realloc function in C causes "realloc(): invalid pointer:" Error

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;
}

The parameters to calloc() and realloc() are wrong - you must fix this. Additionally, "invalid pointer error" usually means that you've corrupted your allocated memory somehow (for example, writing out of bounds). Double-check all the code where you write to the allocated memory. Consider using a tool like valgrind. — paulsm4
– paulsm4, Commented May 3, 2015 at 20:04

Chris Dodd · Accepted Answer · 2015-05-03 20:44:58Z

3

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...

answered May 3, 2015 at 20:44

Chris Dodd

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João Vitor Verona Biazibetti · Accepted Answer · 2015-05-03 19:56:44Z

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));

answered May 3, 2015 at 19:56

João Vitor Verona Biazibetti

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1 Comment

AlexeyGy Over a year ago

Thank you for your help, but I still have the same realloc crash problem

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