Virtual variables and formatting characters in debugging

Pseudo Variables

Probably the most common pseudo variable is $err, which displays the last error value set with the function SetLastError. The value that is displayed represents what would be returned by the GetLastError function.

Pseudo variables can also be used to display the value of a processor register. For example, $ecx displays the value of the ecx register from the x86 architecture. Looking at the values of processor registers might seem a bit enigmatic to the new generation of developers using .NET, but it can really help in diagnosing obscure bugs or just understanding a program's behavior at run time. You can use $ecx to display the address of the "this" pointer, assuming you're using the native C++ calling convention on x86. You can also use $eax to display a 32-bit return value on both architectures. There are many more registers that may come in handy depending on your debugging needs.

Other useful pseudo variables include $handles, which displays the number of open handles to kernel objects in the process, and $user, which displays very detailed information about the current process and thread tokens. The latter can be useful for debugging impersonation-related code. Figure 1 lists some common pseudo variables.

  Figure 1 Pseudo Variables

 

Pseudo Variable	Description
$handles	Number of handles to kernel objects
$vframe	Current stack frame address
$TID	Current thread identifier
$registername	Contents of specified register
$clk	Time in clock cycles
$user	Process and thread token information

 

Format Specifiers

Format specifiers come in handy when you want to change the way a watch window displays the value of a variable, pseudo variable, or expression. In most cases, the watch window will figure out the best format for the value based on its type, but there are cases when you might need to override this. The debugger might not be able to deduce the type of variable, or you might just have a memory address that naturally has no type information. For example, the hr format specifier will display the Win32 or HRESULT corresponding to the value. Other examples include su, which will display the value as a null terminated Unicode string.

Of course, you might want to get at the actual value behind some formatted text and ! will do just that. Finally, given a pointer, you can force the watch window to treat it as an array of n elements using a comma followed by the number of elements to display. Figure 2 lists format specifiers.

 Figure 2 Format Specifiers

Specifier	Description
D	Decimal
U	Unsigned decimal
O	Octal
X	Hexadecimal
F	Floating point
E	Scientific notation
C	Character
S	Character string
Su	Unicode string
s8	UTF-8 string
Hr	HRESULT or Win32 error code
wc	Windows class
wm	Windows message
!	Raw format

To quote http://msdn.microsoft.com/en-us/magazine/dd252945.aspx#id0080009