Thread: struct node pointer dereference printing bug/difficulty

I'm doing a file reading. The contents of the file is saved into a subject node called head. In order to perform dynamic node creation, I equated the temp = *head (on line 48). Upon printing, calling print(head) on line 34, the program crashed.

I debugged it and found a segmentation fault on printing so I added line 50 and 51 for manual debugging. The contents of ctemp can be printed, but not the original node *head. Clearly it is not updated.

What's the problem with line 48? Why can't the value of head be changed? What do you think am I doing wrong?

*added txt && cpp attachments

Code:

#include <iostream>
#include <fstream>
#include <cstring>
#include <cstdlib>
using namespace std;

typedef struct cell {
    int row, bit;
    struct cell *next;
}cell;
typedef struct course{
    int col;
    char name[3];
    struct cell* rcell;
    struct course *next;
}course;
typedef struct subject{
    int col;
    char name[3];
    struct student* student;
    struct subject *next;
}subject;
typedef struct student{
    int index;
    char name[3];
    struct student *next;
}student;
void readfile(subject**);
void print(subject*);
void _free(subject**);
int main(void) {
    subject* head = NULL;
    readfile(&head);
    print(head);
    _free(&head);
    free(head);
}

void readfile(subject** head) {
    int column = 0, row = 0, ctotal = 0;
    char cname[3], sname[3], c;
    ifstream ifile("schedule.txt");
    subject *ctemp = NULL;
    student *stemp = NULL;
    if(ifile.is_open()){
        ctemp = *head;
        while(ifile >> cname) {
            ctemp = new subject;
            strcpy(ctemp->name, cname);
            cout << "\nctemp " << ctemp->name << " ";
            cout << "\nhead "<< (*head)->name << " ";
            ctemp->col = column++;
            stemp = ctemp->student;
            while(ifile >> sname) {
                c = ifile.get();
                if(c == '\n') break;
                stemp = new student;
                stemp->index = row++;
                strcpy(stemp->name, sname);
                cout << stemp->name << " ";
                stemp->next = NULL;
                stemp = stemp->next;
            }
            row = 0;
            ctemp->next = NULL;
            ctemp = ctemp->next;
            ctotal++;
        }
    } else {
        cout << "\nfile error";
    }
    ifile.close();
}

void print(subject *head) {
    while(head) {
        cout << endl << head->name << " ";
        while(head->student) {
            cout << head->student->name << " ";
            head->student = head->student->next;
        }
        head = head->next;
    }
}
void _free(subject** head) {
    subject* ctemp = NULL;
    student* stemp = NULL;
    while(*head) {
        ctemp = *head;
        while((*head)->student) {
                stemp = ctemp->student;
                (*head)->student = (*head)->student->next;
                stemp->next = NULL;
                free(stemp);
        }
        *head = (*head)->next;
        ctemp->next = NULL;
        free(ctemp);
    }
}

Originally Posted by Salem

Well first of all, to store "ABC", you need at least 4 elements in your char arrays.
And since this is C++, you should be using std::string in place of char arrays.

The second point is that you must NOT call free() on objects allocated with new.
malloc / calloc / realloc -> free
new -> delete
new[ ] -> delete [ ]
Never mix and match allocators and deallocators.

Thanks for the heads up, I tend to mix things up all the time. I changed the size to 4 and tried strings(though discouraging due to memory size).

I don't think these have impact on the code flow. Still I am having the same error SIGSEGV segmentation fault on that line.