stat() does not work for any other directory than parent "."

Question

I'm trying to write a simple "ls" script in C in bash (linux). I managed to write a script that gives me all information that I need about files in given directory, However, for some reason it works only for current "." directory, and not for any other. For any other directory it shows files inside (for example file names are correct), but doesn't show correct stats (additional information about files). I have no clue how to repair it. As a guide I used http://man7.org/linux/man-pages/man2/stat.2.html.

My script:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <dirent.h>
#include <sys/types.h>
#include <sys/stat.h> 

int main(int argc, char *argv[]) {
  int opt;
  DIR *dir;
  struct dirent *poz;
  char *katalog;
  char *nazwa, *user_n, *grupa_n, *rozmiar_n, *numer_iw, *typ_p, *output_p;
  nazwa = "Name"; user_n = "UID"; grupa_n = "GID", rozmiar_n = "Size"; numer_iw = "I-node no."; typ_p = "Type"; 
  struct stat sb;

    if (argc == 2) {
    katalog = argv[1];
    dir = opendir(katalog);
    printf("%11s %11s %11s %11s %11s %15s \n", typ_p, user_n, grupa_n, numer_iw, rozmiar_n, nazwa);

    if (dir) {      while ((poz = readdir(dir)) != NULL) {

        if (lstat(poz->d_name, &sb) == 1) {
          perror("lstat");
          exit(EXIT_FAILURE);
        }

           switch (sb.st_mode & S_IFMT) {
           case S_IFBLK:  output_p = "block device";            break;
           case S_IFCHR:  output_p = "character device";        break;
           case S_IFDIR:  output_p = "directory";               break;
           case S_IFIFO:  output_p = "FIFO";                    break;
           case S_IFLNK:  output_p = "symlink";                 break;
           case S_IFREG:  output_p = "file";                    break;
           case S_IFSOCK: output_p = "socket";                  break;
           default:       output_p = "unknown";                 break;
}

  printf("%11s %11llu %11llu %11llu %11llu %15s \n", output_p, (unsigned long long) sb.st_gid, 
  (unsigned long long) sb.st_uid, (unsigned long long) sb.st_ino, (unsigned long long) sb.st_size, poz->d_name);

      }
      closedir(dir);
    }

  }

  return (0);
}

Sample execution:

./a.out .

Works fine (output):

       Type         UID         GID  I-node no.        Size            Name 
       file        1000        1000      274621         248    sample_file2 
       file        1000        1000      274603          36     sample_file 
       file        1000        1000      272921       12776           a.out 
       file        1000        1000      274616        1859        script.c 
  directory        1000        1000      295524        4096               . 
  directory        1000        1000      269568        4096              .. 
  directory        1000        1000      295526        4096      sample_dir 

Sample execution:

./a.out sample_dir

Works with errors (wrong information, e.g. file type or size is wrong for all files/directories):

       Type         UID         GID  I-node no.        Size            Name 
  directory        1000        1000      295524        4096               . 
  directory        1000        1000      295524        4096 sample_file_inside_dir 
  directory        1000        1000      295524        4096 sample_dir_inside_dir 
  directory        1000        1000      295524        4096   sample_file_2 
  directory        1000        1000      269568        4096              .. 

I can't see where is this error... Why stat() works properly only for current directory? Is there a way to change it?

Thank you for your time. B