`_expand`

Article
02/07/2023

Changes the size of a memory block.

Syntax

void *_expand(
   void *memblock,
   size_t size
);

Parameters

memblock
Pointer to previously allocated memory block.

size
New size in bytes.

Return value

_expand returns a void pointer to the reallocated memory block. _expand, unlike realloc, can't move a block to change its size. Thus, if there's sufficient memory available to expand the block without moving it, the memblock parameter to _expand is the same as the return value.

_expand returns NULL when an error is detected during its operation. For example, if _expand is used to shrink a memory block, it might detect corruption in the small block heap or an invalid block pointer and return NULL.

If there isn't sufficient memory available to expand the block without moving it, the function returns NULL. _expand never returns a block expanded to a size less than requested. If a failure occurs, errno indicates the nature of the failure. For more information about errno, see errno, _doserrno, _sys_errlist, and _sys_nerr.

The return value points to a storage space that is suitably aligned for storage of any type of object. To check the new size of the item, use _msize. To get a pointer to a type other than void, use a type cast on the return value.

Remarks

The _expand function changes the size of a previously allocated memory block by trying to expand or contract the block without moving its location in the heap. The memblock parameter points to the beginning of the block. The size parameter gives the new size of the block, in bytes. The contents of the block are unchanged up to the shorter of the new and old sizes. memblock shouldn't be a block that has been freed.

Note

On 64-bit platforms, _expand might not contract the block if the new size is less than the current size; in particular, if the block was less than 16K in size and therefore allocated in the Low Fragmentation Heap, _expand leaves the block unchanged and returns memblock.

When the application is linked with a debug version of the C run-time libraries, _expand resolves to _expand_dbg. For more information about how the heap is managed during the debugging process, see The CRT debug heap.

This function validates its parameters. If memblock is a null pointer, 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. If size is greater than _HEAP_MAXREQ, errno is set to ENOMEM, and the function returns NULL.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Function	Required header
`_expand`	<malloc.h>

For more compatibility information, see Compatibility.

Example

// crt_expand.c

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

int main( void )
{
   char *bufchar;
   printf( "Allocate a 512 element buffer\n" );
   if( (bufchar = (char *)calloc( 512, sizeof( char ) )) == NULL )
      exit( 1 );
   printf( "Allocated %d bytes at %Fp\n",
         _msize( bufchar ), (void *)bufchar );
   if( (bufchar = (char *)_expand( bufchar, 1024 )) == NULL )
      printf( "Can't expand" );
   else
      printf( "Expanded block to %d bytes at %Fp\n",
            _msize( bufchar ), (void *)bufchar );
   // Free memory
   free( bufchar );
   exit( 0 );
}

Allocate a 512 element buffer
Allocated 512 bytes at 002C12BC
Expanded block to 1024 bytes at 002C12BC

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