Crashing when qsorting an array of pointers to strings

Question

So I have searched this forum and when back and read every little detail in this chapter about using qsort(), but I can't seem to figure this out. When I run my code it crashes every time, I have tried casting using every different method I could possibly find and yet I just can't get it to stop crashing.

char *line[MAX_WORDS] <- This is my array I am trying to sort

qsort(line, word_count, sizeof(char*), compare_words);

int compare_words(const void *p, const void *q)
{
    const char *p1 = *(char**)p;
    const char *q1 = *(char**)q;
    return strcmp(p1, q1);
}

here is the full source code

// Chapter 17 Programming Project #6 // Chapter 17 Programming Project #5

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

    #define MAX_WORD_LEN 20
    #define MAX_WORDS 10

    int read_line(char str[], int n);
    int compare_words(const void *p, const void *q);

    int main(void)
    {
        char *line[MAX_WORDS], word_str[MAX_WORD_LEN];
        int i = 0, word_count = 0;

        for (;;) {
            printf("Enter word: ");
            read_line(word_str, MAX_WORD_LEN);
            if (strlen(word_str) == 0)
                break;

            line[i] = malloc(strlen(word_str));
            if (line[i] == NULL) {
                printf("-- No space left --\n");
                break;
            }

            strcpy(line[i], word_str);
            word_count++;
        }
        printf("Word_count: %d\n", word_count);
        qsort(line, word_count, sizeof(char*), compare_words);

        printf("\nIn sorted order:");
        for (i = 0; i <= word_count - 1; i++)
            printf(" %s", line[i]);
        putchar('\n');

        return 0;
    }

    int read_line(char str[], int n)
    {
        int ch, i = 0;

        while ((ch = getchar()) != '\n')
            if (i < n)
                str[i++] = ch;
        str[i] = '\0';
        return i;
    }

    int compare_words(const void *p, const void *q)
    {
        const char *p1 = *(char**)p;
        const char *q1 = *(char**)q;
        return strcmp(p1, q1);
    }

Does the array contain word_count valid strings? If you loop through the first word_count entries and print them does it work correctly? — interjay
– interjay, Commented Jun 5, 2014 at 13:46
Works fine as shown, given a valid word_count and valid line content: ideone.com/qPDpqt — Paul Roub
– Paul Roub, Commented Jun 5, 2014 at 13:52
Yeah I tested the word_count, when I enter 2 words it prints 2 — John Conner
– John Conner, Commented Jun 5, 2014 at 14:08

Paul Roub · Accepted Answer · 2014-06-05 14:21:49Z

1

You're overrunning some buffers:

line[i] = malloc(strlen(word_str));
// ...
strcpy(line[i], word_str);

You need to include space for the terminating '\0' character, either via:

line[i] = malloc(strlen(word_str) + 1);
// ...
strcpy(line[i], word_str);

or

line[i] = strdup(word_str);
if (line[i] == NULL) {
  printf("-- No space left --\n");
  break;
}

And you're never incrementing i when reading the words, so you'll have word_count at 5 or something, but all of the words were temporarily pointed to by line[0]; the rest (line[1]..line[4]) are uninitialized.

Change your first for loop to:

for ( i = 0; i < MAX_WORDS; ++i ) {
  // ..
}

edited Jun 5, 2014 at 14:21

answered Jun 5, 2014 at 14:13

Paul Roub

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3 Comments

John Conner Over a year ago

XD Thanks, I feel like an idiot....The problem was that I wasn't incrementing the i variable. About the buffers though, my word_str already had the '\0' character so when it allocated space for it, I didn't need the +1. Thanks for the help XD

John Conner Over a year ago

yeah what happened, was that the project called for the loop to end when a blank string was entered "", so when I set the for loop to infinite and added a test right under my call to the read_line function, I took out the incrementation for the i variable and commenced to writing my qsort call...and got side tracked

Paul Roub Over a year ago

If you're using strlen(), then yes, you do need the +1 when you allocate space for a copy. The '\0' will not be counted by strlen().

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