Pointer values changes when using realloc() on pointer to pointer - c

Question

I have to read from a file which has a unknown number of students records in it written in binary, then sort the students by their GPA and send to stdout.
Our sort function had to be like
void insertion_sort(Student **, int);
That's why I choose to use an pointer to a pointer to Student (probably not the best solution? I think I could have just sent an pointer to Student like this (&p_to_Student, n) ?)
The code is bellow, the problem is that when I print the first element of what p is pointing to (the first students name) I get gibberish, the other students are fine.
I checked the value of p, and it does change after realloc() is called, and because it's also the address of the first element of p (right?).
Also checked with Valgrind and it returns a bunch of errors about memory leaks!
The code runs fine when there is no realloc() call, also when I initialize p after I'm done reading the file. So it must be something to do with not using realloc() correctly.
Bonus question: is this a proper way to read an unknown number of data entries from a file?
#include <stdio.h>
#include <stdlib.h>
struct student {
char name[30];
char surname[30];
double GPA;
};
typedef struct student Student;
void insertion_sort(Student **arr, int n)
{
int i,j;
for (i=1; i<n; i++)
{
Student *tmp = arr[i];
for (j=i; j>0 && (tmp->GPA > arr[j-1]->GPA); j--)
arr[j] = arr[j-1];
arr[j] = tmp;
}
}
int main(int argc, char **argv)
{
FILE *in;
Student s, *arr, **p;
size_t ret;
int i = 1, n=0, c=2;
in = fopen(argv[1], "rb");
if (in == NULL)
return printf("Can't open file!\n"), 1;
arr = (Student*) malloc(c*sizeof(Student*));
p = (Student**) malloc(c*sizeof(Student*));
do
{
ret = fread(&s, sizeof(Student), 1, in);
if (ret)
{
if (n == c)
{
arr = (Student*) realloc(arr, (c*=2)*sizeof(Student));
p = (Student**) realloc(p, c*sizeof(Student*));
}
// when I print the value of pointer p
// the values is changed when realloc() is called
printf("p = %p\n", p);
arr[n] = s;
p[n] = arr+n;
n++;
}
} while (ret);
fclose(in);
// If I do this instead the program runs correctly
//p = (Student**) malloc(c*sizeof(Student));
//for (int i=0; i<n; i++)
//{
//p[i] = arr+i;
//}
insertion_sort(p, n);
for (i=0; i<n; i++)
{
printf("%2d. %-20s %-20s %7.2lf\n", i+1, p[i]->name,
p[i]->surname, p[i]->GPA);
}
free(arr);
free(p);
return 0;
}

Answer 1

realloc may change the pointer. That means all pointers into that pointer may become invalid. In your case, p holds pointers into arr.
Your problem is not that the value of p changes, but that the old values of p are no longer valid when the value of arr changes.
To illustrate (all pointer and size values are made up):
sizeof(stud) == 16;
allocate arr: arr == 0x00100000;
1st value: arr[0] = stud1; p[0] = &arr[0]; // 0x00100000
2nd value: arr[1] = stud2; p[1] = &arr[1]; // 0x00100010
reallocate arr: arr == 0x00200000;
old address of arr is no longer valid!
3rd value: arr[0] = stud1; p[2] = &arr[2]; // 0x00200020
Now your pointer array looks like this:
p[0] == 0x00100000 // no longer valid!
p[0] == 0x00100010 // no longer valid!
p[0] == 0x00200020 // okay
Because you need p only for your sorting, the approach you have commented out – to allocate p at one go before sorting – is better.
realloc is useful only if you don't now beforehand how big your array is, so you should use it as long as you are building the array. When you are done building the array and you can be sure that arr will stay the same you should create the array of pointers, p.