connected lists...???

hello ppl...
i'm having a bit of a problem here...
to cut a long story longer ... I was trying to make a program to handle cusotmer, product and order files.. but although it works kinda well, the connected structures I did (*next *prev), get filled with a lot of **** and it shows on the files...

what to do???
can anybody help me?
I'm attachin the code here, please be patient about it it's long and annoying.....

thanks in advanced....

You're right, it's long code. Can you show me where you think the problem's are? Just some pre-help tips, maybe you're not allocating the memory (a big problem with pointers and linked lists) and your system/compiler is taking some default values.

yes of course...
I went through all the allocations and still didn't find what I was looking for... anyway I think the problem should be around this function (the one that sorts the data - read from file or input by user into the memory)...

void add_customer(customer *q)
{
customer *p;
if(q==NULL)
{
q=(customer*)malloc(sizeof(customer));
get_customer_details(q);
}
if(q->exist!=1)
{
free(q);
return;
}
if(cust_head==NULL)
{
cust_head=cust_tail=q;
return;
}
else
{
if((q->id)<=(cust_head->id))
{
q->prev=NULL;
q->next=cust_head;
cust_head->prev=q;
cust_head=q;
return;
}
if((q->id)>=(cust_tail->id))
{
q->prev=cust_tail;
cust_tail->next=q;
q->next=NULL;
cust_tail=q;
return;
}
for(p=cust_head;p;p=p->next)
if((p->id)>=(q->id)) break;
p=p->prev;
q->next=p->next;
q->prev=p;
p->next->prev=q;
p->next=q;
return;
}
}


and here's another code that could be problematic:
the function that writes it back to file:

void writefile_cust()
{
FILE *cus;
customer *d;
if(cust_head==NULL) return;
if((cus=fopen("customer.sdl","wb"))==NULL) { printf("can't write file"); getch(); return; }
for(d=cust_head;d;d=d->next)
if(d->exist==1)
fwrite(d,sizeof(customer),1,cus);
else d=d->next;
fclose(cus);
}


thanks for looking at it anyway )