realloc is giving error - invalid next size

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int temp;


int main()
{
    FILE * fp;
    fp = fopen("input2.txt", "r");                      //Open the input
    int counter = 0; 
    int realloc_counter = 10; 

    int *line_array;                                    //Initialize the array
    line_array = malloc(10 * sizeof(int));              //Allocate memory for initial ten numbers, of size int for each 

    while (fscanf(fp, "%d", &temp) > 0)
    {
        line_array[counter] = temp; 
        counter ++;

        if (counter % 10 == 0)
        {       
            realloc_counter = realloc_counter * 2;
            line_array = realloc(line_array, realloc_counter);
        }


    }



    fclose(fp);                                         //Close the input file
    free(line_array);                                   //Free the memory

The above code is what I have. It keeps giving me an error and I can't seem to figure it out. Using valgrind it says there is an invalid write of size 4. Any suggestions or insight?

Solution

The "invalid next size" style of error message when using dynamic memory allocation is usually because you have corrupted the memory arena by writing beyond the end of an allocated buffer.

Have a look at your two allocation lines:

line_array = malloc(10 * sizeof(int));
line_array = realloc(line_array, realloc_counter);

The first is multiplying the element count by the element size so that the number of bytes allocated is correct. The second is just using the element count on its own without multiplying it by the element size.

So the first time you do a re-allocation, realloc_counter is set to 20, so you'll almost certainly shrink the memory allocated (though this depends on the relative sizes of your integers and bytes, of course).

For example, if sizeof(int) == 4, you first allocate the correct forty bytes, then reallocate twenty, when what you need is eighty.

What you should be doing is something like:

line_array = realloc(line_array, realloc_counter * sizeof(int));

As an aside, you should also be checking the return values from both malloc and realloc to see if they fail. It's not really a good idea to assume they'll always work.