用C语言列出。 访问冲突

Question

this is a bit tricky to explain but I'll try anyway. I'm trying to create a program that will get a list that ends with 0, and also has 0s in the middle.The program will check who is the minimum value between each zero (for example: for the list 6 -> 3 -> 15 -> 0 -> 1 ->2 ->0 , the minimum values are 3 and 1) , will delete them from the list, and insert them to another list.

For example, if list1 is 6 -> 3 -> 15 -> 0 -> 1 ->2 ->0 , then after I run the program, list1 will be 6 -> 15 -> 0 -> 2 ->0 and list2 will be 3->1 .

When I run my code, I get an error of access violation. Here's my code:

list* essay(list* anchor1)
{
    list* prev_to_min,*runner,*prev_to_runner,*result,*result_temp;
    int min;
    prev_to_min=prev_to_runner=anchor1;
    result=allocate_list();
    result_temp=result;
    runner=prev_to_runner->address_to_next;
    min=runner->number;
    while(runner!=NULL)
    {
        while(runner->number!=0)
        {
            if(min>=runner->number)
            {
                min=runner->number;
                prev_to_min=prev_to_runner;
            }
            prev_to_runner=runner;
            runner=runner->address_to_next;
        }
        remove_item(prev_to_min);
        result_temp=insert_item(result_temp,min);
        prev_to_runner=runner;
        runner=runner->address_to_next;
        if(runner!=NULL)
            min=runner->number;
    }
    return result;
}

A small explanation since there are so many variables around: result is the pointer to the anchor of list2 (the list of minimums), result_temp is the pointer to the current last item of list2, runner is the pointer which im using to iterate over list1, prev_to_runner is what points to the item before runner in the list, and prev_to_min is what points to the item before a minimum in list1. for example 6 -> 3 -> 15 -> 0 , 3 is the minimum, so prev_to_min is the address of 6.

I tried to run it with a piece of paper, run the program in my head, and I get the needed result. but when I compile it and the computer runs it, I get the error "Unhandled exception at 0x5557700c (msvcr100d.dll) in more lists.exe: 0xC0000005: Access violation reading location 0xfffffffc."

This is the code for inserting and item and deleting an item:

void remove_item(list* prev_position)
{
    list* deleted;
    deleted=prev_position->address_to_next;
    prev_position->address_to_next=deleted->address_to_next;
    free(*deleted);
}

list* insert_item(list* position,listdata x)
{
    list* temp=(list*)malloc(sizeof(list));
    temp->number=x;
    temp->address_to_next=position->address_to_next;
    position->address_to_next=temp;
    return temp;
}

Answer 1

Access violation reading location 0xfffffffc. That's a pretty good clue that you had a NULL pointer that you backuped up by the size of a 32 bit integer and then tried to read it.

If you run in the debugger, it will tell you where.