Thread: What's wrong with this program?

The link of the program is: <<<snipped>>> I use gdb to debug the program. It said that: Program received signal SIGSEGV, Segmentation fault.
0x080491c7 in simplify_polynomial (n=0x804c148) at test7.c:476
476 p=p->head->next;
I don't know what is the problem. Could you help me? Thank you.

[code]

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



typedef struct node /*a node denotes a P_{i,k}^{l}*/
{

int l,i,k;

} node;


typedef struct monomial_node /* a monomial denotes a monomial P_{i,k}^{l} \cdots P_{i',k'}^{l'} */

{
node y;
int coeff; /* coeff is the coefficent of the monomial */
struct monomial_node *next;
struct monomial_node *head;
} monomial_node, *monomial;







typedef struct polynomial_node /* a polynomial denotes a sum of some monomials */

{

monomial m;

struct polynomial_node *next;

struct polynomial_node *head;

} polynomial_node, *polynomial;





monomial insert(node a, monomial b) /* insert a factor a to a monomial b such that the first subscripts i's in the new monomial is in increasing order */

{

monomial p;

monomial_node *s;

monomial_node *p1;


p=b;
p1=p->head->next;

if(p1==NULL)
{

s=malloc(sizeof(monomial_node));
s->y=a;
p->head->next=s;
p->head->next->next=NULL;
p->coeff=1;
}
else
{
while((p1->y).i >= a.i) /* find the position */
{

if(p1->next==NULL)
{

break;

}
p1=p1->next;

}
}


s=malloc(sizeof(monomial_node)); /* create new node */

s->y=a;

s->next=p1->next; /* insert the node a */

p1->next=s;

p->coeff=b->coeff;
return p;

}


monomial create_monomial(int number) /* create a monomial */

{

monomial m;

monomial_node *p, *s;

int i;


m=(monomial)malloc(sizeof(monomial_node));
m->head=(monomial)malloc(sizeof(monomial_node));

if(m->head==NULL)

{
printf("No memory.\n");
return NULL;

}


m->head->next=NULL;

p=malloc(sizeof(monomial_node));

m->head->next=p;

p->next=NULL;


printf("Please input the parameters of the heighest weight monomial(for example, 2, 2, 3 denotes the monomial Y^{2}_{2,3}). The first subscripts i of Y's should be in increase order.");

scanf("%d", &((p->y).l));
scanf("%d", &((p->y).i));
scanf("%d", &((p->y).k));

if(number > 1)
{
for(i=0;i<number-1;i++)

{

s=(monomial)malloc(sizeof(monomial_node));

scanf("%d", &((s->y).l));
scanf("%d", &((s->y).i));
scanf("%d", &((s->y).k));




m=insert(s->y,m);


}

}

m->coeff=1;



return m;



}



polynomial create_polynomial(monomial n) /* create a polynomial with only one monomial n */

{

polynomial m;
polynomial_node *p;

m=(polynomial)malloc(sizeof(polynomial_node));
p=(polynomial)malloc(sizeof(polynomial_node));
m->head=(polynomial)malloc(sizeof(polynomial_node) );

m->head->next=p;
p->m=n;
p->next=NULL;



return m;



}





void print_monomial(monomial m)

{

monomial p;


p=m->head->next;

if(p==NULL)

{

printf("No element. \n");

}

if(m->coeff == -1)
{
printf("-");
}
else if(m->coeff != 1)
{
printf("%d", m->coeff);
}

while(p!=NULL)

{

printf("Y^{%d}_{%d,%d}",(p->y).l,(p->y).i,(p->y).k);

p=p->next;

}

}

> 476 p=p->head->next;
> I don't know what is the problem. Could you help me? Thank you.
You should check p->head isn't NULL before trying to do p=p->head->next


What are these for?

Code:

typedef struct monomial_node /* a monomial denotes a monomial P_{i,k}^{l} \cdots P_{i',k'}^{l'} */
{
 node y;
 int coeff;         /* coeff is the coefficent of the monomial */
 struct monomial_node *next; 
 struct monomial_node *head;
} monomial_node, *monomial;

typedef struct polynomial_node /* a polynomial denotes a sum of some monomials */
{
 monomial m;
 struct polynomial_node *next; 
 struct polynomial_node *head;
} polynomial_node, *polynomial;

If you've got say 5x^2+10x+20, then you have 3 polynomial nodes, where each instance of the 'm' member is initialised to 5x^2, 10x and 20

I would also re-think the idea of storing a head pointer in each node. If you delete the head node of a list, you have to change the entire list to point to the new head node (of the list).

Linked list - Wikipedia, the free encyclopedia
Draw out some pictures so you're clear on paper exactly what is stored in a node, and how all the pointers are arranged.

These are a part of my program. Thank you.