如何在C中对Dirent进行排序

Question

Brand new to C and finding it confusing. What I really want to know about, is taking two separate pieces of code and getting them to work together.

---

Here's some code that simply lists the contents of the current directory:

#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>

int
main (void)
{
  DIR *dp;
  struct dirent *ep;

  dp = opendir ("./");
  if (dp != NULL)
    {
      while (ep = readdir (dp))
        puts (ep->d_name);
      (void) closedir (dp);
    }
  else
    perror ("Couldn't open the directory");

  return 0;
}

The output is unsorted

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Here's some code that sorts an array by length of elements, using a quick sort algorithm:

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

char *array[] = { "XX", "YYY", "Z" };
#define N (sizeof(array) / sizeof(array[0]))

int
cmp(const void *a, const void *b)
{
    size_t lena = strlen(*(const char **)a);
    size_t lenb = strlen(*(const char **)b);
    return lena < lenb ? -1 : lena > lenb;
}

int
main()
{
    size_t i;
    qsort(array, N, sizeof(array[0]), cmp);
    for (i =  0; i < N; i++)
        printf("%s\n", array[i]);
}

---

I'm sure there are better ways to do this, but for purely academic reasons, I'd like to use the output of the first function (directory contents) as an input to the last function (sort by length).

Answer 1

You could store the dirent objects inside an array which you could then pass into qsort. The function cmp should be modified to compare the d_name element inside the dirent pointers. like so

int cmp(const *a, const void *b)
{
  size_t lena = strlen(((struct dirent *) a)->d_name);
  size_t lenb = strlen(((struct dirent *) b)->d_name);
  return lena < lenb ? -1 : lena > lenb;
}