Do i have to malloc for each word?

Question

I am developing a software that use BST (Binary Search Tree), but i don't understand what's going on:

while (1)
{
    word = (char *)malloc(sizeof(char) * wordLength);

    readReturn = scanf("%s", word);
    if (readReturn == 0)
        return 0;
    BSTNode new = newBSTNode(word);
    if (strcmp(word, "END") == 0)
        break;
    TreeInsert(Tree, new);
}

In this way the program it's working and give the input "aaa" "bbb", the tree has the right value. But if i declare the variable outside the program stop working and the output is: "bbb" "bbb".

     word = (char *)malloc(sizeof(char) * wordLength);    
     while (1)
     {
        readReturn = scanf("%s", word);

        if (readReturn == 0)
            return 0;
        BSTNode new = newBSTNode(word);
        if (strcmp(word, "END") == 0)
            break;
        TreeInsert(Tree, new);
     }

Please, explain me why this is happening!

Answer 1

In the second code snippet

 word = (char *)malloc(sizeof(char) * wordLength);    
 while (1)
 {
    readReturn = scanf("%s", word);

    if (readReturn == 0)
        return 0;
    BSTNode new = newBSTNode(word);
    if (strcmp(word, "END") == 0)
        break;
    TreeInsert(Tree, new);
 }

all strings are stored in the same dynamically allocated extent of memory that was allocated once before the while loop.

 word = (char *)malloc(sizeof(char) * wordLength);    

So all nodes refer to it and what was stored last in this extent is outputted for all nodes.

Pay attention to that in the first code snippet

while (1)
{
    word = (char *)malloc(sizeof(char) * wordLength);

    readReturn = scanf("%s", word);
    if (readReturn == 0)
        return 0;
    BSTNode new = newBSTNode(word);
    if (strcmp(word, "END") == 0)
        break;
    TreeInsert(Tree, new);
}

there can be a memory leak if after the loop there is no statement

free( word );