Access data stored in struct within Doubly Linked List?

Question

I'm not sure how specific I have to be but I'll give the breakdown best I can. I'm taking a typedef struct:

typedef struct {
  char name[21];
  int  life;
} pcb_t;

inputting values for the name & life, then storing it in a doubly linked-list.

the linked-list structs in the header file are:

typedef struct list_node {
  void             *data;
  struct list_node *next;
  struct list_node *prev;
} List_node_t;

typedef struct {
  List_node_t *head;
  List_node_t *tail;
} List_t;

In my main I have the first struct variables initialized as:

  char   name[BUF_MAX];
  int    life;
  pcb_t  *pcb;

The input is all correct and the pcb struct is stored as a new node in the list. I tried to run a simple loop after the initial input to print out the Name & Lifetime values for each of the pcb structs. The loop I used is this:

void *context = NULL;
void *data;
while( List_next_node( &the_list, &context, &data) && (data != NULL))
  {

     printf("Name: %s\n", (char *)data);
     printf("Lifetime: %d\n", (int )data); 
   }

Where the List_next_node function transverses the list. the_list is the list, context is what keeps track of where we are in the list, and data is the data.

I'm not sure how to access the information I want as my while loop correctly prints out the Name of the pcb struct, but the lifetime is not.

Crowman · Accepted Answer · 2014-05-22 00:11:26Z

1

Lists, doubly linked or otherwise, are a complete red herring, here. The issues are (1) accessing struct members, which hopefully is trivial; and (2) doing that when all you have is a void *, which is possibly a little less intuitive.

This is the simplest way:

void *data;
while( List_next_node( &the_list, &context, &data) && (data != NULL))
{
    pcb_t * current_data = data;
    printf("Name: %s\n", current_data->name);
    printf("Lifetime: %d\n", current_data->life); 
}

The only reason your current code "works" for printing out the name is because name is the first element of your struct, and so the address of name happens to be the same as the address of the whole struct, so when you cast the address of the struct to char * you get the result you're expecting, even though you're not really getting there the right way.

As ojblass's answer shows, you can do it with a cast and avoid the use of a temporary variable, but I think a temporary variable makes things a lot clearer.

answered May 22, 2014 at 0:11

Crowman

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3 Comments

Crowman Over a year ago

You may have caught it before I just changed data to current_data. No cast is required in the current answer.

Caulay Over a year ago

Thanks for the insight. I knew that the printing of the name wasn't actually the right way to go about things, but left it while I looked into the second element. I had tried something similar to ojblass' solution and decided to come here when my attempt didn't work - not even thinking of a temp variable. I'm relatively new to C and haven't gotten the pointers down. Cheers

Crowman Over a year ago

Don't worry, you'll get it in the end if you keep chipping away at it. It can take a while for some of this stuff to sink in, but once it does, your mental model of what's happening will start matching up with what actually is happening, and the confusion will vanish. Pointers are actually pretty simple once you manage to get past the stumbling block of not being confident of what they're really doing.

ojblass · Accepted Answer · 2014-05-22 00:10:57Z

0

printf("Lifetime: %d\n", ( (pcb_t *) data) ->life);

edited May 22, 2014 at 0:10

answered May 22, 2014 at 0:02

ojblass

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2 Comments

Caulay Over a year ago

;When I do this, I get two errors: "expected expression before 'pcb_t'" and "expected ')' before 'data'"

Crowman Over a year ago

It should be ((pcb_t *) data)->life.

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