C中的realloc函数导致“ realloc（）：无效指针：”错误

Question

I am trying to create a Dynamic Integer Array in C which should automatically double its size once full.

To extend the size of the Array I want to use the realloc function. Unfortunately there seems to be some problem with the pointer to the data of my DynamicArray and GCC crashes :( Does anyone have a clue why? If I realloc just after using malloc, there is no crash. I have checked the pointer address and it is not changed between malloc and realloc.

The following are my header file declarations for the struct:

   struct DynamicArray
    {
        unsigned int size;
        unsigned int capacity;

        int *data;
    };   // -----  end of struct DynamicArray  -----
    typedef struct DynamicArray DynamicArray;


    // #####   EXPORTED FUNCTION DECLARATIONS   

    DynamicArray* dn_new ( unsigned int capacity );
    int dn_append ( DynamicArray *a, int elem );

This is my main class used for testing:

    int
main ( int argc, char *argv[] )
{
    DynamicArray *a1;
    int                     i;

    a1 = dn_new ( 5 );

    // here six integers are inserted, the sixth causes the realloc crash as this is where I want to extend
    dn_append ( a1,  5 );
    dn_append ( a1,  7 );
    dn_append ( a1,  8 );
    dn_append ( a1, 11 );
    dn_append ( a1, 13 );
    dn_append ( a1, 15 );


    return 0;
}   // -----  end of function main  -----

These are the functions dn_append and dn_new which are called by the main:

    DynamicArray*
    dn_new ( unsigned int capacity )
    {
        struct DynamicArray *a = malloc(sizeof(struct DynamicArray));


        a->data = (int *)calloc(capacity,capacity * sizeof(int));
        printf("allocated %d\n",a->data);
        a->size=0;
        a->capacity=capacity;
        printf("array created\n");
        if (a == NULL){
            return NULL;
        }
        else{
            return a;   
        }
    } 
int
dn_append ( DynamicArray *a, int elem )
{   
    short extended = 0;
    if (a->size==a->capacity){
        a->capacity=a->capacity*2;
        printf("allocated %d\n",a->data);
//here is exactly where the error occurs
        a->data = (int *)realloc(a->data,a->capacity * sizeof(int));
        printf("array extended to have capacity for %d integers\n",a->capacity);
    extended=1;
}

    a->data[(a->size-1)] = elem;
    a->size++;
    printf("element %d appended to array\n",elem);

    return extended;
} 

Answer 1

Consider what happens when you first insert a value into an empty array. Your dn_append code has:

a->data[(a->size-1)] = elem;
a->size++;

When a->size is 0, this writes into index -1 , off the start of the array. This apparently overwrites some of malloc's internal data, causing it to think the pointer is invalid when you later realloc it...

Answer 2

a->data = (int *)calloc(capacity,capacity * sizeof(int));

The second parammeter is wrong; you should remove the capacity, and use only sizeof(int) :

a->data = (int*) calloc(capacity, sizeof(int));