Would someone care to identify what might cause a segfault in my code?

Question

This is for my CS50 assignment. I have to make a spell-checker that loads a dictionary into trie data structure. The segfault doesn't happen until the file size increases.

//Trie structure
typedef struct node {

    bool is_word;
    struct node* children[27];
}node;

node* root;

// maximum length for a word
#define LENGTH 45
//global counter for total words in dic
int SIZE = 0;

Loads dictionary into memory.

bool load(const char* dictionary)
{
    //Open dictionary file && safecheck
    FILE* d_file = fopen(dictionary,"r");
    if(d_file == NULL) { return false; }

    //allocate space for the first node
    root = calloc(1,sizeof(node));

    //string buffer
    char word[SIZE+1];
    //loop through each word of the dictionary and store it to word
    while(fscanf(d_file,"%s\n",word)!=EOF){
        //navigation node steping further into trie
        node* nav = root;
        int word_length = strlen(word);
        //Iterate through each letter in a word
        for(int i=0;i<word_length;i++){
            int letter_pos = tolower(word[i]) - 'a';
            //check if letter node exists
            if(nav->children[letter_pos]==NULL){
                //create node for a letter
                node* l_node = calloc(1,sizeof(node));
                if(l_node == NULL) { return false; }
                nav->children[letter_pos] = l_node;
            }
            //proceed to the next node
            if(i < word_length-1)
                nav = nav->children[letter_pos];
        }
        //set word to true;
        if(nav->is_word != true){
            nav->is_word = true;
            // counter for total words in the dictionary
            SIZE++;
        }
    }
    fclose(d_file);
    return true;
}

If you get crashes like segmentation faults, you should run in a debugger to catch the crash, as the debugger will stop at the location of the crash. In the debugger you can then walk up the function call stack to your code (if it's not already there) and examine the values of variables. At the very least, you should edit your question and tell us where the crash happens and the variable values. — Some programmer dude, Nov 03 '15 at 09:12
Thanks for a tip, I tried to debug at first, it's just that I need more practise on it. I don't know how to go through the function once the error occurred and debugger stopped. I will look into it now :) — Marius Buivydas, Nov 03 '15 at 09:28

Paul R · Accepted Answer · 2015-11-03T09:14:22.553

I'm not sure if this is the main problem, but this part of the code is woefully lacking in robustness:

        int letter_pos = tolower(word[i]) - 'a';

If letter_pos is not valid at this point (e.g. because word[i] is not a letter) then all the subsequent operations will result in UB.

I would at the very least add an assert here:

        int letter_pos = tolower(word[i]) - 'a';
        assert(letter_pos >= 0 && letter_pos < sizeof(nav->children) / sizeof(nav->children[0]));

Since your array size is 27 I'm guessing you wanted to use the last element for non-alphabet characters, so you might want to make the code more like this:

        int letter_pos;
        if (isalpha(word[i]))                    // if 'A'..'Z' or 'a'..'z'
            letter_pos = tolower(word[i]) - 'a'; // index = 0..25
        else
            letter_pos = 'z' - 'a' + 1;          // index = 26
        assert(letter_pos >= 0 && letter_pos < sizeof(nav->children) / sizeof(nav->children[0]));

Thanks , a lot that was it ! :) – Marius Buivydas Nov 03 '15 at 09:28 — Marius Buivydas, Nov 03 '15 at 09:28

Would someone care to identify what might cause a segfault in my code?

1 Answers1