Qsort doesn't sort array of pointers to strings

Question

In this C program I read in words typed with my keyboard to a char pointer. The pointer is stored in an pointer array. Then I want to sort the array with qsort by the function comparison. I give it the pointer to my pointer array. It doesnt sort the array at all. I dont know if I got here a UB, or I miss memory by wrong allocation.

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

bool read_word(char ***a, int *length);
int comparison(const void *p, const void *q);


int main()
{
    int *length = malloc(sizeof(int));
    *length = 0;
    char **array = malloc(sizeof(char *));
    bool go = false;

    while(go == false)
    {
        printf("Enter word: ");

        go = read_word(&array,length);
    }

    qsort(array, *length - 1,sizeof(char *), comparison);

    printf("\n");


    for(int i = 0; i < *length; i++)
        printf("%s\n", array[i]);

    return 0;
}

bool read_word(char ***a, int *length)
{
    char ch;

    ++*length;

    char *word = malloc(20 * sizeof(char) + 1);
    char *keep_word;

    char **temp = realloc(*a,*length * sizeof(*a));

    if(!temp)
        exit(EXIT_FAILURE);

    *a = temp;

    keep_word = word;

    while((ch = getchar()) != '\n')
        *keep_word++ = ch;

    *keep_word = '\0';

    if(word == keep_word)
    {
        free(word);
        --*length;
        return true;
    }



    (*a)[*length - 1] = word;

    printf("%s", (*a)[*length -1]);
    printf("\nh\n");
    return false;
}

int comparison(const void *p, const void *q)
{
    const char *p1 = p;
    const char *q1 = q;


    return strcmp(p1,q1);
}

Please tag your language, I'm not proficient enough to determine whether it's C or C++, so I won't do it for you.. — Blorgbeard
– Blorgbeard, Commented May 19, 2016 at 20:33

BLUEPIXY · Accepted Answer · 2016-05-20 03:02:50Z

1

change to

int length = 0;//no need malloc
char **array = NULL;//first value is NULL

qsort(array, length, sizeof(char *), comparison);//not length-1

int ch;//type of return value of getchar is int

for(int i = 0; i < 20 &&  (ch = getchar()) != '\n'; ++i)//i : guard

const char *p1 = *(const char **)p;//qsort pass pointer to object to comparison function  
const char *q1 = *(const char **)q;//if char *, char **

answered May 20, 2016 at 3:02

BLUEPIXY

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2 Comments

BLUEPIXY Over a year ago

go = read_word(&array,length); --> go = read_word(&array, &length);, i < *length --> i < length

David C. Rankin Over a year ago

Or const char *p1 = *(char * const *)p;

user31264 · Accepted Answer · 2016-05-19 21:26:34Z

0

You do

keep_word = word;

and later

if(word == keep_word)

The condition in brackets is always true.

On a side note, your program is error-prone and hard to understand because you use pointers way too much. In main(), length should be int, not int*, array should be char*, not char** . In the read_word, a should be char**, not char***. Don't use pointers unless necessary!

answered May 19, 2016 at 21:26

user31264

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3 Comments

Fenixx Reborn Over a year ago

word != keep_word, if there are chars before a '\n', cause keep_word is in a diff location then and i use **char because its a pointer to a pointer, any other ideas how to malloc a pointer array? was only one i got on my mind :p

user31264 Over a year ago

Wrong. In your code, word==keep_word ALWAYS. If you do foo=bar, then foo==bar ALWAYS unless you change the value of foo or bar later. You don't change the value of word or keep_word, you change the values in the memory where they point. As for the later, does array contain many words or just one word? If it contains just one word, it should be pointer to char, not pointer to pointer.

Mike Over a year ago

You can specify strcmp as the comparison function to qsort.

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