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

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.