Re: Help analyzing a linked list program

aijazbaig1 · #21 01-Aug-2007, 08:09

Hello People. (PLS SEE THIS ONE BEFORE THE ONE ABOVE)
I made an incredible blunder in the post above. I forgot to precede List with the keyword struct. So now i have made some changes to the code above such that I have now declared a variable called mylist which is a structure of type List and declared another pointer variable called mylist_ptr which is of type pointer to List and made it point to the address of mylist
Heres the C equivalent:

CPP / C++ / C Code:

    struct List mylist;
    struct List *mylist_ptr;
    struct Listelmt *tmp_idx;

    mylist_ptr = &mylist;

And replaced all occurences of mylist in the function calls and the subsequent text to mylist_ptr. Doing that i get the following set of error messages:

Code:

gcc -std=c99 -Wall -W -pedantic -c list.c -o list.o
gcc -std=c99 -Wall -W -pedantic -c testlist.c -o testlist.o
testlist.c: In function `main':
testlist.c:7: error: storage size of 'mylist' isn't known
testlist.c:33: error: request for member `size' in something not a structure or union
testlist.c:34: warning: passing arg 1 of `list_ins_nxt' from incompatible pointer type
testlist.c:40: error: request for member `size' in something not a structure or union
testlist.c:45: error: request for member `head' in something not a structure or union
testlist.c:47: error: request for member `next' in something not a structure or union
testlist.c:48: warning: passing arg 1 of `list_ins_nxt' from incompatible pointer type
testlist.c:48: warning: passing arg 2 of `list_ins_nxt' from incompatible pointer type
testlist.c:54: warning: passing arg 1 of `list_destroy' from incompatible pointer type
testlist.c:59: error: request for member `size' in something not a structure or union
testlist.c:65: error: request for member `size' in something not a structure or union
testlist.c:72: error: request for member `head' in something not a structure or union
testlist.c:74: error: request for member `next' in something not a structure or union
testlist.c:75: warning: passing arg 1 of `list_rem_nxt' from incompatible pointer type
testlist.c:75: warning: passing arg 2 of `list_rem_nxt' from incompatible pointer type
testlist.c:7: warning: unused variable `mylist'
make: *** [testlist.o] Error 1

I do not understand why it says that storage size of 'mylist' isn't known where as I have explicitly defined a struct by the name of mylist. If i remove this line and directly define a pointer to a struct of type List, this error disappears. Doesn't doing an explicit variable declaration automatically allocate space for the given variable?

And I don't get as to why it still says that mylist_ptr is NOT pointing to a variable of type structure or union.

Keen to hear from you guys

davekw7x · #22 01-Aug-2007, 10:05

Quote:

Originally Posted by aijazbaig1

CPP / C++ / C Code:

.
.
.
    List *mylist;
 .
.
.
                if (mylist.size == 0)

The most obvious question is ...

From a syntax point of view, mylist is a pointer, not a struct, so the compiler would accept mylist->size. It is telling you that you can't use mylist.size since mylist is not a struct.

However...
From a functionality point of view, since mylist is an unitialized pointer (its value is not the address of a List struct), dereferencing it would be undefined behavior. (So don't do it.)

Why not just declare

CPP / C++ / C Code:

    List mylist;

Now, the mylist.size refers to the element of the struct that you have declared.

However... All of the functions that take a pointer to struct as an argument will require you to give them the address of mylist:

CPP / C++ / C Code:

..
    list_init(&mylist, deallocatelist);
.
.
                list_ins_nxt(&mylist, NULL, &mydata);
.
.
.
                list_ins_nxt(&mylist, tmp_idx, &mydata);
.

Also note that you can't have a "void" data type. Period. Full Stop.

There may be a few other compiler errors to track down and destroy.

Regards,

Dave

davekw7x · #23 01-Aug-2007, 10:19

Quote:

Originally Posted by aijazbaig1

I made an incredible blunder in the post above. I forgot to precede List with the keyword struct

But you had a typedef for List in your "list.h", right? So if you hadn't changed anything else or forgotten some things or something else for which I don't have a clue, it has to work just fine with declarations like the following:

CPP / C++ / C Code:

/* in mylist.h */
.
.

typedef struct Listelmt_ {
    void *data;
    struct Listelmt_ *next;
} Listelmt;

typedef struct List_ {
    int size;
    int (*match) (const void *key1, const void *key2);
    void (*destroy) (void *data);

    Listelmt *head;
    Listelmt *tail;
} List;
.
.
.

CPP / C++ / C Code:

/*in testlist.c */
.
.
#include "mylist.h"
.
.
.
    List mylist;                /* the typedef tells what kind of variable mylist is: a struct */
    List *mylist_ptr = &mylist; /* since it knows what List is, it knows what "List *" is      */
.
.
.
    list_init(mylist_ptr, deallocatelist);
.
.
.
            if (mylist.size == 0)
.
.
.

On the other hand, when you make the typedef for List, then you would not (not) make a declaration like]

CPP / C++ / C Code:

struct List mylist; /* this is an error for your typedef for List */

If you really like to type "struct" a lot, then you could use it with the struct tag:

CPP / C++ / C Code:

struct List_ mylist; /* legal, but why would you do it when you have a typedef */

Regards,

Dave

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