Endless loop in C; for loop seems valid - c

Question

I am a student trying to learn c coming from c++. I wrote the following code and it compiles fine; however, when I execute it I get an endless loop when calling the print function. I looked over the code and it seems to be valid to me, so why is it printing an endless loop?
#include <stdio.h>
#include <stdlib.h>
struct student
{
int id;
int score;
};
void generate(struct student *students, int n)
{
int randomId = rand () % n + 1;
int randomTestScore = rand() % 100 + 1;
students->id = randomId;
students->score = randomTestScore;
}
void sort(struct student *students, int n)
{
/*using insertion sort*/
for (unsigned int i = 1; i < n; ++i)
{
int next = students[i].score;
int j = i;
while(j > 0 && students[j-1].score > next)
{
students[j].score = students[j-1].score;
j--;
}
students[j].score = next;
}
}
void print(struct student *students, int n)
{
for (unsigned int i = 0; i < n; i++)
{
printf("Student at position No: %d Test Score: %d\n", i+1, students[i].score);
}
}
int main()
{
/*user enters num of students to create scores for*/
int num_students;
printf("Enter Num of students\n");
scanf("%d", num_students);
/*allocate memory for the amount of students user wants*/
struct student *userStudents = malloc(num_students*sizeof(struct student));
printf("Randomly filling students IDs & Test Scores...\n");
for (unsigned int i = 0; i < num_students; ++i)
{
generate(&userStudents[i], num_students);
}
printf("Array of students before sorting:\n");
print(userStudents, num_students);
printf("\nNow, sorting students by test scores...\n\n");
sort(userStudents, num_students);
printf("Array of students after sorting:\n");
print(userStudents, num_students);
return 0;
}

Answer 1

To use scanf() correctly it needs to alter the passed variable in place, and since there is no pass by refrence in c, you need to pass the address of the variable, so scanf() is able to modify it though a pointer, hence you need to use the & operator, but that is not enough.
The scanf() family of functions, return a value that must be checked before you can access the scanned values, you should never ignore that value, under any circumstances you should check for it.
What your code is doing is called undefined behavior, it's interpreting the passed integer as if it was a pointer, which is undefined behavior.
To prevent that you can activate compiler warnings, many compilers know what kind of parameter the *f functions expect, i.e. the functions which take a string as a format to be parsed and to allow the function to correctly grab the rest of the parameters passed via variable arguments to it.
The correct way to call scanf() in your program is
if (scanf("%d", &num_students) != 1)
return 1;
that is, from main() and hence it's ending the program, because you can't continue after that condition was true, in that case what actually happens is that num_students is not initialized, that would once again cause undefined behavior.

Answer 2

Change the call to scanf to:
/*
* correct way of calling scanf, passing the address of the wanted variable
*/
scanf("%d", &num_students);
^
This elliminates segmentation faults and makes the code runs OK on my machine.
I had a previous hint that you'd need to change your declaration of userStudents to a pointer to pointers, however I was incorrect. You are clearly correctly allocating enough contiguous memory to hold all of your structs pointed by *userStudents.