help ,tree's

Code:

struct node
{
  char  *string;
  struct node *left;
  struct node *right;
};

struct node *leaf;
main(){
leaf = (struct node *)malloc( sizeof( struct node ) );

FILE *in=fopen("c:\\str6.txt","r");
char str[40];
fscanf(in,"%s",str);
leaf->string=str;
fscanf(in,"%s",str);
(leaf->left)=(struct node *)malloc( sizeof( struct node ) );
((leaf->left)->string)=str;
printf("\n\n");


printf("%s",(leaf->string));
printf("%s",((leaf->left)->string));

}

the first two strings in the file are "qwert" ,"asdfg"


but the output is showing as
asdfgasdfg

when i want
qwertasdfg


whats wrong with the code?,i am just learning tree's.

> leaf = (struct node *)malloc( sizeof( struct node ) );
Don't cast malloc in C, see the FAQ
Also, you should begin with an empty tree with something like

struct node *tree = NULL;

> ((leaf->left)->string)=str;
You're just assigning one pointer to another here. If you update str, then all the nodes which point at str also change as well.

You should really have a separate function, which you would call with
tree = addNode ( tree, str );

addNode then does
- allocates a new node
- allocates space for a copy of str
- finds the correct place to insert the node into the tree
- returns the (possibly changed) tree

Code:

struct node 
{
  char  string[20];
  struct node *left;
  struct node *right;
}
 addnode(struct node **tree,char *str){
struct node *temp;
temp=(struct node *)malloc( sizeof( struct node ) );
memcpy (temp->string,str,strlen(str)+1);
if(*tree==NULL){
(*tree=temp);
}
 else { (((*tree)->left)=temp);(*tree=temp);}
}






main(){


struct node root ;
struct node *tree=NULL;
struct node **tree_p;
tree_p= &tree;
FILE *in=fopen("c:\\str6.txt","r");
char *str=(char *)malloc(30);
for(int inc=1;inc<=10;inc++){
fscanf(in,"%s",str);
addnode(tree_p,str,start);
}
}

addnode function updates tree to point to the latest position.
how do i get to the root now?
please advise me over this,i am just learning tree's and i have very little experience on how to creat functions for handling tree's.
this tree is being used as link list,that is why only adding to the left.

> temp=(struct node *)malloc( sizeof( struct node ) );
OK, did you read the FAQ?

> addnode(tree_p,str,start);
Your function apparently takes only 2 parameters.
Also, you don't need the extra pointer, you should just do
addnode( &tree,str);

> memcpy (temp->string,str,strlen(str)+1);
Better known as
strcpy (temp->string,str);

Most tree functions are recursive, you follow a sequence of left and right nodes until you find an empty one, then you insert the new node.