Resizes a specified block of memory in the heap by expanding or contracting the block (debug version only).


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


Pointer to the previously allocated memory block.

Requested new size for the block (in bytes).

Requested type for resized block: _CLIENT_BLOCK or _NORMAL_BLOCK.

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

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

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

Return Value

On successful completion, _expand_dbg returns a pointer to the resized memory block. Because the memory is not moved, the address is the same as the userData. If an error occurred or the block could not be expanded to the requested size, it returns NULL. If a failure occurs, errno is with information from the operating system about the nature of the failure. For more information about errno, see errno, _doserrno, _sys_errlist, and _sys_nerr.


The _expand_dbg function is a debug version of the _expand function. When _DEBUG is not defined, each call to _expand_dbg is reduced to a call to _expand. Both _expand and _expand_dbg resize a memory block in the base heap, but _expand_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.

_expand_dbg resizes 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 resize is accomplished by either expanding or contracting the original memory block. _expand_dbg does not move the memory block, as does the _realloc_dbg function.

When newSize is greater than the original block size, the memory block is expanded. During an expansion, if the memory block cannot be expanded to accommodate the requested size, NULL is returned. When newSize is less than the original block size, the memory block is contracted until the new size is obtained.

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.

This function validates its parameters. If memblock is a null pointer, or if size is greater than _HEAP_MAXREQ, this function invokes an invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, errno is set to EINVAL and the function returns NULL.


Routine Required header
_expand_dbg <crtdbg.h>

For more compatibility information, see Compatibility.


Debug versions of C run-time libraries only.


// crt_expand_dbg.c
// This program allocates a block of memory using _malloc_dbg
// and then calls _msize_dbg to display the size of that block.
// Next, it uses _expand_dbg to expand the amount of
// memory used by the buffer and then calls _msize_dbg again to
// display the new amount of memory allocated to the buffer.

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

int main( void )
   long *buffer;
   size_t size;

   // Call _malloc_dbg to include the filename and line number
   // of our allocation request in the header
   buffer = (long *)_malloc_dbg( 40 * sizeof(long),
                                 _NORMAL_BLOCK, __FILE__, __LINE__ );
   if( buffer == NULL )
      exit( 1 );

   // Get the size of the buffer by calling _msize_dbg
   size = _msize_dbg( buffer, _NORMAL_BLOCK );
   printf( "Size of block after _malloc_dbg of 40 longs: %u\n", size );

   // Expand the buffer using _expand_dbg and show the new size
   buffer = (long *)_expand_dbg( buffer, size + sizeof(long),
                                 _NORMAL_BLOCK, __FILE__, __LINE__ );

   if( buffer == NULL )
      exit( 1 );
   size = _msize_dbg( buffer, _NORMAL_BLOCK );
   printf( "Size of block after _expand_dbg of 1 more long: %u\n",
           size );

   free( buffer );
   exit( 0 );
Size of block after _malloc_dbg of 40 longs: 160
Size of block after _expand_dbg of 1 more long: 164


The output of this program depends on your computer's ability to expand all the sections. If all sections are expanded, the output is reflected in the Output section.

See also

Debug Routines