Arraylist in C failing on realloc - c

Question

I'm trying to build an arraylist in C with the following code
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
// Declaration of ArrayList structure
typedef struct ArrayList {
int length, capacity;
int *items;
} ArrayList;
// Create a new ArrayList
ArrayList *newList() {
int *items = malloc(4 * sizeof(int));
ArrayList *list = malloc(sizeof(ArrayList));
list->length = 0;
list->capacity = 4;
list->items = items;
return list;
}
// Check and expand list if neccessary
void check(ArrayList *list) {
printf("Check called (%d, %d)\n", list->length, list->capacity);
if (list->length >= list->capacity) {
printf("Expanding\n");
list->capacity = list->capacity * 2;
printf("Reallocating\n");
list->items = realloc(list->items, list->capacity);
if (list->items == NULL) {
printf("realloc failed\n");
exit(1);
}
}
}
// Add a value to the ArrayList
void add(ArrayList *list, int n) {
check(list);
list->items[list->length] = n;
list->length++;
}
// Print the list
void printList(ArrayList *list) {
for (int i=0; i<list->length; i++) {
if (i > 0) printf(", ");
printf("%d", list->items[i]);
}
printf("\n");
}
int main () {
ArrayList *list = newList();
for (int i = 0; i < 20; i++) {
add(list, i);
}
printList(list);
}
When the array is full, the check function is called as it should be. However, the second time the check function is called, the program failes on the call to realloc giving the following error:
*** Error in `./test': realloc(): invalid next size: 0x0000000001d3c010 ***
Aborted (core dumped)
where the size varies every time the program is run.
I have read that this error is caused by a corrupt heap, which is normally caused by pointers going wrong somewhere. However, I cannot see where the problem lies in this example. Any help would be appreciated.

Answer 1

you are reallocating the list->items. realloc() function has 2 parameter
first one is void pointer ,this point to the memory block that previously allocated,and second parameter works for how many bytes have to reallocate.
in your code you added only the capacity...bt it is not .u have to add size of the int with capacity ...cause it only takes (size ) int byte ...
then it works fine
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
// Declaration of ArrayList structure
typedef struct ArrayList {
int length, capacity;
int *items;
} ArrayList;
int i;
// Create a new ArrayList
ArrayList *newList() {
int *items = malloc(4 * sizeof(int));
ArrayList *list = malloc(sizeof(ArrayList));
list->length = 0;
list->capacity = 4;
list->items = items;
return list;
}
// Check and expand list if neccessary
void check(ArrayList *list) {
printf("Check called (%d, %d)\n", list->length, list->capacity);
if (list->length >= list->capacity) {
printf("Expanding\n");
list->capacity = list->capacity * 2;
printf("Reallocating\n");
list->items = realloc(list->items, list->capacity * sizeof(int));
if (list->items == NULL) {
printf("realloc failed\n");
exit(1);
}
}
}
// Add a value to the ArrayList
void add(ArrayList *list, int n) {
check(list);
list->items[list->length] = n;
list->length++;
}
// Print the list
void printList(ArrayList *list) {
for (i=0; i<list->length; i++) {
if (i > 0) printf(", ");
printf("%d", list->items[i]);
}
printf("\n");
}
int main () {
ArrayList *list = newList();
for ( i = 0; i < 20; i++) {
add(list, i);
}
printList(list);
}