Reallocate memory blocks.


void *realloc(
   void *memblock,
   size_t size


Pointer to previously allocated memory block.

New size in bytes.

Return value

realloc returns a void pointer to the reallocated (and possibly moved) memory block.

If there isn't enough available memory to expand the block to the given size, the original block is left unchanged, and NULL is returned.

If size is zero, then the block pointed to by memblock is freed; the return value is NULL, and memblock is left pointing at a freed block.

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



realloc hasn't been updated to implement C17 behavior because the new behavior isn't compatible with the Windows operating system.

The realloc function changes the size of an allocated memory block. The memblock argument points to the beginning of the memory block. If memblock is NULL, realloc behaves the same way as malloc and allocates a new block of size bytes. If memblock isn't NULL, it should be a pointer returned by a previous call to calloc, malloc, or realloc.

The size argument 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, although the new block can be in a different location. Because the new block can be in a new memory location, the pointer returned by realloc isn't guaranteed to be the pointer passed through the memblock argument. realloc doesn't zero newly allocated memory if there's buffer growth.

realloc sets errno to ENOMEM if the memory allocation fails or if the amount of memory requested exceeds _HEAP_MAXREQ. For information on this and other error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

realloc calls malloc in order to use the C++ _set_new_mode function to set the new handler mode. The new handler mode indicates whether, on failure, malloc is to call the new handler routine as set by _set_new_handler. By default, malloc doesn't call the new handler routine on failure to allocate memory. You can override this default behavior so that, when realloc fails to allocate memory, malloc calls the new handler routine in the same way that the new operator does when it fails for the same reason. To override the default, call


early in ones program, or link with NEWMODE.OBJ (see Link options).

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

realloc is marked __declspec(noalias) and __declspec(restrict), meaning that the function is guaranteed not to modify global variables, and that the pointer returned isn't aliased. For more information, see noalias and restrict.

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


Routine Required header
realloc <stdlib.h> and <malloc.h>

For more compatibility information, see Compatibility.


// crt_realloc.c
// This program allocates a block of memory for
// buffer and then uses _msize to display the size of that
// block. Next, it uses realloc to expand the amount of
// memory used by buffer and then calls _msize again to
// display the new amount of memory allocated to buffer.

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

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

   if( (buffer = (long *)malloc( 1000 * sizeof( long ) )) == NULL )
      exit( 1 );

   size = _msize( buffer );
   printf_s( "Size of block after malloc of 1000 longs: %u\n", size );

   // Reallocate and show new size:
   oldbuffer = buffer;     // save pointer in case realloc fails
   if( (buffer = realloc( buffer, size + (1000 * sizeof( long )) ))
        ==  NULL )
      free( oldbuffer );  // free original block
      exit( 1 );
   size = _msize( buffer );
   printf_s( "Size of block after realloc of 1000 more longs: %u\n",
            size );

   free( buffer );
   exit( 0 );
Size of block after malloc of 1000 longs: 4000
Size of block after realloc of 1000 more longs: 8000

See also

Memory allocation