How to set conditional breakpoin based on parameter value using windbg

Question

I want to set a conditional breakpoint based on a member of one parameter, Now I can use command dt to check this parameter, My code looks like this:

void test(const MyClassB &inst, int value)
{
}

Now I can use the dt command to view the first parameter, the result:

0:000:x86> dt MyClassB @esp+4
dbgee!MyClassB
   +0x000 id               : (null) 
   +0x004 m                : 0n2130567168
   +0x008 myClassA         : MyClassA

Now I want to set a conditional breakpoint on this method based on the value of inst.m, anybody can show me how to do that? Thanks a bunch!

And It's appreciated if anybody can give some information about how to use the offset(such as +0x004 for m), thanks!

Answer 1

Snippet Compiled with cl /Zi /nologo /W4 /analyze %1% /link /RELEASE in msvc++2010exp

 #include <stdio.h>
    class MyClass {
        int width,length;
    public:
        void set_val(int,int);
        int  area(); 
    };
    void MyClass::set_val(int x , int y) {
        width  = x;
        length = y;
    }
    int MyClass::area() {
        return width*length;
    }
    void main(void) {  
        MyClass foo;
        for (int i = 0; i < 10; i++) {
            foo.set_val(i,5);
            printf("%d\n",foo.area());
        }
    }

In windbg to set a conditional breakpoint on set_val() to break when width == 7

Conditional Breakpoint syntax Explanation

classtest!MyClass::set_val <module!class::method>
@@c++() using c++ expression evaluator
@ecx holds the this pointer suitably type casted to MyClass *
width is a member of MyClass
we use 6 for comparison purpose in this demo
as we are setting break point prior to execution of set_val() method
(notice the uninitialized garbage when printing the first time
gc go from condition after displaying MyClass if width != 7
.else is implied to break when width == 7

command should be in one line

bp classtest!MyClass::set_val    
".if( @@c++((((MyClass *) @ecx )->width)) != 6 ) {dt MyClass @ecx ; gc }"

results

0:000> bp classtest!MyClass::set_val ".if( @@c++((((MyClass *) @ecx )->width)) != 6 ) {dt MyClass @ecx ; gc }"
0:000> bl
 0 e 00401000     0001 (0001)  0:**** classtest!MyClass::set_val ".if( @@c++((((MyClass *) @ecx )->width)) != 6 ) {dt MyClass @ecx ; gc }"
0:000> g
ModLoad: 5cb70000 5cb96000   C:\WINDOWS\system32\ShimEng.dll
classtest!MyClass
   +0x000 width            : 0n4205541
   +0x004 length           : 0n4208683
classtest!MyClass
   +0x000 width            : 0n0
   +0x004 length           : 0n5
classtest!MyClass
   +0x000 width            : 0n1
   +0x004 length           : 0n5
classtest!MyClass
   +0x000 width            : 0n2
   +0x004 length           : 0n5
classtest!MyClass
   +0x000 width            : 0n3
   +0x004 length           : 0n5
classtest!MyClass
   +0x000 width            : 0n4
   +0x004 length           : 0n5
classtest!MyClass
   +0x000 width            : 0n5
   +0x004 length           : 0n5
eax=00000007 ebx=7ffdf000 ecx=0013ff70 edx=00416680 esi=00000000 edi=0098f6ee
eip=00401000 esp=0013ff60 ebp=0013ff78 iopl=0         nv up ei ng nz ac po cy
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00000293
classtest!MyClass::set_val:
00401000 55              push    ebp
0:000> dd esp l3
0013ff60  0040106c 00000007 00000005
0:000> x @eip
0:000> ?? @eip
unsigned int 0x401000
0:000> lsa . 0,1
>    8: void MyClass::set_val(int x , int y) {
0:000> dv
           this = 0xfffffffe
              x = 0n7
              y = 0n5