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 a call to free or realloc
. Otherwise, the results are undefined.
The reallocation is done by either:
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. 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 may or may not be freed), or some non-null pointer may be returned that may not be used to access storage).
A previous call to free or |
(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 accommodate 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
See also
C++ documentation for realloc
|