Creating a binary tree

[samohtvii]

I am looking to create a binary tree and then print it out. I am getting some strange errors I cant fix.

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

struct animalTree
{
    char data[100];
    struct animalTree *left;
    struct animalTree *right;
};

typedef struct animalTree aTree;

void fillTree(FILE*, struct animalTree*);
int readNext(char*, FILE*, int*);
void printTree(struct animalTree*);
struct aTree *makeNode(char*);

aTree* root;

int main()
{
    FILE *fp;
    fp=fopen("animals", "r");
    char stuff[40];
    if(fp != NULL)
    {
        fillTree(fp, root);
        printTree(root);
    }
    else
    {
        root = (aTree*)makeNode("Horse");
    }   
    fclose(fp);
    return 0;
}


void fillTree(FILE* fp, struct animalTree *node)
{
        char data[100] = {""};
    int isNULL = 1;
    if(readNext(data, fp, &isNULL))
        return;
    if(isNULL == 1)
    {
        node = (aTree*)makeNode(data);
        fillTree(fp, node->left);
        fillTree(fp, node->right);
    }
}

void printTree(struct animalTree *root)
{
    if(root == NULL) 
    {
        printf("%s", "here");
        return;
    }
    printf("%s", root->data);
    printTree(root->left);
    printTree(root->right);
}


int readNext(char* data, FILE* fp, int* isNULL)
{
    fgets(data, 100, fp);
    if(feof(fp))
        return 1;
    else
    {
        if(strcmp(data, "NULL"))
        {
            isNULL = 1;
        }
        else
        {
            isNULL = 0;
        }
        return 0;
    }
}

struct aTree* makeNode(char* data)
{
    aTree *node;
    node = (aTree*)malloc(sizeof(aTree));
    if (node)
    {
        strcpy(node->data, data);
        node->left = NULL;
        node->right = NULL;
    }
    return node;
}

ERRORS:

user@ubuntu:~/Desktop/Assignment2/C$ gcc program.c 
program.c: In function ‘readNext’:
program.c:74:11: warning: assignment makes pointer from integer without a cast [enabled by default]
program.c: In function ‘makeNode’:
program.c:87:17: warning: incompatible implicit declaration of built-in function ‘malloc’ [enabled by default]    
program.c:94:2: warning: return from incompatible pointer type [enabled by default]

I am aware these are warnings and that the program can run which i have done but it seems like the root that it passes to print is sitting at the end on a NULL node and never goes into the print. Other times with minor changes I have got seg faults.
For starters is my root variable doing what it's suppose to. It holds the start of the tree right?
I feel like root is being changed and I can't print from it but then again I'm not really sure what's happening.

The animals file is:

Does it purr?
cat
NULL
NULL
Does it bark?
dog
NULL
NULL
Does it walk?
horse
NULL
NULL
fish
NULL
NULL

Anyway thanks for the help

[deceptikon]

The first warning tells you that isNULL is a pointer yet you're using it like it's not. This suggests a bug wherein you forgot to dereference the pointer. You're also using strcmp() incorrectly; it returns a comparison result rather than a boolean, where 0 means equality:

if (strcmp(data, "NULL") == 0)
    *isNULL = 1;
else
    *isNULL = 0;

But since you're making a boolean test and also assigning to a boolean variable, you can eschew the if statement and do the assignment directly:

*isNULL = (strcmp(data, "NULL") == 0);

C is cool like that. :)

The next two warnings are telling you that you forgot to include stdlib.h and malloc() has no declaration. When a function has no declaration, C assumes that it's a function taking an unknown number of arguments and returning int.