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

main()
{
  DIR *d;
  struct dirent *de;
  struct stat buf;
  int exists;
  int total_size;

  d = opendir(".");
  if (d == NULL) {
    perror("prsize");
    exit(1); // **ОШИБКА**
  }

  total_size = 0;

  for (de = readdir(d); de != NULL; de = readdir(d)) {
    exists = stat(de->d_name, &buf);
    if (exists < 0) {
      fprintf(stderr, "Couldn't stat %sn", de->d_name);
    } else {
      total_size += buf.st_size;
    }
  }
  closedir(d);
  printf("%dn", total_size);
}

#include <stdlib.h>

stat(pathname, &sb);
if ((sb.st_mode & S_IFMT) == S_IFREG) {
     /* Handle regular file */
}