Malloc, when input is too high, it fails but runs smoothly when lower (eg. 3) Why?

Question

My program is to allow user to input no. of students and their preferred sessions, then sort all of them by the session with most students.

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

struct student {  //*** PART 2 STRUCTURE DEFINITION **********
    char studName[50];  // Name of student
    long long int studID;        // Student ID of student
    int session;      // Prefered session of each student
};

struct session {
    int sessionID;
    int NoofStudinSession;
};

FILE * fPointer;


void main(void)
{
    //  Definitions
    struct student *myStudents;
    struct session *sessionsPtr;
    int Noofsessions = 0;
    int NoofStud = 0;
    int i;

    //  User input for number of students and consultation sessions
    printf("Enter Number of Students:\n");
    scanf("%d", &NoofStud);
    printf("Enter number of consultation session:\n");
    scanf("%d", &Noofsessions);

    //  Dynamic allocation for struct
    myStudents = (struct student *) malloc(sizeof(struct student) * NoofStud);

    if (myStudents == NULL) {
        printf("Error, out of memory.\n");
        return;
    }

    sessionsPtr = (int *)malloc(sizeof(struct session) * Noofsessions);

    if (sessionsPtr == NULL) {
        printf("Error, out of memory.\n");
        return;
    }

    // Initializing struct session before using
    for (i = 0; i < Noofsessions; i++) {
        (sessionsPtr + i)->sessionID = i + 1;
        (sessionsPtr + i)->NoofStudinSession = 0;

    }

    //  User input display
    for (i = 0; i < NoofStud; i++) {
        printf("Enter Student %d ID: \n", i + 1); // Prompt user to enter the ID of students
        scanf("%d", &(myStudents + i)->studID); //**


        printf("Enter Student %d Name: \n", i + 1); // Prompt user to enter the Student name
        scanf(" %[^\t\n]s", &(myStudents + i)->studName); //**

        printf("Enter Student %d's prefered Session(Number of session(s) is %d):\n", i + 1, Noofsessions); // Prefered Session
        scanf("%d", &(myStudents + i)->session); //**

        (sessionsPtr + ((myStudents + i)->session - 1))->NoofStudinSession++;
    }


    //  Display summary of Part2 on screen
    printf("Student ID\t Student Name\t\t Chose Which Session\n");
    fprintf(fPointer, "Student ID\t Student Name\t\t Chose Which Session\n"); // Write into Text file
    for (i = 0; i < NoofStud; ++i) {

        printf("%d\t  %15s\t\t\t %d\n", (myStudents + i)->studID, (myStudents + i)->studName, (myStudents + i)->session);
        fprintf(fPointer, "%d\t  %15s\t\t\t %d\n", (myStudents + i)->studID, (myStudents + i)->studName, (myStudents + i)->session);
    }

    // Sorting of sessions from MOST students to LEAST students
    int total = 0, swapped, pos;

    //comparing and sort values
    while (1) {
        pos = 0;

        for (i = 0; i < Noofsessions - 1; ++i) {

            if ((sessionsPtr + i)->NoofStudinSession < (sessionsPtr + i + 1)->NoofStudinSession) {

                swapped = sessionsPtr;
                sessionsPtr = sessionsPtr + 1;
                swapped = sessionsPtr + 1;
                pos = 1;

            }
        }
        if (pos == 0) {
            break;
        }
    }

    //  Display the Sorted Table
    printf("\nSession\t\t\t No. of Students\n");
    fprintf(fPointer, "\nSession\t\t\t No. of Students\n"); // Write into Text file

    for (i = 0; i < Noofsessions; i++) {

        printf("%d\t\t\t    %d\n", (sessionsPtr + i)->sessionID, (sessionsPtr + i)->NoofStudinSession);
        fprintf(fPointer, "%d\t\t\t    %d\n", (sessionsPtr + i)->sessionID, (sessionsPtr + i)->NoofStudinSession); // Write into Text file
    }

    free(myStudents);
    free(sessionsPtr);

    system("pause");
    return 0;

}

When the Noofsession is to be smaller than 3, it runs smoothly. However when it goes higher, this error occurs (HEAP error Invalid address specified to RtlValidateHeap).

EDIT: to better explain the problem, here is the display right after crashing. Output Correct me if I'm wrong, so the output at the sorted tables are actually the address? instead of the sorted values.

Answer 1

Try to avoid multiplying inside calls to malloc. When allocating arrays of structures, use calloc.

That said, heed Michael Walz's comment. Get rid of all your warnings.