_CrtCheckMemory

Article
09/15/2006

Confirms the integrity of the memory blocks allocated in the debug heap (debug version only).

int_CrtCheckMemory(void);

Routine	Required Header	Compatibility
_CrtCheckMemory	<crtdbg.h>	Win NT, Win 95

For additional compatibility information, see Compatibility in the Introduction.

Libraries

LIBCD.LIB	Single thread static library, debug version
LIBCMTD.LIB	Multithread static library, debug version
MSVCRTD.LIB	Import library for MSVCRTD.DLL, debug version

Return Value

If successful, _CrtCheckMemory returns TRUE; otherwise, the function returns FALSE.

Remarks

The _CrtCheckMemory function validates memory allocated by the debug heap manager by verifying the underlying base heap and inspecting every memory block. If an error or memory inconsistency is encountered in the underlying base heap, the debug header information, or the overwrite buffers, _CrtCheckMemory generates a debug report with information describing the error condition. When _DEBUG is not defined, calls to _CrtCheckMemory are removed during preprocessing.

The behavior of _CrtCheckMemory can be controlled by setting the bit fields of the _crtDbgFlag flag using the _CrtSetDbgFlag function. Turning the _CRTDBG_CHECK_ALWAYS_DF bit field ON results in _CrtCheckMemory being called every time a memory allocation operation is requested. Although this method slows down execution, it is useful for catching errors quickly. Turning the _CRTDBG_ALLOC_MEM_DF bit field OFF causes _CrtCheckMemory to not verify the heap and immediately return TRUE.

Because this function returns TRUE or FALSE, it can be passed to one of the _ASSERT macros to create a simple debugging error handling mechanism. The following example will cause an assertion failure if corruption is detected in the heap:

_ASSERTE( _CrtCheckMemory( ) );

For more information about how _CrtCheckMemory can be used with other debug functions, see Heap State Reporting Functions. For an overview of memory management and the debug heap, see Memory Management and the Debug Heap.

Example

/*****************************************************************
 *  EXAMPLE  1                                                   *
 *  This simple program illustrates the basic debugging features *
 *  of the C runtime libraries, and the kind of debug output     *
 *  that these features generate.                                *
 *****************************************************************/

#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include <crtdbg.h>

// This routine place comments at the head of a section of debug output
void OutputHeading( const char * explanation )
{
   _RPT1( _CRT_WARN, "\n\n%s:\n**************************************\
************************************\n", explanation );
}

// The following macros set and clear, respectively, given bits
// of the C runtime library debug flag, as specified by a bitmask.
#ifdef   _DEBUG
#define  SET_CRT_DEBUG_FIELD(a) \
            _CrtSetDbgFlag((a) | _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG))
#define  CLEAR_CRT_DEBUG_FIELD(a) \
            _CrtSetDbgFlag(~(a) & _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG))
#else
#define  SET_CRT_DEBUG_FIELD(a)   ((void) 0)
#define  CLEAR_CRT_DEBUG_FIELD(a) ((void) 0)
#endif


void main( )
{
   char *p1, *p2;
   _CrtMemState s1, s2, s3;

   // Send all reports to STDOUT
   _CrtSetReportMode( _CRT_WARN, _CRTDBG_MODE_FILE );
   _CrtSetReportFile( _CRT_WARN, _CRTDBG_FILE_STDOUT );
   _CrtSetReportMode( _CRT_ERROR, _CRTDBG_MODE_FILE );
   _CrtSetReportFile( _CRT_ERROR, _CRTDBG_FILE_STDOUT );
   _CrtSetReportMode( _CRT_ASSERT, _CRTDBG_MODE_FILE );
   _CrtSetReportFile( _CRT_ASSERT, _CRTDBG_FILE_STDOUT );

   // Allocate 2 memory blocks and store a string in each
   p1 = malloc( 34 );
   strcpy( p1, "This is the p1 string (34 bytes)." );

   p2 = malloc( 34 );
   strcpy( p2, "This is the p2 string (34 bytes)." );


   OutputHeading(
      "Use _ASSERTE to check that the two strings are identical" );
   _ASSERTE( strcmp( p1, p2 ) == 0 );

   OutputHeading(
      "Use a _RPT macro to report the string contents as a warning" );
   _RPT2( _CRT_WARN, "p1 points to '%s' and \np2 points to '%s'\n", p1, p2 );

   OutputHeading(
      "Use _CRTMemDumpAllObjectsSince to check the p1 and p2 allocations" );
   _CrtMemDumpAllObjectsSince( NULL );

   free( p2 );

   OutputHeading(
      "Having freed p2, dump allocation information about p1 only" );
   _CrtMemDumpAllObjectsSince( NULL );

   // Store a memory checkpoint in the s1 memory-state structure
   _CrtMemCheckpoint( &s1 );

   // Allocate another block, pointed to by p2
   p2 = malloc( 38 );
   strcpy( p2, "This new p2 string occupies 38 bytes.");

   // Store a 2nd memory checkpoint in s2
   _CrtMemCheckpoint( &s2 );

   OutputHeading(
      "Dump the changes that occurred between two memory checkpoints" );
   if ( _CrtMemDifference( &s3, &s1, &s2 ) )
      _CrtMemDumpStatistics( &s3 );

   // Free p2 again and store a new memory checkpoint in s2
   free( p2 );
   _CrtMemCheckpoint( &s2 );

   OutputHeading(
      "Now the memory state at the two checkpoints is the same" );
   if ( _CrtMemDifference( &s3, &s1, &s2 ) )
      _CrtMemDumpStatistics( &s3 );

   strcpy( p1, "This new p1 string is over 34 bytes" );
   OutputHeading( "Free p1 after overwriting the end of the allocation" );
   free( p1 );

   // Set the debug-heap flag so that freed blocks are kept on the
   // linked list, to catch any inadvertent use of freed memory
   SET_CRT_DEBUG_FIELD( _CRTDBG_DELAY_FREE_MEM_DF );

   p1 = malloc( 10 );
   free( p1 );
   strcpy( p1, "Oops" );

   OutputHeading( "Perform a memory check after corrupting freed memory" );
   _CrtCheckMemory( );

   // Use explicit calls to _malloc_dbg to save file name and line number
   // information, and also to allocate Client type blocks for tracking
   p1 = _malloc_dbg( 40, _NORMAL_BLOCK, __FILE__, __LINE__ );
   p2 = _malloc_dbg( 40, _CLIENT_BLOCK, __FILE__, __LINE__ );
   strcpy( p1, "p1 points to a Normal allocation block" );
   strcpy( p2, "p2 points to a Client allocation block" );

   // You must use _free_dbg to free a Client block
   OutputHeading(
      "Using free( ) to free a Client block causes an assertion failure" );
   free( p1 );
   free( p2 );

   p1 = malloc( 10 );
   OutputHeading( "Examine outstanding allocations (dump memory leaks)" );
   _CrtDumpMemoryLeaks( );

   // Set the debug-heap flag so that memory leaks are reported when
   // the process terminates. Then, exit.
   OutputHeading( "Program exits without freeing a memory block" );
   SET_CRT_DEBUG_FIELD( _CRTDBG_LEAK_CHECK_DF );
}

Output

Use _ASSERTE to check that the two strings are identical:

************************************************************************ C:\DEV\EXAMPLE1.C(56) : Assertion failed: strcmp( p1, p2 ) == 0

Use a _RPT macro to report the string contents as a warning:

************************************************************************ p1 points to 'This is the p1 string (34 bytes).' and p2 points to 'This is the p2 string (34 bytes).'

Use _CRTMemDumpAllObjectsSince to check the p1 and p2 allocations:

************************************************************************ Dumping objects -> {13} normal block at 0x00660B5C, 34 bytes long Data: 54 68 69 73 20 69 73 20 74 68 65 20 70 32 20 73 {12} normal block at 0x00660B10, 34 bytes long Data: 54 68 69 73 20 69 73 20 74 68 65 20 70 31 20 73 Object dump complete.

Having freed p2, dump allocation information about p1 only:

************************************************************************ Dumping objects -> {12} normal block at 0x00660B10, 34 bytes long Data: 54 68 69 73 20 69 73 20 74 68 65 20 70 31 20 73 Object dump complete.

Dump the changes that occurred between two memory checkpoints:

************************************************************************ 0 bytes in 0 Free Blocks. 38 bytes in 1 Normal Blocks. 0 bytes in 0 CRT Blocks. 0 bytes in 0 IgnoreClient Blocks. 0 bytes in 0 (null) Blocks. Largest number used: 4 bytes. Total allocations: 38 bytes.

Now the memory state at the two checkpoints is the same:

************************************************************************

Free p1 after overwriting the end of the allocation:

************************************************************************ memory check error at 0x00660B32 = 0x73, should be 0xFD. memory check error at 0x00660B33 = 0x00, should be 0xFD. DAMAGE: after Normal block (#12) at 0x00660B10.

Perform a memory check after corrupting freed memory:

************************************************************************ memory check error at 0x00660B10 = 0x4F, should be 0xDD. memory check error at 0x00660B11 = 0x6F, should be 0xDD. memory check error at 0x00660B12 = 0x70, should be 0xDD. memory check error at 0x00660B13 = 0x73, should be 0xDD. memory check error at 0x00660B14 = 0x00, should be 0xDD. DAMAGE: on top of Free block at 0x00660B10. DAMAGED located at 0x00660B10 is 10 bytes long.

Using free( ) to free a Client block causes an assertion failure:

************************************************************************ dbgheap.c(1039) : Assertion failed: pHead->nBlockUse == nBlockUse

Examine outstanding allocations (dump memory leaks):

************************************************************************ Detected memory leaks! Dumping objects -> {18} normal block at 0x00660BE4, 10 bytes long Data: < > CD CD CD CD CD CD CD CD CD CD Object dump complete.

Program exits without freeing a memory block:

Debug Functions

See Also _crtDbgFlag, _CrtSetDbgFlag

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