Another C pointers question from a newbie

chris.hicks · 11-10-2003, 05:02 AM

I have now completed a small example that uses a double linked list to place events in a list in the correct order. The program works fine. However, when I compile the program I get some warnings and I don't know why. (I am using gcc 3.2.1 on RH9, the warnings are listed after my code).

My program is:
/*look at the use of structures and pointers to produce a
double linked list that lists events in time order*/
#include <stdio.h>
#include <malloc.h>
#include <string.h>

int main (void)
{
int cont,time;
typedef struct
{
int time;
struct event *fptr,*bptr;
} event;

event *baseptr,*newptr,*tempptr,*tf,*tb;

/* create the base element for the queue */
baseptr = (event *) malloc (sizeof (event));

/* set up the base of the queue to point to itself with time = 0 */
baseptr->time = 0;
baseptr->fptr = baseptr;
baseptr->bptr = baseptr;

/*enter a loop to collect events */
time = 999;
while (time > 0)
{
printf("Enter the time for the next event (t = 0 for quit) ");
scanf("%i",&time);

if (time == 0) printf("quiting\n"); else
{

/*create a new pointer */
newptr = (event *) malloc (sizeof (event));
newptr->time = time;
newptr->bptr = newptr; /* initialise */
newptr->fptr = newptr;

/* place the event into the double linked list in the correct place */
tempptr = baseptr; /*start off at the root */
cont = 1;
do /* loop to work back until correct entry point found */
{
tempptr = tempptr->bptr;
if (newptr->time > tempptr->time) cont = 0;
if (tempptr == baseptr) cont = 0;
} while (cont == 1);

/* rearrange the pointers to enter the new event */
newptr->fptr = tempptr->fptr;
newptr->bptr = tempptr;
tb = tempptr->fptr;
tb->bptr = newptr;
tempptr->fptr = newptr;
}
} /* end of entering new events */

/* now we will write the events out in order */
tempptr = baseptr->bptr;
printf("Event pointer %p time = %i\n",tempptr,tempptr->time);
while (tempptr != baseptr)
{
tempptr = tempptr->bptr;
printf("Event pointer %p time = %i\n",tempptr,tempptr->time);
}
}

The warning messages are:
[nmansys@mmme1 c_examples]$ gcc doublelinkedlist.c
doublelinkedlist.c: In function `main':
doublelinkedlist.c:24: warning: assignment from incompatible pointer type
doublelinkedlist.c:25: warning: assignment from incompatible pointer type
doublelinkedlist.c:40: warning: assignment from incompatible pointer type
doublelinkedlist.c:41: warning: assignment from incompatible pointer type
doublelinkedlist.c:48: warning: assignment from incompatible pointer type
doublelinkedlist.c:55: warning: assignment from incompatible pointer type
doublelinkedlist.c:56: warning: assignment from incompatible pointer type
doublelinkedlist.c:57: warning: assignment from incompatible pointer type
doublelinkedlist.c:58: warning: assignment from incompatible pointer type
doublelinkedlist.c:63: warning: assignment from incompatible pointer type
doublelinkedlist.c:67: warning: assignment from incompatible pointer type

I can't see anything wrong with my code. Has anybody got any ideas please?

Best regards,
Chris

Quis · 11-10-2003, 06:35 AM

You should try to link your pointers, like this:

24: baseptr->fptr = baseptr->bptr; //forwarptr=forwardptr or forwardptr=backwardptr
25: baseptr->bptr = baseptr->fptr; // see above (make a loop)
..
40: and 41: try the same

..

chris.hicks · 11-10-2003, 06:52 AM

Thanks for the response. I'm not sure I understand the suggestion. If I print out baseptr, baseptr->fptr and baseptr->bptr they are all have the same memory address, which is what I am wanting. On these lines the pointers are of the same type, namely event, so why is the compiler producing the warning "warning: assignment from incompatible pointer type"?

Any further help would be very gratefully received. Thanks. Chris.

Quis · 11-10-2003, 07:33 AM

ok, forget my post, i tried to debug the program with my suggestion, the warnings are gone, but the address also.

Sorry, for sure this is not what a linked list should do.

MartinN · 11-10-2003, 08:01 AM

Your declaration of the event type should look something like this:

typedef struct eventnode *eventptr;

typedef struct eventnode
{
int time;
eventptr fptr,bptr;
} event;

Regards
Martin

chris.hicks · 11-11-2003, 03:12 AM

I have checked with a book "C a Software Engineering Approach", Darnell and Margolis (1996) and think that my definition of the type event is OK. The program also works fine. However, I am clearly missing something, because my next example to process trees is also coming up with similar warnings. (I have raised this issue in another thread).

mr_segfault · 11-11-2003, 05:14 AM

Ok I'm a C++ programmer and am not 100% sure this is correct since it has been many years since I have done any C programming (I know, I know, lame excuse

) So correct me if I'm wrong.

I think that the code:

Code:

typedef struct
{
  int time;
  struct event *fptr,*bptr; 
} event;

only creates an instance (called event) of an unnamed struct.

The syntax (from memory) for definining a struct:

Code:

typedef struct <typename>
{
// body
} <instance>;

so to create a new type called myNewType but no instances, you would use the syntax:

Code:

typedef struct myNewType
{
  int uselessStructBody;
};

Now to create an instance of this use:

Code:

myNewType myInstance;

The previous two code segments are equivalent to:

Code:

typedef struct myNewType
{
  int uselessStructBody;
} myInstance;

note: Please learn to use the [C0DE] tags.. as it makes it soooooo much easier to read your code..

Cheers..