In C, how to I get to a specified directory?

Question

I have to do a program where I need to index the files in a specified directory. I've gotten the indexing part down, but what I'm having trouble with is how to navigate to the directory. For example, say when I start the program, it will ask "What directory would you like to index," And then the input would be "usr/Documents/CS/Assignment4," how do I get to the "Assignment4" directory? I know recursion is needed, but I'm really confused as to how directories work in C. Say my source file is in "usr/Documents/SourceCode," then what should I do to get to Assignment4?

I know I sound like I want all the answers, but I'm completely lost as to how directories work, and the book I have sucks. So even if all you have is a link to a good tutorial on this, that would be fantastic.

Thanks in advance for your help.

EDIT: I'm running Linux, Ubuntu to be exact. GCC is the compiler.

Answer 1

The C programming language doesn't have a notion of a file system. This is instead an operating system specific question.

Based on the style of directory in your question though it sounds like you're on a unix / linux style system. If that's the case then you're looking for the opendir function

• http://linux.die.net/man/3/opendir

Answer 2

man 2 chdir :-)

Answer 3

ls to list current locations files/directories, and then cd to change to that directory.

Eg.

lynx:~ Mark$ ls
Class     Documents Library   Music     Public
Desktop   Downloads Movies    Pictures  Sites

lynx:~ Mark$ cd Class

lynx:Class Mark$ ls
08G-001 22C-21  22C-60

Answer 4

The name of the directory is only a string.

So opendir("filename"); will make it possible to read the directory "file".

However you should perhaps start thinking in filenames and pathes.

"usr/Documents/SourceCode" + "/../CS/Assignment4" is the same as "usr/Documents/CS/Assignment4" however I assume you are missing the leading "/".

Well, I don't get how you can be lost how directories work. A directory is nothing different than a "folder" in Windows or in Mac OS X. Bottom line a hard disk has a filesystem and a filesystem only consists out of folders/directories that "contain" files (and special files like named sockets etc., this should not interest you right now).

Hope this helped at least a bit.

Angelo

Answer 5

Recursively traversing a directory in C goes something like this:

Use opendir and readdir to list the directory entries. I probably shouldn't be doing this, but I'm posting a full code sample (sans error handling) because there are a bunch of little things you have to do to ensure you're using the API correctly:

DIR           *dir;
struct dirent *de;
const char    *name;

dir = opendir(dirpath);
if (dir == NULL) {
    /* handle error */
}

for (;;) {
    errno = 0;
    de = readdir(dir);
    if (de == NULL) {
        if (errno != 0) {
            /* handle error */
        } else {
            /* no more entries left */
            break;
        }
    }

    /* name of file (prefix it with dirpath to get a usable file path) */
    name = de->d_name;

    /* ignore . and .. */
    if (name[0] == '.' && (name[1] == '\0' || (name[1] == '.' && name[2] == '\0')))
        continue;

    /* do something with the file */
}

if (closedir(dir) != 0) {
    /* handle error */
}

When working with each file, be sure to prepend the dirpath to it (along with a slash, if needed). You could also use chdir to descend and ascend, but it introduces complications in practice (e.g. you can't traverse two directories simultaneously), so I personally recommend keeping your current working directory stationary and using string manipulation to concatenate paths.

To find out if a path is a directory or not (and hence whether you should opendir() it), I recommend using lstat() rather than stat(), as the latter follows symbolic links, meaning your directory traversal could get caught in a loop and you'll end up with something like this ctags output.

Of course, since directory structure is recursive in nature, recursion plays a natural role in the traversal process: make a recursive call when a child path is a directory.