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_CrtSetDbgFlag

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

Syntax

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 bit fields, the application can instruct the debug heap manager to perform special debugging operations. There are several possible operations:

  • 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,
  • Verifying the integrity of the heap by inspecting each memory block at every allocation request.

When _DEBUG isn't 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 aren't set (turned off) by default. For more information about these bit fields, see Heap state reporting functions.

Bit field Default Description
_CRTDBG_ALLOC_MEM_DF ON ON: Enable debug heap allocations and use of memory block type identifiers, such as _CLIENT_BLOCK. OFF: Add new allocations to heap's linked list, but set block type to _IGNORE_BLOCK.

Can also be combined with any of the heap-frequency check macros.
_CRTDBG_CHECK_ALWAYS_DF OFF ON: Call _CrtCheckMemory at every allocation and deallocation request. OFF: _CrtCheckMemory must be called explicitly.

Heap-frequency check macros have no effect when this flag is set.
_CRTDBG_CHECK_CRT_DF OFF ON: Include _CRT_BLOCK types in leak detection and memory state difference operations. OFF: Memory used internally by the run-time library is ignored by these operations.

Can also be combined with any of the heap-frequency check macros.
_CRTDBG_DELAY_FREE_MEM_DF OFF ON: Keep freed memory blocks in the heap's linked list, assign them the _FREE_BLOCK type, and fill them with the byte value 0xDD. OFF: Don't keep freed blocks in the heap's linked list.

Can also be combined with any of the heap-frequency check macros.
_CRTDBG_LEAK_CHECK_DF OFF ON: Perform automatic leak checking at program exit through a call to _CrtDumpMemoryLeaks and generate an error report if the application failed to free all the memory it allocated. OFF: Don't automatically perform leak checking at program exit.

Can also be combined with any of the heap-frequency check macros.

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.

Macro Number of malloc, realloc, free, and _msize calls between calls to _CrtCheckMemory
_CRTDBG_CHECK_EVERY_16_DF 16
_CRTDBG_CHECK_EVERY_128_DF 128
_CRTDBG_CHECK_EVERY_1024_DF 1024
_CRTDBG_CHECK_DEFAULT_DF 0 (by default, no heap checks)

By default, _CrtCheckMemory isn't called during memory operations. You can change that by sending the flags shown above to _CrtSetDbgFlag().

For example, you can 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 frequency
    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 a bitwise "or" (|) of the temporary variable with the corresponding bitmasks (represented in the application code by manifest constants).

  3. Turn off the other bits with a bitwise "and" (&) of 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 CRT debug heap details.

To disable a flag with the _CrtSetDbgFlag function, use a bitwise "and" (&) of the variable with the bitwise "not" (~) of the bitmask.

If newFlag isn't 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

Routine Required header
_CrtSetDbgFlag <crtdbg.h>

For more compatibility information, see Compatibility.

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 );
}

See also

Debug routines
_crtDbgFlag
_CrtCheckMemory