How to use scandir() in C for case insensitive?

Question

I'm learning C and I have this implementation to sort the files and folders, but this isn't case insensitive:

#include <dirent.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>

int main(void) {
    struct dirent **namelist;
    int n;

    n = scandir(".", &namelist, NULL, alphasort);
    if (n < 0)
        perror("scandir");
    else {
        printf("Inside else, n = %d\n", n);
        while (n--) {
            printf("%s\n", namelist[n]->d_name);
            free(namelist[n]);
        }
        free(namelist);
    }
}

And if I have a.txt, b.txt, C.txt and z.txt it will sort in this order: C.txt, a.txt, b.txt, z.txt. I want this to be sorted case insensitive like this: a.txt, b.txt, C.txt, z.txt

Answer 1

scandir is defined with this prototype:

int scandir(const char *restrict dirp,
            struct dirent ***restrict namelist,
            int (*filter)(const struct dirent *),
            int (*compar)(const struct dirent **,
                          const struct dirent **));

The function alphasort sorts the filenames in lexicographical order, hence case-sensitive order. If you want case insensitive sorting, use a different comparison function:

int alphasort_no_case(const struct dirent **a, const struct dirent **b) {
    return strcasecmp((*a)->d_name, (*b)->d_name);
}

Both scandir and strcasecmp are POSIX functions: strcasecmp is highly likely to be available on systems that support scandir and defined in <strings.h>.

Modifier version:

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

int alphasort_no_case(const struct dirent **a, const struct dirent **b) {
    return strcasecmp((*a)->d_name, (*b)->d_name);
}

int main(void) {
    struct dirent **namelist;
    int n;

    n = scandir(".", &namelist, NULL, alphasort_no_case);
    if (n < 0) {
        perror("scandir");
    } else {
        printf("Inside else, n = %d\n", n);
        while (n--) {
            printf("%s\n", namelist[n]->d_name);
            free(namelist[n]);
        }
        free(namelist);
    }
    return 0;
}