Help getting this to compile

[Allan M. Bruce]

I have a small implementation of a queue which I am trying to get to compile
but cant figure out why it doesnt work. I have copied the minimum
compilable code below. On gcc I get

temp2.c: In function `pop':
temp2.c:24: warning: initialization from incompatible pointer type
temp2.c:25: warning: assignment from incompatible pointer type
temp2.c: In function `destroy':
temp2.c:38: error: dereferencing pointer to incomplete type

In line 24, I am trying to intialise e which is an (entry *) to *xiQueue
which is a *(entry **) which should be an (entry *), no?
In line 38, I am dereferencing xiQueue which should give me an (entry *) so
I can access the members of the struct with ->, no?

In Visual C I get even more errors:

queue.c(24): error C2275: 'entry' : illegal use of this type as an
expression
queue.c(24): error C2065: 'e' : undeclared identifier
queue.c(25): error C2223: left of '->nextNode' must point to struct/union
queue.c(26): error C2223: left of '->data' must point to struct/union
queue.c(27): warning C4022: 'free' : pointer mismatch for actual parameter 1
queue.c(38): error C2037: left of 'nextNode' specifies undefined
struct/union 'entry'

Perhaps I am misunderstanding how to use typedef to make the queue
effectively an entry *. Can anybody help?
Thanks
Allan



#include <stdio.h>
#include <stdlib.h>

/*****************************************/
enum ERROR_CODES {ERROR, SUCCESS};

typedef struct entry *queue;

int pop(queue *, int *);
void destroy(queue *);
/*****************************************/

typedef struct tagEntry
{
int data;
struct tagEntry *nextNode;
} entry;

int pop(queue *xiQueue, int *xoData)
{
if (xiQueue == NULL)
return ERROR;

entry *e = *xiQueue;
*xiQueue = e->nextNode;
*xoData = e->data;
free(e);

return SUCCESS;
}

void destroy(queue *xiQueue)
{
if (xiQueue == NULL)
return;

int lDummy;
while ((*xiQueue)->nextNode != NULL)
pop(xiQueue, &lDummy);
xiQueue = NULL;
}

int main(void)
{
return 0;
}

 

[Richard Bos]

In Visual C I get even more errors:

queue.c(24): error C2275: 'entry' : illegal use of this type as an
expression
queue.c(24): error C2065: 'e' : undeclared identifier
queue.c(25): error C2223: left of '->nextNode' must point to struct/union
queue.c(26): error C2223: left of '->data' must point to struct/union
queue.c(27): warning C4022: 'free' : pointer mismatch for actual parameter 1

Compiling as C++, are you?

queue.c(38): error C2037: left of 'nextNode' specifies undefined
struct/union 'entry'

#include <stdio.h>
#include <stdlib.h>

/*****************************************/
enum ERROR_CODES {ERROR, SUCCESS};

typedef struct entry *queue;

^^^^^^^^^^^^

int pop(queue *, int *);
void destroy(queue *);
/*****************************************/

typedef struct tagEntry ^^^^^^^^^^^^^^^
{
int data;
struct tagEntry *nextNode;
} entry;

^^^^^

That's your problem. There's no such thing as a struct entry. It's
either an entry, but you can't use that before it's defined; or it's a
struct tagEntry. It should be

typedef struct tagEntry *queue;

Alternatively, you could replace struct tagEntry with struct entry in
the struct definition. Structure tags live in a namespace of their own,
so struct entry and typedef entry do not clash.

Richard

 

[Allan M. Bruce]

Compiling as C++, are you?


^^^^^

That's your problem. There's no such thing as a struct entry. It's
either an entry, but you can't use that before it's defined; or it's a
struct tagEntry. It should be

typedef struct tagEntry *queue;

Alternatively, you could replace struct tagEntry with struct entry in
the struct definition. Structure tags live in a namespace of their own,
so struct entry and typedef entry do not clash.

Richard

Hehe, as easy as that - I thought it might be!
Thanks Richard
Allan

 

[Martin Ambuhl]

Perhaps I am misunderstanding how to use typedef to make the queue
effectively an entry *.

Yes, you are:

typedef struct entry *queue;

The above is wrong. 'struct entry' is meaningless. Try, if you _must_
use promiscuous typedefs,

typedef struct tagEntry *queue;

 

[Peter Shaggy Haywood]

Groovy hepcat Allan M. Bruce was jivin' on Thu, 20 Jul 2006 11:31:31
+0100 in comp.lang.c.
Help getting this to compile's a cool scene! Dig it!

In Visual C I get even more errors:

queue.c(24): error C2275: 'entry' : illegal use of this type as an
expression
queue.c(24): error C2065: 'e' : undeclared identifier
queue.c(25): error C2223: left of '->nextNode' must point to struct/union
queue.c(26): error C2223: left of '->data' must point to struct/union

These errors result from the fact that you are declaring a variable
after executing statements. Move declarations to the top of the block
in which they are declared.

--

Dig the even newer still, yet more improved, sig!

http://alphalink.com.au/~phaywood/
"Ain't I'm a dog?" - Ronny Self, Ain't I'm a Dog, written by G. Sherry & W. Walker.
I know it's not "technically correct" English; but since when was rock & roll "technically correct"?