realloc
Defined in header <stdlib.h>
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void *realloc( void *ptr, size_t new_size ); |
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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 |
(until C23) |
If |
(since C23) |
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 in bytes |
Return value
On success, returns the pointer to the beginning of newly allocated memory. To avoid a memory leak, 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
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 <assert.h> #include <stdio.h> #include <stdlib.h> #include <string.h> void print_storage_info(const int* next, const int* prev, int ints) { if (next) { printf("%s location: %p. Size: %d ints (%ld bytes).\n", (next != prev ? "New" : "Old"), (void*)next, ints, ints * sizeof(int)); } else { printf("Allocation failed.\n"); } } int main(void) { const int pattern[] = {1, 2, 3, 4, 5, 6, 7, 8}; const int pattern_size = sizeof pattern / sizeof(int); int *next = NULL, *prev = NULL; if ((next = (int*)malloc(pattern_size * sizeof *next))) { // allocates an array memcpy(next, pattern, sizeof pattern); // fills the array print_storage_info(next, prev, pattern_size); } else { return EXIT_FAILURE; } // Reallocate in cycle using the following values as a new storage size. const int realloc_size[] = {10, 12, 512, 32768, 65536, 32768}; for (int i = 0; i != sizeof realloc_size / sizeof(int); ++i) { if ((next = (int*)realloc(prev = next, realloc_size[i] * sizeof(int)))) { print_storage_info(next, prev, realloc_size[i]); assert(!memcmp(next, pattern, sizeof pattern)); // is pattern held } else { // if realloc failed, the original pointer needs to be freed free(prev); return EXIT_FAILURE; } } free(next); // finally, frees the storage return EXIT_SUCCESS; }
Possible output:
New location: 0x144c010. Size: 8 ints (32 bytes). Old location: 0x144c010. Size: 10 ints (40 bytes). New location: 0x144c450. Size: 12 ints (48 bytes). Old location: 0x144c450. Size: 512 ints (2048 bytes). Old location: 0x144c450. Size: 32768 ints (131072 bytes). New location: 0x7f490c5bd010. Size: 65536 ints (262144 bytes). Old location: 0x7f490c5bd010. Size: 32768 ints (131072 bytes).
References
- C17 standard (ISO/IEC 9899:2018):
- 7.22.3.5 The realloc function (p: 254)
- 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