Frees a block of memory in the heap (debug version only).
void _free_dbg( void *userData, int blockType );
| Routine | Required Header | Compatibility |
| _free_dbg | <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
None
Parameters
userData
Pointer to the allocated memory block to be freed
blockType
Type of allocated memory block to be freed: _CLIENT_BLOCK, _NORMAL_BLOCK, or _IGNORE_BLOCK
Remarks
The _free_dbg function is a debug version of the free function. When _DEBUG is not defined, calls to _free_dbg are removed during preprocessing. Both free and _free_dbg free a memory block in the base heap, but _free_dbg accomodates two debugging features: the ability to keep freed blocks in the heap’s linked list to simulate low memory conditions and a block type parameter to free specific allocation types.
_free_dbg performs a validity check on all specified files and block locations before performing the free operation?the application is not expected to provide this information. When a memory block is freed, the debug heap manager automatically checks the integrity of the buffers on either side of the user portion and issues an error report if overwriting has occurred. If the _CRTDBG_DELAY_FREE_MEM_DF bit field of the _crtDbgFlag flag is set, the freed block is filled with the value 0xDD, assigned the _FREE_BLOCK block type, and kept in the heap’s linked list of memory blocks.
For information about how memory blocks are allocated, initialized, and managed in the debug version of the base heap, see Memory Management and the Debug Heap. For information about the allocation block types and how they are used, see Types of Blocks on the Debug Heap. For information on the differences between calling a standard heap function versus its debug version in a debug build of an application, see Using the Debug Version Versus the Base Version.
Example
/*****************************************************************
* EXAMPLE 2 *
* ---------- *
* This program illustrates several ways to use debugging hook *
* functions with the new debug versions of the C runtime *
* libraries. To add some realism, it has a few elements of an *
* actual application, including two bugs. *
* *
* The program stores birthdate information in a linked list *
* of Client blocks. A Client-dump hook function validates the *
* birthday data and reports the contents of the Client blocks. *
* An allocation hook function logs heap operations to a text *
* file, and the report hook function logs reports to the same *
* text file. *
* *
* NOTE: The allocation hook function explicitly excludes CRT *
* blocks (the memory allocated internally by the C *
* runtime library) from its log. It is important to *
* understand why! The hook function uses fprintf( ) to *
* write to the log file, and fprintf( ) allocates a CRT *
* block. If CRT blocks were not excluded in this case, *
* an endless loop would be created in which fprintf( ) *
* would cause the hook function to be called, and the *
* hook would in turn call fprintf( ), which would cause *
* the hook to be called again, and so on. The moral is: *
* *
* --> IF YOUR ALLOCATION HOOK USES ANY C RUNTIME FUNCTION *
* THAT ALLOCATES MEMORY, THE HOOK MUST IGNORE CRT-TYPE *
* ALLOCATION OPERATIONS! *
* *
* HINT: If you want to be able to report CRT-type blocks in *
* your allocation hook, use Windows API functions for *
* formatting and output, instead of C runtime functions. *
* Since the Windows APIs do not use the CRT heap, they *
* will not trap your hook in an endless loop. *
* *
* BUGS: There are two bugs in the program below, which the *
* debug heap features identify in several ways. One bug *
* is that the birthDay.Name field is not large enough *
* to hold several of the test names. The field should *
* be larger, and strncpy( ) should be used in place of *
* strcpy( ). The second bug is that the while( ) loop *
* in the printRecords( ) function should not end until *
* HeadPtr itself == NULL. This bug results not only in *
* an incomplete display of birthdays, but also in a *
* memory leak. In addition to these two bugs, Gauss' *
* birthday data is out of range (April 30, not 32). *
* *
*****************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <malloc.h>
#include <time.h>
#include <crtdbg.h>
/*****************************************************************
* DATA DECLARATIONS AND DEFINES *
*****************************************************************/
// The following arrays provide test data for the example program:
const char * Names[] =
{
"George Washington",
"Thomas Jefferson",
"Carl Friedrich Gauss",
"Ludwig van Beethoven",
"Thomas Carlyle"
} ;
const int Dates[] =
{
1732, 2, 11,
1743, 4, 13,
1777, 4, 32,
1795, 12, 4,
1770, 12, 16
} ;
#define TEST_RECS 5
// A generic sort of linked-list data structure, in this case for birthdays:
typedef struct BirthdayStruct
{
struct BirthdayStruct * NextRec;
int Year;
int Month;
int Day;
char Name[20];
} birthDay;
birthDay * HeadPtr;
birthDay * TailPtr;
#define FILE_IO_ERROR 0
#define OUT_OF_MEMORY 1
#define TRUE 7
#define FALSE 0
// Macros for setting or clearing bits in the CRT debug flag
#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
/*****************************************************************
* SPECIAL-PURPOSE ROUTINES *
*****************************************************************/
/* ERROR HANDLER
-------------
Handling serious errors gracefully is a real test of craftsmanship.
This function is just a stub; it doesn't really handle errors.
*/
void FatalError( int ErrType )
{
exit( 1 );
}
/* MEMORY ALLOCATION FUNCTION
--------------------------
The createRecord function allocates memory for a new birthday record,
fills in the structure members, and then adds the record to a linked list.
In debug builds, it makes these allocations in Client blocks. If memory
is not available, it calls the error handler.
*/
void createRecord(
const int Year,
const int Month,
const int Day,
const char * Name
#ifdef _DEBUG
, const unsigned char * szFileName, int nLine
#endif
)
{
birthDay * ptr;
size_t n;
n = sizeof( struct BirthdayStruct );
ptr = (birthDay *) _malloc_dbg( n, _CLIENT_BLOCK, szFileName, nLine );
if( ptr == NULL )
FatalError( OUT_OF_MEMORY );
ptr->Year = Year;
ptr->Month = Month;
ptr->Day = Day;
strcpy( ptr->Name, Name );
ptr->NextRec = NULL;
if ( HeadPtr == NULL ) // If this is the first record in the linked list
HeadPtr = ptr;
else
TailPtr->NextRec = ptr;
TailPtr = ptr;
}
/* BIRTHDAY DISPLAY FUNCTION
-------------------------
This function traverses the linked list, displays the birthday data,
and then frees the memory blocks used to store the birthdays.
*/
void printRecords( )
{
birthDay * ptr;
char *months[] = {
"", "January", "February", "March", "April", "May", "June", "July",
"August", "September", "October", "November", "December" };
if ( HeadPtr == NULL ) // Do nothing if list is empty
return;
printf( "\n\nThis is the birthday list:\n" );
while ( HeadPtr->NextRec != NULL )
{
printf( " %s was born on %s %d, %d.\n",
HeadPtr->Name, months[HeadPtr->Month], HeadPtr->Day, HeadPtr->Year );
ptr = HeadPtr->NextRec;
_free_dbg( HeadPtr, _CLIENT_BLOCK );
HeadPtr = ptr;
}
}
/*****************************************************************
* DEBUG C RUNTIME LIBRARY HOOK FUNCTIONS AND DEFINES *
*****************************************************************/
#ifdef _DEBUG
#define createRecord(a, b, c, d) \
createRecord(a, b, c, d, __FILE__, __LINE__)
FILE *logFile; // Used to log allocation information
const char lineStr[] = { "---------------------------------------\
--------------------------------------\n" };
/* CLIENT DUMP HOOK FUNCTION
-------------------------
A hook function for dumping a Client block usually reports some
or all of the contents of the block in question. The function
below also checks the data in several ways, and reports corruption
or inconsistency as an assertion failure.
*/
void __cdecl MyDumpClientHook(
void * pUserData,
size_t nBytes
)
{
birthDay * bday;
bday = (birthDay *) pUserData;
_RPT4( _CRT_WARN, " The birthday of %s is %d/%d/%d.\n",
bday->Name, bday->Month, bday->Day, bday->Year );
_ASSERTE( ( bday->Day > 0 ) && ( bday->Day < 32 ) );
_ASSERTE( ( bday->Month > 0 ) && ( bday->Month < 13 ) );
_ASSERTE( ( bday->Year > 0 ) && ( bday->Year < 1996 ) );
}
/* ALLOCATION HOOK FUNCTION
-------------------------
An allocation hook function can have many, many different
uses. This one simply logs each allocation operation in a file.
*/
int __cdecl MyAllocHook(
int nAllocType,
void * pvData,
size_t nSize,
int nBlockUse,
long lRequest,
const unsigned char * szFileName,
int nLine
)
{
char *operation[] = { "", "allocating", "re-allocating", "freeing" };
char *blockType[] = { "Free", "Normal", "CRT", "Ignore", "Client" };
if ( nBlockUse == _CRT_BLOCK ) // Ignore internal C runtime library allocations
return( TRUE );
_ASSERT( ( nAllocType > 0 ) && ( nAllocType < 4 ) );
_ASSERT( ( nBlockUse >= 0 ) && ( nBlockUse < 5 ) );
fprintf( logFile,
"Memory operation in %s, line %d: %s a %d-byte '%s' block (# %ld)\n",
szFileName, nLine, operation[nAllocType], nSize,
blockType[nBlockUse], lRequest );
if ( pvData != NULL )
fprintf( logFile, " at %X", pvData );
return( TRUE ); // Allow the memory operation to proceed
}
/* REPORT HOOK FUNCTION
--------------------
Again, report hook functions can serve a very wide variety of purposes.
This one logs error and assertion failure debug reports in the
log file, along with 'Damage' reports about overwritten memory.
By setting the retVal parameter to zero, we are instructing _CrtDbgReport
to return zero, which causes execution to continue. If we want the function
to start the debugger, we should have _CrtDbgReport return one.
*/
int MyReportHook(
int nRptType,
char *szMsg,
int *retVal
)
{
char *RptTypes[] = { "Warning", "Error", "Assert" };
if ( ( nRptType > 0 ) || ( strstr( szMsg, "DAMAGE" ) ) )
fprintf( logFile, "%s: %s", RptTypes[nRptType], szMsg );
retVal = 0;
return( TRUE ); // Allow the report to be made as usual
}
#endif // End of #ifdef _DEBUG
/*****************************************************************
* MAIN FUNCTION *
*****************************************************************/
void main( )
{
int i, j;
#ifdef _DEBUG
_CrtMemState checkPt1;
char timeStr[10], dateStr[10]; // Used to set up log file
// Send all reports to STDOUT, since this example is a console app
_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);
// Set the debug heap to report memory leaks when the process terminates,
// and to keep freed blocks in the linked list.
SET_CRT_DEBUG_FIELD( _CRTDBG_LEAK_CHECK_DF | _CRTDBG_DELAY_FREE_MEM_DF );
// Open a log file for the hook functions to use
logFile = fopen( "MEM-LOG.TXT", "w" );
if ( logFile == NULL )
FatalError( FILE_IO_ERROR );
_strtime( timeStr );
_strdate( dateStr );
fprintf( logFile,
"Memory Allocation Log File for Example Program, run at %s on %s.\n",
timeStr, dateStr );
fputs( lineStr, logFile );
// Install the hook functions
_CrtSetDumpClient( MyDumpClientHook );
_CrtSetAllocHook( MyAllocHook );
_CrtSetReportHook( MyReportHook );
#endif // End of #ifdef _DEBUG
HeadPtr = NULL;
// Create a trial birthday record.
createRecord( 1749, 3, 23, "Pierre de Laplace" );
// Check the debug heap, and dump the new birthday record. --Note that
// debug C runtime library functions such as _CrtCheckMemory( ) and
// _CrtMemDumpAllObjectsSince( ) automatically disappear in a release build.
_CrtMemDumpAllObjectsSince( NULL );
_CrtCheckMemory( );
_CrtMemCheckpoint( &checkPt1 );
// Since everything has worked so far, create more records
for ( i = 0, j = 0; i < TEST_RECS; i++, j+=3 )
createRecord( Dates[j], Dates[j+1], Dates[j+2], Names[i] );
// Examine the results
_CrtMemDumpAllObjectsSince( &checkPt1 );
_CrtMemCheckpoint( &checkPt1 );
_CrtMemDumpStatistics( &checkPt1 );
_CrtCheckMemory( );
// This fflush needs to be removed...
fflush( logFile );
// Now try displaying the records, which frees the memory being used
printRecords( );
// OK, time to go. Did I forget to turn out any lights? I could check
// explicitly using _CrtDumpMemoryLeaks( ), but I have set
// _CRTDBG_LEAK_CHECK_DF, so the C runtime library debug heap will
// automatically alert me at exit of any memory leaks.
#ifdef _DEBUG
fclose( logFile );
#endif
}Output
Screen output:
Dumping objects ->
C:\DEV\EXAMPLE2.C(327) : {13} client block at 0x00661B38, subtype 0, 36 bytes long:
The birthday of Pierre de Laplace is 3/23/1749.
Object dump complete.
Dumping objects ->
C:\DEV\EXAMPLE2.C(338) : {18} client block at 0x00661CB4, subtype 0, 36 bytes long:
The birthday of Thomas Carlyle is 12/16/1770.
C:\DEV\EXAMPLE2.C(338) : {17} client block at 0x00661C68, subtype 0, 36 bytes long:
The birthday of Ludwig van Beethoven is 12/4/1795.
C:\DEV\EXAMPLE2.C(338) : {16} client block at 0x00661C1C, subtype 0, 36 bytes long:
The birthday of Carl Friedrich Gauss is 4/32/1777.
C:\DEV\EXAMPLE2.C(219) : Assertion failed: ( bday->Day > 0 ) && ( bday->Day < 32 )
C:\DEV\EXAMPLE2.C(338) : {15} client block at 0x00661BD0, subtype 0, 36 bytes long:
The birthday of Thomas Jefferson is 4/13/1743.
C:\DEV\EXAMPLE2.C(338) : {14} client block at 0x00661B84, subtype 0, 36 bytes long:
The birthday of George Washington is 2/11/1732.
Object dump complete.
0 bytes in 0 Free Blocks.
0 bytes in 0 Normal Blocks.
6442 bytes in 12 CRT Blocks.
0 bytes in 0 IgnoreClient Blocks.
216 bytes in 6 (null) Blocks.
Largest number used: 6658 bytes.
Total allocations: 6658 bytes.
memory check error at 0x00661C8C = 0x00, should be 0xFD.
DAMAGE: after (null) block (#17) at 0x00661C68.
(null) allocated at file C:\DEV\EXAMPLE2.C(338).
(null) located at 0x00661C68 is 36 bytes long.
memory check error at 0x00661C40 = 0x00, should be 0xFD.
DAMAGE: after (null) block (#16) at 0x00661C1C.
(null) allocated at file C:\DEV\EXAMPLE2.C(338).
(null) located at 0x00661C1C is 36 bytes long.
memory check error at 0x00661C40 = 0x00, should be 0xFD.
DAMAGE: after (null) block (#16) at 0x00661C1C.
memory check error at 0x00661C8C = 0x00, should be 0xFD.
DAMAGE: after (null) block (#17) at 0x00661C68.
This is the birthday list:
Pierre de Laplace was born on March 23, 1749.
George Washington was born on February 11, 1732.
Thomas Jefferson was born on April 13, 1743.
Carl Friedrich Gauss was born on April 32, 1777.
Ludwig van Beethoven was born on December 4, 1795.
Detected memory leaks!
Dumping objects ->
C:\DEV\EXAMPLE2.C(338) : {18} client block at 0x00661CB4, subtype 0, 36 bytes long:
The birthday of Thomas Carlyle is 12/16/1770.
Object dump complete.
Log file output:
Memory Allocation Log File for Example Program, run at 14:11:01 on 04/28/95.
-----------------------------------------------------------------------------
Memory operation in C:\DEV\EXAMPLE2.C, line 327:
allocating a 36-byte 'Client' block (# 13)
Memory operation in C:\DEV\EXAMPLE2.C, line 338:
allocating a 36-byte 'Client' block (# 14)
Memory operation in C:\DEV\EXAMPLE2.C, line 338:
allocating a 36-byte 'Client' block (# 15)
Memory operation in C:\DEV\EXAMPLE2.C, line 338:
allocating a 36-byte 'Client' block (# 16)
Memory operation in C:\DEV\EXAMPLE2.C, line 338:
allocating a 36-byte 'Client' block (# 17)
Memory operation in C:\DEV\EXAMPLE2.C, line 338:
allocating a 36-byte 'Client' block (# 18)
Assert: C:\DEV\EXAMPLE2.C(219) : Assertion failed:
( bday->Day > 0 ) && ( bday->Day < 32 )
Warning: DAMAGE: after (null) block (#17) at 0x00661C68.
Warning: DAMAGE: after (null) block (#16) at 0x00661C1C.
Memory operation in (null), line 0: freeing a 0-byte 'Client' block (# 0)
at 661B38Memory operation in (null), line 0:
freeing a 0-byte 'Client' block (# 0)
at 661B84Memory operation in (null), line 0:
freeing a 0-byte 'Client' block (# 0)
at 661BD0Memory operation in (null), line 0:
freeing a 0-byte 'Client' block (# 0)
at 661C1CError: DAMAGE: after (null) block (#16) at 0x00661C1C.
Memory operation in (null), line 0: freeing a 0-byte 'Client' block (# 0)
at 661C68Error: DAMAGE: after (null) block (#17) at 0x00661C68.
See Also _malloc_dbg