unmount USB drive in C++

Question

I am developing a Qt application for Linux (Ubuntu) in which I use USB drive to backup some content. Application should unmount the target drive after copying stuff. I have a udev rule file to mount USB at a specific location with ENV{mount_options}="relatime,users,umask=0,uid=user,gid=user" where user represents my user name.

I tried using this without any luck.

const char* usb = "/mnt/mountpoint/usbdrive";
if (!umount(usb))
{
  qDebug() << "Device unmounted";
}
else
{
  qDebug() << "Can't unmount" << strerror(errno); //this prints Operation not permitted
}

Could someone please help me here? Am I using umount right?

Thanks in advance.

Answer 1

Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to unmount file systems.

Per umount the code is fine. However you need privilege to umount devices.

The CAP_SYS_ADMIN capability allows a process to perform various administrative tasks, namely: calling mount(), umount(). There are two worth articles about capabilities here:

• CAP_SYS_ADMIN: the new root
• Overview of Linux capabilities (List of capabilities)