Accessing attributes of structures stored in linked lists - c

Question

I'm trying to find a specific location in a linked list and then be able to access its attributes. I know how to sort through the linked list but I can not figure out how to access the name attribute of the Locations.
I define my Location * structure as (These locations are stored into the list later):
#ifndef NESW_STRUCT
#define NESW_STRUCT
typedef struct location{
char *name;
char *longer;
char *shorter;
char *tip;
char *north;
char *south;
char *east;
char *west;
char *logic;
int visited;
char *items[20];
} Location;
#endif
My instructor provides us with a module to create a linked list as well as various functions to manipulate the list. The linked list is comprised of Node * which I believe hold the Locations as well as point to the next node in the list.
typedef struct node
{
Location *loc;
struct node *next;
} Node;
So in my game loop I create a global variable 'world' that is my linked list of (I think) Locations:
Node *world;
and
extern Node* world;
In other modules that also access it.
I then run a simple while loop in my main that creates a Location structure and then joins it to the Linked list(excluded from this post), world, using join(location,world) with the following functions my instructor provided, modified by me to work with Locations rather than void objects. I don't initialize world to anything before joining the first location to it, I think I may need to, but since its a core dump and crashes either way, I can't tell if it makes a difference/is necessary:
Node *
newNode(Location *place,Node *next)
{
Node *n = malloc(sizeof(Node));
if (n == 0)
{
fprintf(stderr,"newNode: out of memory!\n");
exit(1);
}
n->loc = place;
n->next = next;
return n;
}
Node *
join(Location *s,Node *rest)
{
return newNode(s,rest);
}
This all works perfectly fine so far and I create my list successfully. However, elsewhere in my program I created a function that maps through the world list and find the location that has a matching name to whichever name I pass to the function, which, logically, works correctly . I created a temp list that is equal to 'world', and then compared the name attribute of the head of the list to the name of the location I was looking for using, strcmp, returning that location if it matches, and setting the list = to the tail of the list if it doesn't.
Head and Tail are defined here, again provided in the module from my instructor:
Location *
head(Node *items)
{
return items->loc;
}
Node *
tail(Node *items)
{
return items->next;
}
If I understand these functions correctly, using head(list) should return a Location right, not a pointer? Then I should be able to just use 'location->name' to access the name? Apparently not though...
To save time of running through all the game logic just get to the part where it needs to compare the names, I tried writing some temporary code similar to how it would be in the mapping function, to test getting a location from the list and then accessing the attributes.
The probably wrong code I'm using to try and test accessing the list is:
Location *test = 0; //creating an empty location, (not sure if it needs to be initialized to 0 before assigning the desired value but I think I remember a mention of that during class)
test = head(world); //I would like to believe this sets test equal to the location of the head of the list world, but I am fairly certain this is where my error occurs because what is getting assigned to test really isn't a location
printf("%s",test->name); //basic print of the name attribute, I know this works logically because I use it elsewhere when dealing with locations not accessed through world, however this is what causes the core dump because I think I'm trying to access a garbage value so to speak
The program compiles with no errors and successfully reads all the the locations based on a debugging print statement I added. Any help, advice, or tips are greatly welcomed. I know people hate on kids that post here because they think they are trying to get their work done for free, but this is a very small part of an immersive project and once I figure this out, the game is essentially done other than content. I'm at a minor roadblock that is a major inhibitor and have tried everything my friend and I could think of and have even just started changing random data types in the Node struct and join/newNode functions as well as the Location struct hoping to either get lucky or figure out a solution through different error messages that occurred but as you can guess, no luck.

Answer 1

Solution by OP.
Because each new item is joined to the front of the list, the very last Node read into (which is the first node read when going through the list because the list is filled backwards) is the null pointer.
Example: Because the join function used stores a pointer to a Location and then points to the next Node in the list, the list is reversed after reading in.
Say you want to read alpha, bravo, charlie, delta epsilon into the list and you read them in that order, the list in memory looks like this with null being the head in the list:
null<-epsilon<-delta<-charlie<-bravo<-alpha
Therefore when I was trying to print with
Node *spot = world;
Location *loc = 0;
loc = head(spot);
printf("%s",loc->name);
I was attempting the print the name for the location which was really just a null pointer and obviously doesn't exist...So a very easy fix was to set spot equal to the tail before actually working with any of the Nodes in the list.
Node *spot = world;
Location *loc = 0;
spot = tail(spot);
loc = head(spot);
printf("%s",loc->name);