Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here. ArchiveDisclaimer


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

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


Requested size of memory block (bytes).
Requested type of memory block: _CLIENT_BLOCK or _NORMAL_BLOCK.
Pointer to name of source file that requested allocation operation or NULL.
Line number in source file where allocation operation was requested or NULL.

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

Return Value

Upon successful completion, this function either returns a pointer to the user portion of the allocated memory block, calls the new handler function, or returns NULL. See the following Remarks section for a complete description of the return behavior. See the malloc function for more information on how the new handler function is used.


_malloc_dbg is a debug version of the malloc function. When _DEBUG is not defined, each call to _malloc_dbg is reduced to a call to malloc. Both malloc and _malloc_dbg allocate a block of memory in the base heap, but _malloc_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, and filename/linenumber information to determine the origin of allocation requests.

_malloc_dbg allocates the 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 together 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.

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.


Routine Required header Compatibility
_malloc_dbg <crtdbg.h> Win 98, Win Me, Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.


Debug versions of C run-time libraries only.


For a sample of how to use _malloc_dbg, see crt_dbg1.

See Also

Debug Functions | Run-Time Routines and .NET Framework Equivalents

© 2015 Microsoft