_CrtSetDbgFlag

Retrieves or modifies the state of the _crtDbgFlag flag to control the allocation behavior of the debug heap manager (debug version only).

int _CrtSetDbgFlag( 
   int newFlag 
);

Parameters

• newFlag
  New state for _crtDbgFlag.

Return Value

Returns the previous state of _crtDbgFlag.

Remarks

The _CrtSetDbgFlag function allows the application to control how the debug heap manager tracks memory allocations by modifying the bit fields of the _crtDbgFlag flag. By setting the bits (turning on), the application can instruct the debug heap manager to perform special debugging operations, including checking for memory leaks when the application exits and reporting if any are found, simulating low-memory conditions by specifying that freed memory blocks should remain in the heap's linked list, and verifying the integrity of the heap by inspecting each memory block at every allocation request. When _DEBUG is not defined, calls to _CrtSetDbgFlag are removed during preprocessing.

The following table lists the bit fields for _crtDbgFlag and describes their behavior. Because setting the bits results in increased diagnostic output and reduced program execution speed, these bits are not set (turned off) by default. For more information about these bit fields, see Using the Debug Heap.

Heap-Check Frequency Macros

You can specify how often the C run-time library performs validation of the debug heap (_CrtCheckMemory) based on the number of calls to malloc, realloc, free, and _msize.

_CrtSetDbgFlag then inspects the upper 16 bits of the newFlag parameter for a value. The value specified is the number of malloc, realloc, free, and _msize calls between _CrtCheckMemory calls. Four predefined macros are provided for this purpose.

By default, _CrtCheckMemory is called once every 1,024 times you call malloc, realloc, free, and _msize.

For example, you could specify a heap check every 16 malloc, realloc, free, and _msize operations with the following code:

#include <crtdbg.h>
int main( )
{
int tmp;

// Get the current bits
tmp = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);

// Clear the upper 16 bits and OR in the desired freqency
tmp = (tmp & 0x0000FFFF) | _CRTDBG_CHECK_EVERY_16_DF;

// Set the new bits
_CrtSetDbgFlag(tmp);
}

The upper 16 bits of the newFlag parameter are ignored when _CRTDBG_CHECK_ALWAYS_DF is specified. In this case, _CrtCheckMemory is called each time you call malloc, realloc, free, and _msize.

newFlag is the new state to apply to the _crtDbgFlag and is a combination of the values for each of the bit fields.

To change one or more of these bit fields and create a new state for the flag

1. Call _CrtSetDbgFlag with newFlag equal to _CRTDBG_REPORT_FLAG to obtain the current _crtDbgFlag state and store the returned value in a temporary variable.

2. Turn on any bits by OR-ing the temporary variable with the corresponding bitmasks (represented in the application code by manifest constants).

3. Turn off the other bits by AND-ing the variable with a bitwise NOT of the appropriate bitmasks.

4. Call _CrtSetDbgFlag with newFlag equal to the value stored in the temporary variable to set the new state for _crtDbgFlag.

The following code demonstrates how to simulate low-memory conditions by keeping freed memory blocks in the heap's linked list and prevent _CrtCheckMemory from being called at every allocation request:

// Get the current state of the flag
// and store it in a temporary variable
int tmpFlag = _CrtSetDbgFlag( _CRTDBG_REPORT_FLAG );

// Turn On (OR) - Keep freed memory blocks in the
// heap's linked list and mark them as freed
tmpFlag |= _CRTDBG_DELAY_FREE_MEM_DF;

// Turn Off (AND) - prevent _CrtCheckMemory from
// being called at every allocation request
tmpFlag &= ~_CRTDBG_CHECK_ALWAYS_DF;

// Set the new state for the flag
_CrtSetDbgFlag( tmpFlag );

For an overview of memory management and the debug heap, see Memory Management and the Debug Heap.

To disable a flag with the _CrtSetDbgFlag function, you should AND the variable with the bitwise NOT of the bitmask.

If newFlag is not a valid value, this function invokes the invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, this function sets errno to EINVAL and returns the previous state of _crtDbgFlag.

Requirements

For more compatibility information, see Compatibility in the Introduction.

Libraries

Debug versions of C run-time libraries only.

Example

// crt_crtsetdflag.c
// compile with: /c -D_DEBUG /MTd -Od -Zi -W3 /link -verbose:lib /debug
/*
 * This program concentrates on allocating and freeing memory
 * blocks to test the functionality of the _crtDbgFlag flag..
 */

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

int main( )
{
        char *p1, *p2;
        int tmpDbgFlag;

        _CrtSetReportMode( _CRT_ERROR, _CRTDBG_MODE_FILE );
        _CrtSetReportFile( _CRT_ERROR, _CRTDBG_FILE_STDERR );
        /*
         * Set the debug-heap flag to keep freed blocks in the
         * heap's linked list - This will allow us to catch any
         * inadvertent use of freed memory
         */
        tmpDbgFlag = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);
        tmpDbgFlag |= _CRTDBG_DELAY_FREE_MEM_DF;
        tmpDbgFlag |= _CRTDBG_LEAK_CHECK_DF;
        _CrtSetDbgFlag(tmpDbgFlag);

        /*
         * Allocate 2 memory blocks and store a string in each
         */
        p1 = malloc( 34 );
        p2 = malloc( 38 );
        strcpy_s( p1, 34, "p1 points to a Normal allocation block" );
        strcpy_s( p2, 38, "p2 points to a Client allocation block" );

        /*
         * Free both memory blocks
         */
        free( p2 );
        free( p1 );

        /*
         * Set the debug-heap flag to no longer keep freed blocks in the
         * heap's linked list and turn on Debug type allocations (CLIENT)
         */
        tmpDbgFlag = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);
        tmpDbgFlag |= _CRTDBG_ALLOC_MEM_DF;
        tmpDbgFlag &= ~_CRTDBG_DELAY_FREE_MEM_DF;
        _CrtSetDbgFlag(tmpDbgFlag);

        /*
         * Explicitly call _malloc_dbg to obtain the filename and 
         * line number of our allocation request and also so we can 
         * allocate CLIENT type blocks specifically for tracking
         */
        p1 = _malloc_dbg( 40, _NORMAL_BLOCK, __FILE__, __LINE__ );
        p2 = _malloc_dbg( 40, _CLIENT_BLOCK, __FILE__, __LINE__ );
        strcpy_s( p1, 40, "p1 points to a Normal allocation block" );
        strcpy_s( p2, 40, "p2 points to a Client allocation block" );

        /*
         * _free_dbg must be called to free the CLIENT block
         */
        _free_dbg( p2, _CLIENT_BLOCK );
        free( p1 );

        /*
         * Allocate p1 again and then exit - this will leave unfreed
         * memory on the heap
         */
        p1 = malloc( 10 );
}

.NET Framework Equivalent

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

See Also

Reference

Debug Routines

_crtDbgFlag

_CrtCheckMemory

Concepts

Heap State Reporting Functions