Runtime error! Please help

[Garfield]

I wrote this program (it is unfinished, due to the runtime error). I compile fine, but then I get a runtime error that causes the program to end and crash.

Please look at the code and, if you have time, compile and look for the problem. I believe that I comment very well, so it shouldn't be bad to read. The .c file is the attachment. Thanks.

--Garfield

[Unregistered]

Ack!?!? GOTO!?!?! I stopped looking right then and there.

Quzah.

[Garfield]

Oh, I'm sorry about the goto. I'll find a way around it later. But for now, can you or anybody find my problem with this runtime error? Please help.

--Garfield

[zen]

I'm not sure if the rest of your program is correct but you're getting a runtime error when searching because you're not initialising the the next pointer to NULL when you create your initial student node, so in schstud() walk_stud is never equal to NULL and you are attempting to access an unintialised pointer.

Evertime, you create a node you should initialise it's next pointer to NULL, so it can be used as a test for the end of the list.

[andy bee]

It runs fine on my compiler,,complete the whole program then send to me ,,I'll get a better idea on why it crashes.
Does not give out any errors with me??

[Garfield]

So, zen, what I have to do in main is correct it from:

int main()
{
	int test;
	char strline[10];

	/* first head struct of student */
	struct student *init_stud, *temp;
	init_stud = malloc(sizeof(struct student));
/* etc..... */

to this:

int main()
{
	int test;
	char strline[10];

	/* first head struct of student */
	struct student *init_stud, *temp;
	init_stud = malloc(sizeof(struct student));
        init_stud = NULL; /* Must make NULL, right? */
/* etc... */

And then in the function where I add the new list to linked list, I'll have to change it from:

void addstud(struct student *head_stud, struct student *insert_stud)
{
	struct student *temp_stud;	/* temporary stud to walk with */
	temp_stud = malloc(sizeof(struct student));
	/* loop to get to the last list */
	while (1)
	{
		if (temp_stud == NULL)
			break;
		
		else
			temp_stud = temp_stud->next_student;
	}

	temp_stud->next_student = insert_stud;
}

to this:

void addstud(struct student *head_stud, struct student *insert_stud)
{
        insert_stud->next_stud = NULL; /* next instance NULL */
	struct student *temp_stud;	/* temporary stud to walk with */
	temp_stud = malloc(sizeof(struct student));
	/* loop to get to the last list */
	while (1)
	{
		if (temp_stud == NULL)
			break;
		
		else
			temp_stud = temp_stud->next_student;
	}

	temp_stud->next_student = insert_stud;
}

Right? So then when I add another student, the next pointer (*next_stud) will equal NULL until another linked list is added on. Is this correct? Are these changes valid? Thanks so much. I really owe you one, zen.

--Garfield

[Salem]

You've got a basic problem initialising the linked list, since you are forcing yourself to have a dummy node at the front of the list (this is the malloc in main).
The reason for this difficulty is that addstud doesn't return a new head pointer (in the case when the list is currently empty).

This is my version.

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

/* struct grades -- the list of grades per student */
struct grades {
    int grade;  /* the actual grade for this struct */
    struct grades *next_grade;
};

/* struct student -- the basis for the student data */
struct student {
    char name[50];  /* holds the name of the student */
    struct grades *grade_list;  /* the student's grade list */
    struct student *next_student;   /* pointer to next student */
};

struct student *addstud ( struct student *head_stud, struct student *insert_stud )
{
    struct student *new_node;
    new_node = malloc(sizeof(struct student));

    /* copy data to the new node */
    strcpy( new_node->name, insert_stud->name );
    new_node->grade_list = NULL;
    new_node->next_student = NULL;

    /* Add new node to the appropriate point in the list */
    if ( head_stud == NULL ) {
        /* empty list - the new node is the whole list */
        return new_node;
    } else {
        /* Find the last node in the list */
        struct student *end = head_stud;
        while ( end->next_student != NULL ) {
            end = end->next_student;
        }

        /* make the end node point to the new node */
        /* it will be the end node next time around */
        end->next_student = new_node;

        /* no change to the head of the list */
        return head_stud;
    }
}

int main ( ) {
    struct student *init_stud = NULL;   /* an empty list */
    struct student data;                /* values from somewhere */
    init_stud = addstud( init_stud, &data );
    return 0;
}

The key part is this
  init_stud = addstud( init_stud, &data );

1. By returning the current list head each time, you allow addstud to return a proper first node as the new head of the list in the case when the list is initially empty.

2. data is a normal variable - you don't malloc new nodes in main, its all done in addstud, where a new node is allocated, and the data copied to it.

[Garfield]

Thanks! I understand. Thanks again.

--Garfield