C Doubly Linked List Program

I have been writing a C program to work with Doubly Linked Lists. But it does not work on Dev Cpp. Can anybody point out any error? It works until the part where I add a new record and then it crashes. I'm guessing there is a problem with the scanf() function.

//C double-linked list program struct stu { int roll,ph,ch,ma; stu *plink,*nlink; }; void addstu(stu *p,stu *n) { //add stu *s; int roll,ph,ch,ma; printf("HELLO STUDENT!\nYour roll no.? "); scanf("%d",&roll); printf("\nPhysics score? "); scanf("%d",&ph); printf("\nChemistry? "); scanf("%d",&ch); printf("\nMath? "); scanf("%d",&ma); s->roll=roll; s->ph=ph; s->ch=ch; s->ma=ma; s->plink=p; s->nlink=n; } void remstu(stu *s) { //remove stu *next; next->plink=s->nlink; free(s); } // Main function int main() { stu *s; int choice=0, ct=0; while(choice!=4) { printf("\n\t\tWELCOME TO RECORDS\n\n1. ADD\n2. REMOVE\n3. SEE\n4. EXIT\nEnter your choice(1-4): "); scanf("%d",&choice); switch(choice) { case 1: stu *t; addstu(s,t); s=t; free(t); ++ct; break; case 2: remstu(s); break; case 3: for(int i=0;i<ct;++i) { printf("\n\nRoll no. %d\nPhysics %d\nChemistry %d\nMath %d",s->roll,s->ch,s->ph,s->ma); s=s->plink; } break; case 4: exit(0); break; default: printf("Please enter correct choice(1-4)!"); } } return 0; }

-------------Problems Reply------------


The pointer s in function addstu() is never initialized and dereferencing it leads to undefined behavior hence the crash.


s = malloc(sizeof(struct stu)); /* Allocate memory */

I hope you have

typedef struct stu stu;

After reading your code, you have a lot of undefined behaviors going on.

Lets take case 1 as example:

stu *t;

The first problem is that when you call addstu the variable s is not initialized. Uninitialized (non-static) local variables have an indeterminate value, and will in practice seem to be random. The same goes for the variable t. So when in addstu you will set the nodes links to uninitialized pointers.

You call addstu which should create a new node, and add to the list. But what happens to that node you should create? Nothing, it's not passed on from the addstu function, the function doesn't even link it into the list properly (disregarding the uninitialized variables for a moment) so no new nodes will be added.

You then assign t to s, thereby initializing s but with an indeterminate value so nothing really changes.

You then continue to call free the memory for t (even though you haven't allocated any memory), and as s == t at that moment, that's the same as freeing s. Assignment of pointer only copies the pointer and not the memory that it points to.

There are probably many other problems with the code, but I only got that far. I suggest you start over, and with something simpler to learn how pointers and dynamic memory allocations work. Then come back to this problem, but before you write any code, try to draw the operations on paper. For example, what should an empty list look like? How would it look after adding the first node? After the second and third node? How would it look if you add a node to the end of the list? To the beginning of the list? Once you got all that clear, you can start writing your code.

Category:c# Views:0 Time:2018-09-13
Tags: linked list

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