How to get only *.txt filenames from directory in c and write it to char ** array?

Question

Its an extension of my previous question: How can I get only txt files from directory in c? . Now I want to save those filenames (I dont really know how many of them are in the dir) to char ** array. I figured out a solution (kind of) but then I realized that I need not a char* but char ** (I know, stupid me :])

Anyway, I got segmentation fault [core dumped] with this code:

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <dirent.h>
#include <stdbool.h>

char* allocMemory(int n)
{
    char *tab = (char *) malloc(n*sizeof(char));
    return tab;
}

void freeMemory(char **tab, int n, int m)
{
    int i=0;
    for(i=0; i<m; i++)
        free(tab[i]);
    free(tab);
    tab = NULL;
}

bool hasTxtExtension(char const *filename)
{
    size_t len = strlen(filename);
    return len > 4 && strcmp(filename + len - 4, ".txt") == 0;
}

char** getTxtFilenames(const char *dirname)
{
    DIR *directory = NULL;
    struct dirent *ent = NULL;
    int fileCounter = 0;

    char **txtFiles = allocMemory(1);
    char **moreTxtFiles = allocMemory(1);

    directory = opendir (dirname);
    if(directory == NULL)
        return NULL;
    int i = 0;

     while ((ent = readdir (directory)) != NULL)
     {
         if(hasTxtExtension(ent->d_name))
         {
             fileCounter ++;
             moreTxtFiles = (char**) realloc (txtFiles, fileCounter * sizeof(char*));

             if(moreTxtFiles[i] != NULL)
             {
                 txtFiles = moreTxtFiles;
                 txtFiles[i] = allocMemory(strlen(ent->d_name));
                 txtFiles[i][fileCounter - 1] = ent->d_name;
             }
             else
             {
                 freeMemory(txtFiles, 1, fileCounter);
                 return NULL;
             }
         }
         i ++;
     }

     if(closedir(directory) < 0)
        return NULL;

    return txtFiles;
}

int main(int argc, char **argv)
{
    char **txtFilenames = getTxtFilenames("dir");
    if(txtFilenames == NULL)
        return -1;
    printf("%s\n", txtFilenames[0][1]);
    return 0;
}

EDIT:

I also tried this: (notice that Im a little bit confused with those not-lovely char arrays in C, argh :/)

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <dirent.h>
#include <stdbool.h>

char* allocMemory(int n)
{
    char *tab = (char *) malloc(n*sizeof(char));
    return tab;
}

void freeMemory(char *tab)
{
    free(tab);
    tab = NULL;
}

bool hasTxtExtension(char const *filename)
{
    size_t len = strlen(filename);
    return len > 4 && strcmp(filename + len - 4, ".txt") == 0;
}

char* getTxtFilenames(const char *dirname)
{
    DIR *directory = NULL;
    struct dirent *ent = NULL;
    int fileCounter = 0;

    char *txtFiles = NULL;
    char *moreTxtFiles = NULL;

    directory = opendir (dirname);
    if(directory == NULL)
        return NULL;

     while ((ent = readdir (directory)) != NULL)
     {
         if(hasTxtExtension(ent->d_name))
         {
             fileCounter ++;
             moreTxtFiles = (char*) realloc (txtFiles, fileCounter * sizeof(char));

             if(moreTxtFiles != NULL)
             {
                 txtFiles = moreTxtFiles;
                 txtFiles[fileCounter - 1] = ent->d_name;
             }
             else
             {
                 freeMemory(txtFiles);
                 return NULL;
             }
         }
     }

     if(closedir(directory) < 0)
        return NULL;

    return txtFiles;
}

int main(int argc, char **argv)
{
    char **txtFilenames = getTxtFilenames("dir");
    if(txtFilenames == NULL)
        return -1;
    printf("%s\n", txtFilenames[0]);
    return 0;
}

Answer 1

• txtFilenames[0][1] is a character, not a string.
• txtFiles[i][fileCounter - 1] is a character, but ent->d_name is a string.

I don't understand why you are using two index ( i and fileCounter ). Just use an array of strings. You should also include <string.h> .

The solution is simpler:

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

char **f(const char *s)
{
    char **p = NULL;
    DIR *dir;
    struct dirent *ent;
    size_t i = 0;

    dir = opendir(s);

    while ((ent = readdir(dir)) != NULL) {
        if (hasTxtExtension(ent->d_name)) {
            p = realloc(p, (i + 1) * sizeof(char *));
            p[i] = malloc(strlen(ent->d_name) + 1);
            strcpy(p[i], ent->d_name);
            ++i;
        }
    }

    closedir(dir);
    return p;
}

Here is a non-exhaustive list to improve this code:

• Handle reallocation errors: if realloc fail, there is a memory leak.
• Handle directory errors: especially, treat the case where s is a bad directory path.
• Optimize reallocations (don't use realloc with each iteration).

Answer 2

Aside from the questionable indexing Kirilenko pointed out, your first (and core) error is:

char **txtFiles = allocMemory(1);
char **moreTxtFiles = allocMemory(1);

Which seems innocent enough, but allocMemory is defined as:

char* allocMemory(int n)
{
    char *tab = (char *) malloc(n*sizeof(char));
    return tab;
}

Both txtFiles and moreTxtFiles are pointers to pointers ; not pointers to chars. You're returning a pointer to a single allocated char, then storing it in a pointer that expects it to be a pointer to a pointer .

For your case, don't use allocMemory for allocating list entries. Just use it for allocating your strings.