C++ Debugger Tips

C++ Debugger Tips

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Hi, my name is Sergey Grankin and I'm a developer on the VC++ IDE Team.  I mostly work on the C++ debugger expression evaluators -- the code responsible for the magic behind the debugger watch windows. 


Visual Studio's (native) C++ debugger has many useful features that can make your debugging much more pleasant, if you know what they are.  These tend to accumulate over releases, and get forgotten and unused, unless you happen upon an archaic piece of documentation. On this topic, then, there are special expression and format specifiers that you can use to better examine the content in the debugger's watch windows.


For example, say we break after the following bit of code:


          int i = 0x12345678;


You can use the by, wo, and dw operators to view contents of a variable as an unsigned byte, word, or dword:


          i                 0x12345678             int

          by i            0x78 'x'                    unsigned char

          wo i           0x5678                    unsigned short

          dw i           0x12345678            unsigned long


You can also use the operators on a register to do the same to the destination of the register:


          eax 0x0012ff2c                       unsigned long

          by eax       0x78 'x'                 unsigned char

          wo eax      0x5678                 unsigned short

          dw eax      0x12345678         unsigned long


These come in handy when debugging through assembly.


Another way to change debugger output is through format specifiers.  These are directives passed after the expression, separated by a comma. For example, to change the radix out the output, you can append ',o' for octal, ',d' for decimal, or ',x' for hex:


          i       42                                  int

          i,o     052                              int

          i,d     42                                int

          i,x     0x0000002a                 int


To interpret a pointer expression as a string, you can use ',s' for an simple null-terminated string, ',s8' for a UTF-8 string, or ',su' for a Unicode string. (Note that the expression has to be a pointer type for this to work).


          char str[] = "hello";

          wchar_t str2[] = L"world";


    str                          0x0012ff00 "hello"                        char [6]

          str,s                        "hello"                                            char [6]

          str2                         0x0012fee8 "world"                     wchar_t [6]

          (void*)str2,su      "world"                                           void *


The memory operators can be used to display up to 64 bytes of memory in the preview line, as bytes, words, dwords, quads, or ascii characters. 


      str,m          0x0012ff00   68 65 6c 6c 6f 00 cc cc cc cc cc cc cc cc cc cc  hello.                   char [6]

       str,mb        0x0012ff00    68 65 6c 6c 6f 00 cc cc cc cc cc cc cc cc cc cc  hello.                   char [6]

       str,mw       0x0012ff00    6568 6c6c 006f cccc cccc cccc cccc cccc                                      char [6]

       str,md        0x0012ff00    6c6c6568 cccc006f cccccccc cccccccc                                          char [6]

       str2,mu      0x0012feec    0077 006f 0072 006c 0064 0000 cccc cccc  world.??              wchar_t [6]

       str,mq        0x0012ff00    cccc006f6c6c6568 cccccccccccccccc                                            char [6]

       str,ma         0x0012ff00    hello.(..(......T..                                                                     char [6]


You can use ,wc ,wm and ,hr to view data as a window class, window message, or HRESULT.


       0x00400000,wc     WS_OVERLAPPEDWINDOW                                                   int

       0x10,wm               WM_CLOSE                                                                                   int

       0x10,hr                 0x00000010 The directory cannot be removed.                              int


Finally, you can use ,! to turn off STL visualizations on the expression:


      str       "hello world"                                                                                                       std::basic_string< ... >

       str,!     {_Bx={...} _Mysize=0x0000000b _Myres=0x0000000f}                               std::basic_string<...>


All of these operators can be used to ease the way you get to data while debugging, and become necessary whern creating custom visualizations. You can check-out the autoexp.dat file in your Visual Studio directory for examples of how to combine these operators and the visualization language to create custom visualizers for your own data.


-- sergey grankin // vc++ dev team

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  • Is there anyway to customize the debug string that appears for a class?  Like maybe implement a __repr__() function that returns the string will appears in the watch window?

    Is there a way to do this for windbg?
  • C++ guy:

    Look for a file called autoExp.dat. Native expression evaluators are contained in here.
    It contains (very basic) documentation on the formating plus many examples. Imitating it you should be able to implement something suitable for your custom types.
  • Excellent, thanks for the tips..I knew about and use some of these but many are new to me.  

  • In fact, I think that the most useful "format specifier" is one allowing to represent pointers as arrays.

    You just add ,N to the evaluation expression and the debugger print first N elements of array

  • heh:
    i,o     052                              int

    ..that's the weirdest way to implement ioctls I've ever seen :)

  • Hi,

    I just learned that I can use "x, 10" to express a pointer as an array. Now the problem is how could I evaluate a double pointer as a matrix? Thanks
  • hello people is good day!!! http://autonewsportal.info http://autdotcar.info

  • Thanks a lot for such a valuable information. Can you kindly point us to an official documentation of things we can do with the debugger in VS 2k3+.

  • PingBack from http://www.pcdebug.com/windows-debugging/visual-c-debugging-tips.html

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