Question

I’m trying to list up my local file-system recursively using dirent.h. In order to prevent from following sym-links, I’m using the sys/stat.h header. In the following you can find my SSCCE program.

/**
 * coding: utf-8
 *
 * Copyright (C) 2013, Niklas Rosenstein
 *
 * listdir.c - List up directories and file-content recursively.
 */

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

void list_dir(const char* directory_name) {
    DIR* directory_handle = opendir(directory_name);
    if (directory_handle == NULL) {
        fprintf(stderr, "Could not open directory %s.\n", directory_name);
        return;
    }

    char buffer[1024];
    struct dirent* dentry;
    int directory_name_length = strlen(directory_name);

    memcpy(buffer, directory_name, directory_name_length);
    buffer[directory_name_length] = '/';

    while ((dentry = readdir(directory_handle)) != NULL) {
        char* name = dentry->d_name;
        int length = strlen(name);

        // Skip the dotted elements.
        if (strcmp(name, ".") == 0 || strcmp(name, "..") == 0) continue;

        // Concatenate the directory name with the element name.
        memcpy(buffer + directory_name_length + 1, name, length);
        buffer[directory_name_length + 1 + length] = 0;

        printf("%s\n", buffer);

        // Proceed recursively if the element is a directory.
        struct stat s;
        if (stat(buffer, &s) != 0) {
            fprintf(stderr, "WARNING: stat() failed on %s\n", buffer);
            continue;
        }

        mode_t mode = s.st_mode;
        if (mode & S_IFDIR && !(mode & S_IFLNK)) {
            list_dir(buffer);
        }

    }
    closedir(directory_handle);
}

int main(int argc, char** argv) {
    if (argc != 2) {
        fprintf(stderr, "Expected exactly 2 arguments.\n");
        return -1;
    }
    list_dir(argv[1]);
    return 0;
}

I just can’t get it to work to detect symbolic links correctly. When it comes accross a symbolic link, linking to it’s parent directory, it goes on and on and on. It seems like there are a few folders like this on my system, eg. /usr/bin/X11:

/usr/bin/X11/
    X11/ -> .

This line must not be entirely correct: if (mode & S_IFDIR && !(mode & S_IFLNK)) {. Is it propabaly an issue with the stat() function or am I missing something obvious here?

Here’s a picture of my terminal after invoking ./listdir /usr/bin/X11, stopping the program after about a second by pressing ^C.

Answer 1

Try lstat instead of stat: the latter, when done on symbolic link, returns information on its target (the final one, which is not a symbolic link).

S_IFDIR and S_IFLNK are not supposed to be used as if they were exclusive bit flags. For directory, use S_ISDIR(mode); for symbolic link, you’ll need no test: lstat won’t report symbolic link as a directory, and that’s enough when all you want is to skip symbolic links.