * Microsoft Windows 95

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SUMMARY
The following is an example of a valid optimization that may take programmers by surprise.


 * 1) A variable (temp) used only within the try-except body is declared outside it, and therefore is global with respect to the try.
 * 2) Assignment to the variable (temp) is in the program only for a possible side effect of doing a read memory access through the pointer.

MORE INFORMATION
For example:

VOID puRoutine( PULONG pu ) {  ...   ULONG temp;       // Just for probing ...  try { temp = *pu;   // See if pu is a valid argument }

except { // Handle exception } } The compiler optimizes and eliminates the entire try-except statement because temp is not used later.

If the value of temp were used globally, the compiler should treat the assignment to temp as volatile and do the assignment immediately even if it is overwritten later in the body of the try. The reasoning is that, at almost any point in the try body, control may jump to the except (or an exception filter). Presumably the programmer accessing the variable outside the try wants to get the current (most recently assigned) value.

The way to prevent the compiler from performing the optimization is:

  temp = (volatile ULONG) *pu; If a temporary variable is not needed, given the example, the read access should still be specified as volatile, for example:

  *(volatile PULONG) pu; Additional query words: 3.10 3.50 4.00

Keywords : kbOSWinNT310 kbOSWinNT350 kbOSWinNT351 kbOSWin95

Issue type :

Technology : kbAudDeveloper kbWin32sSearch kbWin32API