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Showing content from https://cplusplus.com/reference/cstdlib/realloc/ below:

function

<cstdlib>

realloc 
void* realloc (void* ptr, size_t size);

Reallocate memory block

Changes the size of the memory block pointed to by ptr.

The function may move the memory block to a new location (whose address is returned by the function).

The content of the memory block is preserved up to the lesser of the new and old sizes, even if the block is moved to a new location. If the new size is larger, the value of the newly allocated portion is indeterminate.

In case that ptr is a null pointer, the function behaves like malloc, assigning a new block of size bytes and returning a pointer to its beginning.

Otherwise, if

size

is zero, the memory previously allocated at

ptr

is deallocated as if a call to

free

was made, and a

null pointer

is returned.


If size is zero, the return value depends on the particular library implementation: it may either be a null pointer or some other location that shall not be dereferenced.


If the function fails to allocate the requested block of memory, a null pointer is returned, and the memory block pointed to by argument ptr is not deallocated (it is still valid, and with its contents unchanged).

Parameters
ptr
Pointer to a memory block previously allocated with malloc, calloc or realloc.
Alternatively, this can be a null pointer, in which case a new block is allocated (as if malloc was called).
size
New size for the memory block, in bytes.
size_t is an unsigned integral type.

Return Value A pointer to the reallocated memory block, which may be either the same as ptr or a new location.
The type of this pointer is void*, which can be cast to the desired type of data pointer in order to be dereferenceable.

A null-pointer indicates either that size was zero (an thus ptr was deallocated), or that the function did not allocate storage (and thus the block pointed by ptr was not modified).

A null-pointer indicates that the function failed to allocate storage, and thus the block pointed by ptr was not modified.


Example 
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/* realloc example: rememb-o-matic */
#include <stdio.h>      /* printf, scanf, puts */
#include <stdlib.h>     /* realloc, free, exit, NULL */

int main ()
{
  int input,n;
  int count = 0;
  int* numbers = NULL;
  int* more_numbers = NULL;

  do {
     printf ("Enter an integer value (0 to end): ");
     scanf ("%d", &input);
     count++;

     more_numbers = (int*) realloc (numbers, count * sizeof(int));

     if (more_numbers!=NULL) {
       numbers=more_numbers;
       numbers[count-1]=input;
     }
     else {
       free (numbers);
       puts ("Error (re)allocating memory");
       exit (1);
     }
  } while (input!=0);

  printf ("Numbers entered: ");
  for (n=0;n<count;n++) printf ("%d ",numbers[n]);
  free (numbers);

  return 0;
}

The program prompts the user for numbers until a zero character is entered. Each time a new value is introduced the memory block pointed by numbers is increased by the size of an int.

Data races Only the storage referenced by ptr and by the returned pointer are modified. No other storage locations are accessed by the call.
If the function releases or reuses a unit of storage that is reused or released by another allocation or deallocation function, the functions are synchronized in such a way that the deallocation happens entirely before the next allocation.

Exceptions (C++)No-throw guarantee: this function never throws exceptions.

See also
free
Deallocate memory block (function)
calloc
Allocate and zero-initialize array (function)
malloc
Allocate memory block (function)

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