Allocates a number of memory blocks in the heap with additional space for a debugging header and overwrite buffers (debug version only).


void *_calloc_dbg(
   size_t num,
   size_t size,
   int blockType,
   const char *filename,
   int linenumber


Requested number of memory blocks.

Requested size of each memory block (bytes).

Requested type of memory block: _CLIENT_BLOCK or _NORMAL_BLOCK.

For information about the allocation block types and how they are used, seeTypes of blocks on the debug heap.

Pointer to name of the source file that requested allocation operation or NULL.

Line number in the source file where allocation operation was requested or NULL.

The filename and linenumber parameters are only available when _calloc_dbg has been called explicitly or the _CRTDBG_MAP_ALLOC preprocessor constant has been defined.

Return Value

On successful completion, this function returns a pointer to the user portion of the last allocated memory block, calls the new handler function, or returns NULL. For a complete description of the return behavior, see the Remarks section. For more information about how the new handler function is used, see the calloc function.


_calloc_dbg is a debug version of the calloc function. When _DEBUG is not defined, each call to _calloc_dbg is reduced to a call to calloc. Both calloc and _calloc_dbg allocate number memory blocks in the base heap, but _calloc_dbg offers several debugging features:

  • Buffers on either side of the user portion of the block to test for leaks.

  • A block type parameter to track specific allocation types.

  • filename/linenumber information to determine the origin of allocation requests.

_calloc_dbg allocates each memory block with slightly more space than the requested size. The additional space is used by the debug heap manager to link the debug memory blocks and to provide the application with debug header information and overwrite buffers. When the block is allocated, the user portion of the block is filled with the value 0xCD and each of the overwrite buffers are filled with 0xFD.

_calloc_dbg sets errno to ENOMEM if a memory allocation fails; EINVAL is returned if the amount of memory needed (including the overhead mentioned previously) exceeds _HEAP_MAXREQ. For information about this and other error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

For information about how memory blocks are allocated, initialized, and managed in the debug version of the base heap, see CRT Debug Heap Details. For information about the differences between calling a standard heap function versus its debug version in a debug build of an application, see Debug Versions of Heap Allocation Functions.


Routine Required header
_calloc_dbg <crtdbg.h>

For more compatibility information, see Compatibility.


// crt_callocd.c
// This program uses _calloc_dbg to allocate space for
// 40 long integers. It initializes each element to zero.

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

int main( void )
    long *bufferN, *bufferC;

    // Call _calloc_dbg to include the filename and line number
    // of our allocation request in the header and also so we can
    // allocate CLIENT type blocks specifically
    bufferN = (long *)_calloc_dbg( 40, sizeof(long), _NORMAL_BLOCK, __FILE__, __LINE__ );
    bufferC = (long *)_calloc_dbg( 40, sizeof(long), _CLIENT_BLOCK, __FILE__, __LINE__ );
    if( bufferN != NULL && bufferC != NULL )
        printf( "Allocated memory successfully\n" );
        printf( "Problem allocating memory\n" );

    / _free_dbg must be called to free CLIENT type blocks
    free( bufferN );
    _free_dbg( bufferC, _CLIENT_BLOCK );
Allocated memory successfully

See also

Debug Routines