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Table 17-2: Debug C Run.Time Library Flags Flag Description requirements can easily double because the deallocated memory isn't reclaimed by the heap. Check for memory leaks at the end of the program. Turning on this extremely useful flag is mandatory.
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After building your application with the appropriate #includes and #defines, and calling _CrtSetDbgFlag, you now have the DCRT library, with its slew of functions that help you control and report on memory usage, fully available. You can call these functions at any time. Many of them lend themselves to being used in assertions, so you can sprinkle them around freely and catch your memory problems close to the source. The Bug in the DCRT If you follow the instructions I just gave you in the previous section, your source code will look something like that shown in Listing 17-1. At the top of the listing, after the rest of the headers, you see the _CRTDBG_MAP_ALLOC define and the include file for CRTDBG.H. The first call inside main is to _CrtSetDbgFlag to set up the DCRT. At the bottom, I've done three allocations, a malloc and two new, and all three pieces of memory are leaked. Listing 17-1: Setting up the DCRT and leaking memory // This define must occur before any headers are included. #define _CRTDBG_MAP_ALLOC #include <stdio.h> #include <stdlib.h> #include <string.h> #include <tchar.h> // Include CRTDBG.H after all other headers #include <crtdbg.h> void main ( void ) { // Turn on the full heap checking _CrtSetDbgFlag ( _CRTDBG_ALLOC_MEM_DF _CRTDBG_CHECK_ALWAYS_DF _CRTDBG_DELAY_FREE_MEM_DF _CRTDBG_LEAK_CHECK_DF // Allocate some more memory. TCHAR * pNew = new TCHAR[ 200 ] ; TCHAR * pNew2 = new TCHAR[ 200 ] ; TCHAR * pMemLeak = (TCHAR*)malloc ( 100 ) ; 619 | | | ) ;
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_tcscpy ( pNew , _T ( "New'd memory..." ) ) ; _tcscpy ( pNew2 , _T ( "More new'd memory..." ) ) ; _tcscpy ( pMemLeak , _T ( "Malloc'd memory..." ) ) ; }
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Since I've turned on leak checking in the code for Listing 17-1, you'll see detected memory leak output like the following in the Output window when the program executes: Detected memory leaks! Dumping objects -> NewProblem.cpp(22) : {62} normal block at 0x002F2E58, 100 bytes long. Data: <M a l l o c ' d > 4D 00 61 00 6C 00 6C 00 6F 00 63 00 27 00 64 00 G:\vsnet\Vc7\include\crtdbg.h(692) : {61} normal block at 0x002F2C88, 400 bytes long. Data: <M o r e 77 00 n e w > 4D 00 6F 00 72 00 65 00 20 00 6E 00 65 00
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G:\vsnet\Vc7\include\crtdbg.h(692) : {60} normal block at 0x002F4DC0, 400 bytes long. Data: <N e w ' d 65 00 m e > 4E 00 65 00 77 00 27 00 64 00 20 00 6D 00
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Object dump complete. What's very nice is that the leak output format is such that double-clicking on the source line displayed will automatically jump to that line in the source file. The first leak is on line 22 of NEWPROBLEM.CPP (shown in Listing 17-1), and double-clicking jumps you to the line that did the malloc. If you double-click on the second source line listed, you jump to CRTDBG.H (line 692). While you're staring at a new call, it's certainly not the new in your source code. Since I had multiple calls to new in the program, you can see that logically all calls to new are going to show up as coming from CRTDBG.H. As you can imagine, this doesn't help much when you're looking for leaks in a huge program! The problem lies in the declaration of new in CRTDBG.H, as shown here: inline void* __cdecl operator new[](size_t s) { return __LINE__); } ::operator new[](s, _NORMAL_BLOCK, __FILE__,
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If you've never experienced the joy of the placement syntax new, the preceding code could appear a little strange at first. The new operator is a very special construct in C++ in that it can take all sorts of different parameters. The call to ::operator new in the code shows that it's passing three additional parameters to new. This version of new is defined as part of CRTDBG.H. At first glance, the problem with the code isn't obvious. The __FILE__ and __LINE__ macros expand to the source name and line when compiled. However, as you've seen, they don't expand into your source and lines. The problem is with the first word in the code: inline. In release builds, inline indicates to the compiler that it's supposed to take the code inside the function and plop it directly into where it is being used 620
instead of making a function call. However, in a debug build, inline-specified functions, as everyone should realize, don't get expanded and are treated as real functions. Hence, the __FILE__ and __LINE__ macros expand into CRTDBG.H and 692. Since this puts a severe damper on using the DCRT, I certainly had to figure out a way to get any detected memory leaks to point to the allocation location in your source code. My original thought was to change CRTDBG.H, but because it's a system header, that didn't seem like such a great idea. After a lot of pondering, I finally came up with the following macro to be placed immediately after including CRTDBG.H in your precompiled header file. This simply turns any use of new into the placement syntax version of new, which takes the extra parameters. #ifdef _DEBUG #ifndef NEW_INLINE_WORKAROUND #define NEW_INLINE_WORKAROUND new ( _NORMAL_BLOCK ,\ __FILE__ , __LINE__ ) #define new NEW_INLINE_WORKAROUND #endif #endif // _DEBUG
Sharp-eyed readers might immediately think there's a problem with my NEW_INLINE_WORKAROUND trick. The one big problem with my approach is that if you have a class that defines the new operator, my macro will cause problems because your declarations will be all messed up. Although you might not define new operators for your classes, libraries such as MFC and STL certainly do. The whole trick is to define this only inside your precompiled header file and to include only libraries like MFC and STL inside your precompiled header as well as taking advantage of the improved #pragma push_macro/#pragma pop_macro directives. To see exactly what to do, read the Common Debugging Question appearing later in this section, titled "What are the rules for properly precompiling headers " If you do have your own classes or use classes that define the new operator, you can still use the NEW_INLINE_WORKAROUND macro if you're willing to add some code and a few directives to your class. The following code shows a stripped-down class with the necessary parts in it to make NEW_INLINE_WORKAROUND work like a champ. // Save off the definition of new on the macro stack. #pragma push_macro ( "new" ) // Undefine new so I don't blow the class declaration. #ifdef new #undef new #endif class TestClass { public : // The placement syntax version of new with the proper prototype needed 621
// to ensure the source and line information is recorded for the // allocation. #ifdef _DEBUG // iSize // iBlockType // nLine - The size of the allocation. - The DCRT block type. - The source line. , , ) int int { // Put any special code in here you need. // Allocate the actual memory with _malloc_dbg and pass in all // the parameters to record the allocation location. return ( _malloc_dbg ( nSize iBlockType lpszFileName nLine } #endif } ; // Restore the new macro to the saved value (i.e., the // NEW_INLINE_WORKAROUND). #pragma pop_macro ( "new" ) The big trick that allows this to work is the new #pragma push_macro and #pragma pop_macro directives. These save off an existing macro definition to an internal compiler stack and restore the previous value, respectively. You'll have to include the pragma directives around each class that has the new operator overloaded since pragmas cannot be macroized. The extra new operator you'll have to include is the one that the NEW_INLINE_WORKAROUND macros call to affect the actual allocation. Even though the extra new operator is a little inconvenient, at least you're going to get all your leaks completely reported. If you want to see how everything works, check out the FixedNewProblem project included with this book's sample files. Common Debugging Question: What are the rules for properly precompiling headers As I mentioned when discussing how to work around the bug in the DCRT so that you can properly get leaks from memory allocated with the new operator and display the correct source line, getting your precompiled headers straight is critical. Not only will this make your 622 // _DEBUG , , , ) ) ; iBlockType nLine
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