Reallocates a specified block of memory in the heap by moving and/or resizing the block (debug version only).


void *_realloc_dbg(
   void *userData,
   size_t newSize,
   int blockType,
   const char *filename,
   int linenumber


Pointer to the previously allocated memory block.

Requested size for the reallocated block (bytes).

Requested type for the reallocated block: _CLIENT_BLOCK or _NORMAL_BLOCK.

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

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

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

Return Value

On successful completion, this function either returns a pointer to the user portion of the reallocated 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 realloc function.


_realloc_dbg is a debug version of the realloc function. When _DEBUG is not defined, each call to _realloc_dbg is reduced to a call to realloc. Both realloc and _realloc_dbg reallocate a memory block in the base heap, but _realloc_dbg accommodates 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.

_realloc_dbg reallocates the specified memory block with slightly more space than the requested newSize. newSize might be greater or less than the size of the originally allocated memory block. 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. The reallocation might result in moving the original memory block to a different location in the heap, as well as changing the size of the memory block. If the memory block is moved, the contents of the original block are overwritten.

_realloc_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.


Routine Required header
_realloc_dbg <crtdbg.h>

For more compatibility information, see Compatibility.


Debug versions of C run-time libraries only.


See the example in the _msize_dbg topic.

See also

Debug Routines