Crazy Linked List Problem

MAX_POWER · #1 06-Nov-2004, 14:38

Hello :-o
I am new to this forum and this is my first post so excuse me for any errors in my posting. I have been learning c for a few months now and in the 3rd or so rewrite of my first program. I am starting to get into doing more with structs and linked lists and using pointers to hopefully limit the amount of memory used since not all of them need everything. I am at a point where I need to add a struct that hasn't been defined at the point where I need it because it has to be below since it uses the other structs. I have tried to use a void pointer to the struct as a member but it doesn't work and I even tried to type cast it but that didn't help anything other than adding more errors. I'm not sure if I was doing it right since my book I'm learning from never did a cast to a struct so I thought it may have just been outside the scope of the book.

My question is:

Is it possible to have a void pointer to a struct or union so I don't have to nest them all in the base linked list so I'm not wasting memory and if not is there a way to do what I'm trying to do?

ps.
Keep in mind that I don't know much yet and my terminology is kinda weak so try not to confuse me past the point I am at now.

A BIG THANX in advance for any help you can provide.

dsmith · #2 06-Nov-2004, 14:52

Hello MAX_POWER. Welcome to GIDForums™. Did you get your name off a hair dryer

I don't follow your question exactly. Do you have any code snippet that you could post that would explain what you mean?

If you post code, keep in mind to use the c-code posting tags

MAX_POWER · #3 06-Nov-2004, 15:16

hehehe
nope
got the name from a simpsons episode ;-)
my code is kinda messed up so I would have to do some tweaking since I only wrote it for testing it out and not for use in the program I needed it in since that program has a long way before I can compile it...
I'll go and tweak my code and post it

davekw7x · #4 06-Nov-2004, 15:19

Quote:

Originally Posted by MAX_POWER

Hello :-o
I am new to this forum and this is my first post so excuse me for any errors in my posting. I have been learning c for a few months now and in the 3rd or so rewrite of my first program. I am starting to get into doing more with structs and linked lists and using pointers to hopefully limit the amount of memory used since not all of them need everything. I am at a point where I need to add a struct that hasn't been defined at the point where I need it because it has to be below since it uses the other structs. I have tried to use a void pointer to the struct as a member but it doesn't work and I even tried to type cast it but that didn't help anything other than adding more errors. I'm not sure if I was doing it right since my book I'm learning from never did a cast to a struct so I thought it may have just been outside the scope of the book.

My question is:

Is it possible to have a void pointer to a struct or union so I don't have to nest them all in the base linked list so I'm not wasting memory and if not is there a way to do what I'm trying to do?

ps.
Keep in mind that I don't know much yet and my terminology is kinda weak so try not to confuse me past the point I am at now.

A BIG THANX in advance for any help you can provide.

Instead of boring you with termonilogy, here's an example with a couple of things you might be interested in.

The bottom line is that you can refer to a pointer to a struct even before the struct has been defined. You can have a struct with members that are pointers to the type of struct being defined. That's how linked lists are implemented.

Have fun with this:

CPP / C++ / C Code:

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

struct tnode {
  char *word;
  int count;
  struct tnode *left;
  struct tnode *right;
};

struct t {
  int i1;
  int i2;
  struct s *p;
};

struct s {
  int xx;
  struct t *tpointer;
  struct tnode *tnode_pt;
};

int main()
{

  struct tnode st1;
  struct t     stt;
  struct s     sts;
  struct tnode tar[10];

  st1.word = malloc(100);
  if (st1.word == NULL) {
    printf("Can't allocate memory\n");
    return 0;
  }
  else {
    printf("Allocated memory\n");
  }
  strcpy(st1.word, "Hello, world!");
  printf("Here's the scoop: <%s>\n", st1.word);

  sts.tnode_pt = &st1;

  printf("Here's some more of the same:<%s>\n", sts.tnode_pt->word);
  
  free(st1.word);

  return 0;
}

Any questions? (This stuff is kind of fun when it works.)

Regards,

Dave

MAX_POWER · #5 06-Nov-2004, 15:35

thanx for that info dave
I'm going to try making it work using that info before I try to fix my unfixable test code and post it
but if I understand it properly as long as it is a pointer to the struct then it doesn't need to be defined at the time the pointer to it is used in another struct
but that is if I understand it properly

davekw7x · #6 06-Nov-2004, 15:40

Quote:

Originally Posted by MAX_POWER

thanx for that info dave
... if I understand it properly as long as it is a pointer to the struct then it doesn't need to be defined at the time the pointer to it is used in another struct
but that is if I understand it properly

I think you've got it! (Lots of people do this for a lot longer than you've been at it and never seem to get it.)

Enjoy!

Regards,

Dave

MAX_POWER · #7 06-Nov-2004, 17:07

I would just like to say THANX again for the help
I rewrote the test code and it worked...
at least after I fixed all my other errors ;-)
at least now I know where to go if I have any other questions
and thanks for the quickness of the help
I been sitting on my code(not literally) for weeks trying to think of a way to get past my problem
all I can say is thanks(but not enough) and woohoo

I can now continue with the real code

and a big
THANK YOU
to dave for setting me straight

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