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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 the memory block (in bytes).


Requested type of the memory block: _CLIENT_BLOCK or _NORMAL_BLOCK.


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


Line number in the source file where the 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.

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

_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 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.

_malloc_dbg sets errno to ENOMEM if a memory allocation fails or 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 allocation block types and how they are used, see Types of Blocks on the Debug Heap. For information about the differences between calling a standard heap function and its debug version in a debug build of an application, see Debug Versions of Heap Allocation Functions.


Required header



For more 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.

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

© 2014 Microsoft