How to use qsort for an array of strings?

Question

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

int sortstring(const void *str1, const void *str2) {
    const char *rec1 = str1;
    const char *rec2 = str2;
}

void sortutil(char* lines[]) {
    qsort(lines, 200, sizeof(char), sortstring);
}

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

int getarray(char *lines[]) {
    int i = 0;
    char *text = (char *)malloc(200);
    while (fgets(text, 200, stdin) != NULL) {
        lines[i] = text;
        i++;
        text = (char *)malloc(200);
    }
    return i;
}

void printarray(char *lines[], int max) {
    for (int i = 0; i < max; i++)
        printf("%s\n\n", lines[i]);
}

int main(int argc, char* argv[]) {
    char* arr[100];
    int numlines = getarray(arr);
    printf("There are %d lines\n", numlines);
    printarray(arr, numlines);

    for (int i = 1; i < argc;  i++) {
        if (strcmp(argv[i], "-s") == 0) {
            sortutil(arr);
            printarray(arr, numlines);
        }
    }
}

When I send in a file with arbitrary text, It'll read the file and print it out, but when i call -s and call the qsort function, it comes back with nulls. I'm sure I am using qsort incorrectly, what is the right way to use it for an array to char pointers?

Answer 1

Your comparator is being sent each pair by-address. I.e. they're pointer-to-pointer-to-char.

Change the comparator to:

int sortstring( const void *str1, const void *str2 )
{
    char *const *pp1 = str1;
    char *const *pp2 = str2;
    return strcmp(*pp1, *pp2);
}

Likewise, your sortutil needs to know the number of items being sorted, as well as pass the correct size of each item. Change that to:

void sortutil(char* lines[], int count)
{
    qsort(lines, count, sizeof(*lines), sortstring);
}

Finally, the call from main() should look like this:

sortutil(arr, numlines);

That should do it.

Answer 2

What the compar function gets are pointers to the elements in your array, which in this case, are pointers to char. So the parameters str1 and str2 are actually pointers to pointers to char. You must cast them like this:

int sortstring( const void *str1, const void *str2 )
{
    const char *rec1 = *(char**)str1;
    const char *rec2 = *(char**)str2;
    int val = strcmp(rec1, rec2);

    return val;
}

Then you have to use the proper element size in qsort.

qsort(lines, 200, sizeof(char*), sortstring);

Answer 3

This line is incorrect.

    qsort(lines, 200, sizeof(char), sortstring);

Change it to

    qsort(lines, 200, sizeof(char*), sortstring);