Why is __stack_chk_fail happening in my code?

Question

Regarding following function, my debugger shows me __stack_chk_fail when finishing this function.

My system is Mac OS.

It is because my stack overflowed itself by checking references.

Also based on my experiment, if set vocab_size = 30000 it shows __stack_chk_fail error but when vocab_size = 20000 it is fine.

So I believe

vocab = (struct vocab_word *)malloc ((size_t) ((vocab_size + 1) * sizeof(struct vocab_word)));

is the issue. But malloc allocate memory on the heap rather than stack, so I am wondering where I goes wrong?

void populate_vocab(){
    FILE *fin;
    fin = fopen(word_file, "rb");
    vocab = (struct vocab_word *)malloc ((size_t) ((vocab_size + 1) * sizeof(struct vocab_word)));
    char word[MAX_STRING];
    int word_idx = 0;
    int num = 0;
    boolean word_mode = 1;
    long long cur_vocab_size = 0;

    while (!feof(fin)) {
        ch = fgetc(fin);

        if(ch == ' '){
            word_mode = 0;
        }else if(ch == '\n'){
            word_mode = 1;
            word[word_idx] = 0;
            vocab[cur_vocab_size].word = (char *)calloc(word_idx, sizeof(char));
            strcpy(vocab[cur_vocab_size].word,word);
            vocab[cur_vocab_size].cn = num;
            cur_vocab_size++;
            if (cur_vocab_size >= vocab_size){
                break;
            }
            //fresh var
            word_idx = 0;
            num = 0;

        }else{
            if(word_mode){
                word[word_idx] = ch;
                word_idx ++;
            }else{
                num = num * 10;
                num += ch - '0';
            }
        }
    }
    fclose(fin);
}

Answer 1

Based on comments, I figured out the reason. One of words exceed MAX_STRING which cause stack overflowed.