关于如何使用C语言从文本文件读取和写入

Question

问题是要使用给定的大整数数组文件中的归并排序来计算反转次数。虽然我可以编写用于反转计数的代码，但无法编写如何读取和打印给定文件的反转次数。有什么帮助吗？

long Merge(int arr[], int temp[], int low, int mid, int high)
{
    int k = low, i = low, j = mid + 1;
    long inversionCount = 0;


    while (i <= mid && j <= high)
    {
        if (arr[i] <= arr[j]) {
            temp[k++] = arr[i++];
        }
        else {
            temp[k++] = arr[j++];
            inversionCount += (mid - i + 1);
        }
    }


    while (i <= mid)
        temp[k++] = arr[i++];




    for (int i = low; i <= high; i++)
        arr[i] = temp[i];

    return inversionCount;
}


long MergeSort(int arr[], int temp[], int low, int high)
{

    if (high == low)
        return 0;


    int mid = (low+high)/2;
    long inversionCount = 0;



    inversionCount = inversionCount + MergeSort(arr, temp, low, mid);
    inversionCount =inversionCount + MergeSort(arr, temp, mid + 1, high);
    inversionCount =inversionCount + Merge(arr, temp, low, mid, high);
    return inversionCount;
int main()
{
    FILE *in_file;
    int arr[1000000000];
    int N = sizeof(arr)/sizeof(arr[0]);

        int temp[N];
        for (int i = 0; i < N; i++)
            temp[i] = arr[i];


    in_file = fopen("integerarray.txt", "r");

    if(in_file == NULL){
        printf("Error");}
    else{
        printf("Inversion Count is %ld", mergesort(arr, temp, 0, N - 1));
    return 0;
    }
}

收到未定义mergesort的错误。