realloc

Defined in header `<stdlib.h>`
void realloc( void ptr, size_t new_size );

Reallocates the given area of memory. It must be previously allocated by malloc(), calloc() or realloc() and not yet freed with free, otherwise, the results are undefined.

The reallocation is done by either:

a) expanding or contracting the existing area pointed to by ptr, if possible. The contents of the area remain unchanged up to the lesser of the new and old sizes. If the area is expanded, the contents of the new part of the array are undefined.

b) allocating a new memory block of size new_size bytes, copying memory area with size equal the lesser of the new and the old sizes, and freeing the old block.

If there is not enough memory, the old memory block is not freed and null pointer is returned.

If ptr is NULL, the behavior is the same as calling malloc(new_size).

If new_size is zero, the behavior is implementation defined (null pointer may be returned (in which case the old memory block is not freed), or some non-null pointer may be returned that may not be used to access storage).

realloc is thread-safe: it behaves as though only accessing the memory locations visible through its argument, and not any static storage.

A previous call to free or realloc that deallocates a region of memory synchronizes-with a call to any allocation function, including realloc that allocates the same or a part of the same region of memory. This synchronization occurs after any access to the memory by the deallocating function and before any access to the memory by realloc. There is a single total order of all allocation and deallocation functions operating on each particular region of memory.

(since C11)

Parameters

ptr	-	pointer to the memory area to be reallocated
new_size	-	new size of the array

Return value

On success, returns the pointer to the beginning of newly allocated memory. The returned pointer must be deallocated with free() or realloc(). The original pointer ptr is invalidated and any access to it is undefined behavior (even if reallocation was in-place).

On failure, returns a null pointer. The original pointer ptr remains valid and may need to be deallocated with free() or realloc().

Notes

Support for zero size is deprecated as of C11 DR 400.

Originally (in C89), support for zero size was added to accomodate code such as.

OBJ *p = calloc(0, sizeof(OBJ)); // "zero-length" placeholder
...
while(1) { 
    p = realloc(p, c * sizeof(OBJ)); // reallocations until size settles
    ... // code that may change c or break out of loop
}

Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int *pa = malloc(10 * sizeof *pa); // allocate an array of 10 int
    if(pa) {
        printf("%zu bytes allocated. Storing ints: ", 10*sizeof(int));
        for(int n = 0; n < 10; ++n)
            printf("%d ", pa[n] = n);
    }
 
    int *pb = realloc(pa, 1000000 * sizeof *pb); // reallocate array to a larger size
    if(pb) {
        printf("\n%zu bytes allocated, first 10 ints are: ", 1000000*sizeof(int));
        for(int n = 0; n < 10; ++n)
            printf("%d ", pb[n]); // show the array
        free(pb);
    } else { // if realloc failed, the original pointer needs to be freed
        free(pa);
    }
}

Output:

40 bytes allocated. Storing ints: 0 1 2 3 4 5 6 7 8 9
4000000 bytes allocated, first 10 ints are: 0 1 2 3 4 5 6 7 8 9

References

C11 standard (ISO/IEC 9899:2011):
- 7.22.3.5 The realloc function (p: 349)
C99 standard (ISO/IEC 9899:1999):
- 7.20.3.4 The realloc function (p: 314)
C89/C90 standard (ISO/IEC 9899:1990):
- 4.10.3.4 The realloc function