Question

I’m trying to get the corresponding /dev file given a path name, kind of like df command

I currently use this command, but I’d rather not use system() function or system-specific commands.

Here’s the current code :

const string PathStr (Path); // Path is input value

// get the /dev/* files
string Cmd ("df 2> /dev/null | grep /dev | awk '{print $1, $6}'");

FILE * Pipe = popen (Cmd.c_str(), "r");
if (!Pipe)
{
    errorLog("Cannot execute command : \"" + Cmd + '"');
    return string();
}

char Buf [BufSz];
stringstream sstr;
while (!feof(Pipe))
{
    if (fgets(Buf, BufSz, Pipe))
        sstr << Buf;
}
pclose(Pipe);

// get the actual /dev file
pair<string, string> DiskPaths;
while (!sstr.eof())
{
    string Dev, MountPoint;
    sstr >> Dev >> MountPoint;
    if (string::npos != PathStr.find(MountPoint) &&
        MountPoint.size() > DiskPaths.second.size())
        DiskPaths = make_pair(Dev, MountPoint);
}

Answer 1

Calling system() and parsing the output may well be the more reliable and portable way to do what you are trying to do, because the df command may use all sorts of system-dependant tricks to resolve device numbers to names in /dev which you’d have to implement yourself otherwise.

You can easily get the device NUMBER of the filesystem on which a file is located: it’s the st_dev field of the stat() structure. But how to find what file in /dev goes with that number is left as an exercise for the df command. In particular, remember that not every filesystem is mounted from a file in /dev: think NFS, and special filesystems like /proc.

Here are some other things you can do:

• Scan every file in /dev looking for a block device whose st_rdev is the same number
• Parse the table of mounted filesystem (/etc/mtab or /etc/mnttab, it’s system-dependant) looking for a mount prefix that looks like it might be a parent of the pathname you’re interested in.