Delete entry from a list in ISO C89 - list

Question

I have to develop a library function for a phonebook which uses lists.
This function has to delete the nth entry from the list.
It is a linked list,and the entries are structs which contain the strings name,srnm,nmbr,addr,the int number and the next pointer.
Every time I call the function,however,VS gives me lot of exceptions and triggered breakpoints,and says I've corrupted the heap.
I have no idea of where I could have made this mistake that corrupts the heap.Please Help.
Here's what I've done so far :
typedef struct list {
char name[20];
char srnm[20];
char nmbr[20];
char addr[20];
unsigned int number;/*Contact index*/
struct list *next;
} entry;
static unsigned int count = 0;
static entry *hol = NULL;/*pointer to the head of the list*/
int delete_contact(unsigned int n) {
int i=0,k=0;
if(n>count) return -1;
hol[n-1].next = hol[n-1].next->next;
for(i=n;i<count;i++) {
hol[i]=hol[i+1];
}
for(i=n;i<count;i++)
hol[i].number = hol[i].number-1; /*Updates the contact index*/
count--; /*Updates the global variable that gives the number of contacts */
return 0;
}

Answer 1

Suggested solution
hol is a pointer, so, hol[n-1] etc. might not refer to a entry struct in memory, consider when n = 0. Either way, this is not how you should access different entries in your list structure.
For a singly-linked list you have to consider 3 cases,
If n = 0 (i.e. the first entry) is being deleted
If the entry to be deleted is somewhere inside the list
If the entry is the last entry in the list.
It is my understanding that number is the index of the list entry and that you've used dynamic memory allocation for the list entries. Furthermore, since you appear to be using the C89 standard (which does not allow loop variable initialization) I have adapted the code below as best as I could to C89, but I do not use that standard myself:
int delete_contact(unsigned int n) {
int i = 0;
int k = 0;
// Check if the index refers to an element not included in the list
if (n > count) { return -1; }
entry* tmp = hol;
if (n == count - 1) {
// Iterate to the last entry
while (tmp->next) {
tmp = tmp->next;
}
free(tmp); // Free the list entry
// Decrement the global counter keeping track of the
// amount of list elements
count--;
// No need to edit the contact indexes
return;
}
entry* remaining_list = hol;
if (n == 0) {
// Free the head of the list
hol = hol->next;
remaining_list = hol;
free(tmp); // tmp points to the head of the list already
} else {
// The list entry is somewhere inside the list
int idx = 0;
// Iterate until tmp points to the n-1:th entry
while (idx < n - 1) {
tmp = tmp->next;
idx++;
}
entry *to_be_freed = tmp->next; // n:th entry
tmp->next = tmp->next->next;
remaining_list = tmp->next;
free(to_be_freed);
}
// Decrement the contact index on all the remaining entries
while (remaining_list) {
remaining_list->number--;
remaining_list = remaining_list->next;
}
// Decrement the global counter keeping track of the
// amount of list elements
count--;
return 0;
}
Tips
You'd be better served by creating a more expressive list interface, possibly one that isn't bound to the values stored inside it. That way you could create a
list_size()
function and remove the global counter.
Here is a little something to get you started:
typedef struct node {
void* value;
struct node* next;
} node;
typedef struct list {
node* head;
node* tail;
size_t len;
} list;
Reference material
Linked list basics
Linked list exercises