c++ - Where is a file mounted?

Question

Given a path to a file or directory, how can I determine the mount point for that file? For example, if /tmp is mounted as a tmpfs filesystem then given the file name /tmp/foo/bar I want to know that it's stored on a tmpfs rooted at /tmp.

This will be in C++ and I'd like to avoid invoking external commands via system(). The code should be robust--not necessarily against deliberate tampering but definitely in the face of nested mountpoints, symlinks, etc.

I haven't been able to find a simple system call to do this. It looks like I'll have to write the check myself. Here's a rough outline of what I'm planning.

1. Canonicalize the file name a la the readlink shell command. How?
2. Read /etc/mtab with getmntent() & co.
3. Determine the corresponding mount entry for the file. How?

For #1 is there a simple system call or do I need to read each directory component of the path and resolve them with readlink(2) if they are symlinks? And handle . and .. myself? Seems like a pain.

For #3 I've got various ideas on how to do this. Not sure which is best.

1. open() the file, its parent, its parent's parent, etc. using openat(fd, "..") until I reach one of the /etc/mtab entries. (How do I know when I do? fstat() them and compare the inode numbers?)
2. Find the longest directory name in the mount table which is a substring of my file name.

I'm leaning towards the first option but before I code it all up I want to make sure I'm not overlooking anything--ideally a built-in function that does this already!

Answer 1

This is what I've come up with. It turns out there's usually no need to iterate through the parent directories. All you have to do is get the file's device number and then find the corresponding mount entry with the same device number.

struct mntent *mountpoint(char *filename, struct mntent *mnt, char *buf, size_t buflen)
{
    struct stat s;
    FILE *      fp;
    dev_t       dev;

    if (stat(filename, &s) != 0) {
        return NULL;
    }

    dev = s.st_dev;

    if ((fp = setmntent("/proc/mounts", "r")) == NULL) {
        return NULL;
    }

    while (getmntent_r(fp, mnt, buf, buflen)) {
        if (stat(mnt->mnt_dir, &s) != 0) {
            continue;
        }

        if (s.st_dev == dev) {
            endmntent(fp);
            return mnt;
        }
    }

    endmntent(fp);

    // Should never reach here.
    errno = EINVAL;
    return NULL;
}

Thanks to @RichardPennington for the heads up on realpath(), and on comparing device numbers instead of inode numbers.