structs help

[Dave Cooke]

Hi I am very new to C. I am trying to figure out how to initialize
a struct in my main program.
The struct is declared in anouther header file like this...
typedef struct ln {
int key;
int data;
struct ln *next;
} listNode, *listNodePtr;

just to test in my main method I tried to initialize the
listNodePtr variable to null like this...

listNodePtr *head = NULL;

First as far as I understand listNode and *listNodePtr are
variables of type ln. However I have also learned, so I thought,
that listNodePtr is an alias to the struct ln. So my statement
above should be legal. Is It?

When I try to pass my new listNodePtr variable *head to my
listInsert method like........

listInsert(head ,1,2);

it doesn't work. It builds but at run time my program hangs
when inside the listInsert method the first if condition
tries to determine of the listNodePtr *list is null or not.
i.e. if( *list == NULL )..... this is where the program quits.

The method definition looks like...

void listInsert(list NodePtr *list, int key, int value)

I am assuming the body of the method is correct as this code
was given to us to use by my Prof.

Please help what am I doing wrong during my initialization?
How do I use structs defined like the one above?

Thanks in advance.

[Al Bowers]

Barry Schwarz wrote:

On Sat, 03 Apr 2004 22:36:05 -0600, Dave Cooke
<dc****@ee.uman itoba.ca> wrote:

Hi I am very new to C. I am trying to figure out how to initialize
a struct in my main program.
The struct is declared in anouther header file like this...
typedef struct ln {
int key;
int data;
struct ln *next;
} listNode, *listNodePtr;

just to test in my main method I tried to initialize the

C does not have methods. It has functions.

listNodePtr variable to null like this...

listNodePtr *head = NULL;
I suspect the OP wants:
listNodePtr head = NULL;

When I try to pass my new listNodePtr variable *head to my
listInsert method like........

listInsert(he ad,1,2);
The call:
listInsert(&hea d,1,2);

The method definition looks like...

void listInsert(list NodePtr *list, int key, int value)

Since C passes by value, how do you expect the results of this
function to be available to the calling function. Anything you do to
list will disappear as soon as listInsert returns.

Actually this prototype looks correct. It can modify a variable
in the calling function.
For example in the following, the value of head will be modified
should the function listInsert successfully allocate storage.

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

typedef struct ln {
int key;
int data;
struct ln *next;
} listNode, *listNodePtr;

void listInsert( listNodePtr *p, int key, int data);

int main(void)
{
listNodePtr head = NULL;

printf("In Main: Before call to listInsert function head"
" is %sNULL\n", head?"not ":"");
listInsert(&hea d,1,31);
printf("In Main: After call to listInsert function head"
" is %sNULL\n", head?"not ":"");
free(head);
return 0;
}
void listInsert( listNodePtr *p, int key, int data)
{
listNodePtr new;

new = malloc(sizeof *new);
if(new != NULL)
{
new->key = key;
new->data = data;
new->next = *p;
*p = new;
}
return;
}

--
Al Bowers
Tampa, Fl USA
mailto: xa******@myrapi dsys.com (remove the x to send email)
http://www.geocities.com/abowers822/

[Barry Schwarz]

On Sun, 04 Apr 2004 21:00:09 -0400, Al Bowers <xa******@rapid sys.com>
wrote:

Barry Schwarz wrote:

On Sat, 03 Apr 2004 22:36:05 -0600, Dave Cooke
<dc****@ee.uman itoba.ca> wrote:

Hi I am very new to C. I am trying to figure out how to initialize
a struct in my main program.
The struct is declared in anouther header file like this...
typedef struct ln {
int key;
int data;
struct ln *next;
} listNode, *listNodePtr;

just to test in my main method I tried to initialize the

C does not have methods. It has functions.

listNodePt r variable to null like this...

listNodePt r *head = NULL;
I suspect the OP wants:
listNodePtr head = NULL;

When I try to pass my new listNodePtr variable *head to my
listInsert method like........

listInsert(h ead,1,2);
The call:
listInsert(&he ad,1,2);

The method definition looks like...

void listInsert(list NodePtr *list, int key, int value)

Since C passes by value, how do you expect the results of this
function to be available to the calling function. Anything you do to
list will disappear as soon as listInsert returns.

Actually this prototype looks correct. It can modify a variable
in the calling function.

Of course it can but only if the argument is of the form &variable or
equivalent. The OP's argument wasn't and therefore it couldn't. All
of which was covered in the subsequent paragraphs of my response which
you chose to omit.
<<Remove the del for email>>

[Barry Schwarz]

On Sun, 04 Apr 2004 21:00:09 -0400, Al Bowers <xa******@rapid sys.com>
wrote:

Barry Schwarz wrote:

On Sat, 03 Apr 2004 22:36:05 -0600, Dave Cooke
<dc****@ee.uman itoba.ca> wrote:

Hi I am very new to C. I am trying to figure out how to initialize
a struct in my main program.
The struct is declared in anouther header file like this...
typedef struct ln {
int key;
int data;
struct ln *next;
} listNode, *listNodePtr;

just to test in my main method I tried to initialize the

C does not have methods. It has functions.

listNodePt r variable to null like this...

listNodePt r *head = NULL;
I suspect the OP wants:
listNodePtr head = NULL;

When I try to pass my new listNodePtr variable *head to my
listInsert method like........

listInsert(h ead,1,2);
The call:
listInsert(&he ad,1,2);

The method definition looks like...

void listInsert(list NodePtr *list, int key, int value)

Since C passes by value, how do you expect the results of this
function to be available to the calling function. Anything you do to
list will disappear as soon as listInsert returns.

Actually this prototype looks correct. It can modify a variable
in the calling function.

Of course it can but only if the argument is of the form &variable or
equivalent. The OP's argument wasn't and therefore it couldn't. All
of which was covered in the subsequent paragraphs of my response which
you chose to omit.
<<Remove the del for email>>