C：从文件读取int并存储在指针数组中

Question

I am trying to read integers from a file and store them in an array. Then, I want to write those values to a different file. Reading an writing are done in a separate file from the main file, where the array was created. I cannot change the function's arguments (this is for an assignment, so the array argument must be int** ppPerm). The function is called, and the array originally created, in another file's main function. The file I am reading from looks like this:

15
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Where 15 is how many numbers will follow. So the chronological order of functions is:

Array Perm is created in the main function of the main file (int* Perm = NULL). It is passed into readP()

readP(In, &Perm);

The numbers from the file are read and stored in Perm. The same variable Perm is then passed into writeP().

writeP(Out, Perm, permLength);

Perm is read and written to a different file. I cannot change either of those lines. Somewhere along the road the array is screwed up. Here is readP().

int readP(FILE* In, int** ppPerm) {
   int numElements = 0;

   fscanf(In, "%d", &numElements);

   *ppPerm = (int*)calloc(numElements, sizeof(int));

   int i;
   for (i = 0; i < numElements; i++) {
       fscanf(In, "%p", &ppPerm[i]);
   }

   return numElements;
}

Right now, the array is completely unreadable. For whatever reason, the numbers stored are something like 0x0 then a random jumble of hexadecimals. The array is then used in writeP() to write the values to a different file:

void writeP(FILE* Out, const int* pPerm, int permLength) {

    int i = 2;
    for (i = 0; i < permLength; i++) {
        fprintf(Out, "%d ", pPerm[i]);
    }

    return;
}

int* pPerm is the same array that was passed into readP(). For some reason, using debugging, I see that pPerm contains completely different values than ppPerm did, and in some cases it seems half empty. What exactly is wrong with my functions? Why can I not store the numbers in the array correctly? And why does the array keep messing up between readP() and writeP()?

Answer 1

Don't cast the result of calloc() or malloc() in C! It can hide errors.

 int i; for (i = 0; i < numElements; i++) { fscanf(In, "%p", &ppPerm[i]); } 

Since you want to read integers the format string should be "%i" or "%d" . To get to the Pointer to the memory you have allocated use *ppPerm :

size_t i;  // variables that hold an index into or the size
           // of objects in memory should be of type size_t.
for (i = 0; i < numElements; i++) {
    fscanf(In, "%p", &(*ppPerm)[i]);
}