Creating Dynamically Allocated Strings from a file in C - c

Question

I am having some issues with dynamically allocating a string for a node in a tree. I have included my node structure below for reference.
struct node
{
char *string;
struct node *left;
struct node *right;
};
typedef struct node node;
I am supposed to read words from a text file and then store those words into a tree. I am able to store char arrays that have been defined, such as char string[20] without problems, but not strings that are supposed to be dynamically allocated.
I am only going to post the code I am using to read my file and try to create the dynamically allocated array. I have already created the file pointer and checked that it is not NULL. Every time I try to run the program, it simply crashes, do I need to try and read the words character by character?
//IN MAIN
node *p, *root ;
int i;
int u;
root = NULL;
char input[100];
while(fscanf(fp, "%s", &input) != EOF)
{
//Create the node to insert into the tree
p = (node *)malloc(sizeof(node));
p->left = p->right = NULL;
int p = strlen(input); //get the length of the read string
char *temp = (char*) malloc(sizeof(char)*p);
//malloc a dynamic string of only the length needed
strcpy(local, input);
strcpy(p->word,local);
insert(&root, p);
}
To be completely clear, I only want advice regarding the logic of my code, and only would like someone to help point me in the right direction.

Answer 1

You are invoking many undefined behaviors by
passing pointer to object having wrong type to scanf(). i.e. In fscanf(ifp, "%s", &input), char(*)[100] is passed where char* is expected
accessing out-of-range of allocated buffer when storeing terminating null-character in strcpy(local, input);
using value of buffer allocated via malloc() and not initialized in strcpy(curr->word,local);
Your code should be like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
typedef struct node_t {
struct node_t* left, *right;
int count;
char* word;
} node;
void insert(node ** tree, node * item);
int main(void) {
FILE* ifp = stdin;
node * curr, * root;
int i;
int u;
root = NULL;
char input[100];
/* you should specify the maximum length to read in order to avoid buffer overrun */
while(fscanf(ifp, "%99s", input) != EOF)
{
//Create the node to insert into the tree
curr = malloc(sizeof(node));
if(curr == NULL) /* add error check */
{
perror("malloc 1");
return 1;
}
curr->left = curr->right = NULL;
curr->count = 1;
int p = strlen(input); //get the length of the read string
char *local = malloc(sizeof(char)*(p + 1)); /* make room for terminating null-character */
if (local == NULL) /* add error check again */
{
perror("malloc 2");
return 1;
}
//malloc a dynamic string of only the length needed
//To lowercase, so Job and job is considered the same word
/* using strlen() in loop condition is not a good idea.
* you have already calculated it, so use it. */
for(u = 0; u < p; u++)
{
/* cast to unsigned char in order to avoid undefined behavior
* for passing out-of-range value */
input[u] = tolower((unsigned char)input[u]);
}
strcpy(local, input);
curr->word = local; /* do not use strcpy, just assign */
insert(&root, curr);
}
/* code to free what is allocated will be here */
return 0;
}
//Separate insert function
void insert(node ** tree, node * item)
{
if(!(*tree))
{
*tree = item;
return;
}
if(strcmp(item->word,(*tree)->word) < 0)
insert(&(*tree)->left, item);
else if(strcmp(item->word,(*tree)->word) > 0)
insert(&(*tree)->right, item);
/* note: memory leak may occur if the word read is same as what is previously read */
}