C struct / pointer confusion

Hello,
I'm having a lot of trouble implementing a linked-list style stack in C. I am getting the following compiler error:

Code:

stack.c:28: error: request for member "data" in something not a structure or union.

I know what you're thinking... he used a '.' when he should have used a '->'. I don't think that this is my problem in this case, as I have attempted to compile with every combination in a "stab in the dark" debugging style (haha). Here is the code:

Code:

char* pop (stack_t *stack) {
...
node *back;
back = stack->back;
...
char *return_val;
return_val = back->data; //the line causing the error
...
}

Here is the defenition for stack_t and node:

Code:

typedef struct node *node; //so I can use self referencing structure members

struct node {
node *next;
node *prev;
char *data;
};

typedef struct  {
node* front;
node*back;
int size;
} stack_t;

Can anyone help me out?

Originally Posted by ChipsHandon

Hello,
I'm having a lot of trouble implementing a linked-list style stack in C. I am getting the following compiler error:

Code:

stack.c:28: error: request for member "data" in something not a structure or union.

I know what you're thinking... he used a '.' when he should have used a '->'. I don't think that this is my problem in this case, as I have attempted to compile with every combination in a "stab in the dark" debugging style (haha). Here is the code:

Code:

char* pop (stack_t *stack) {
...
struct node *back;
back = stack->back;
...
char *return_val;
return_val = back->data; //the line causing the error
...
}

Here is the defenition for stack_t and node:

Code:

typedef struct node *node; //so I can use self referencing structure members

struct node {
struct node *next;
struct node *prev;
char *data;
};

typedef struct  {
struct node* front;
struct node*back;
int size;
} stack_t;

Can anyone help me out?

"node" is not a type. typedef like you did for "stack_t" or prefix with "struct ".

Originally Posted by zacs7

"node" is not a type. typedef like you did for "stack_t" or prefix with "struct ".

So the problem is in my definition of the node structure? I had a lot of trouble getting it to compile the node structure with a pointer to a node inside of it. What am I doing wrong?

Edit:

Code:

typedef struct {
  node *next;  //compiler error on this line: expected specifier qualifier list before 'node'
  node *prev;
  char *data;
} node;

This gives me a compiler error. I am so confused

Originally Posted by zacs7

"node" is not a type. typedef like you did for "stack_t" or prefix with "struct ".

Sure it is. @Chips: ignore zac

This is the problem:

Code:

typedef struct node *node; //so I can use self referencing structure members
[....]
node *back;

node is defined as pointer already, so node *back is equivalent to a pointer to a struct node pointer:

Code:

struct node **ptp;

You have three options:

1) change the typedef.
2) dereference back appropriately:

Code:

return_val =  (*back)->data;

3) don't declare back as a pointer to a pointer type:

Code:

node back;

I'd try the third one first but give some thought to the first one.

Code:

typedef struct _node node;
struct _node
{
  node *next;  
  node *prev;
  char *data;
};

Originally Posted by vart

Code:

typedef struct _node node;
struct _node
{
  node *next;  
  node *prev;
  char *data;
};

Any identifier beginning with a single underscore is reserved for use by the implementation (for identifiers with file scope, IIRC).

In other words, user code should not have identifiers that begin with a single underscore.