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I'm using libudev to detect when a USB drive is attached in a C program. Once I detect that it has been attached via a signal from the kernel (through a udev_monitor structure), can I assume that systemd has mounted the drive?

If not, how can I deterministically wait until the drive has mounted i.e. not wait an arbitrary time X seconds, but wait on a specific event.

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  • The udev signal from the kernel is given both to systemd and your C program, so that's a race condition, and you can't assume it is mounted by the time you receive the signal. I don't know if there's a standard mechanism to get a "mounted" signal, or have systemd deliver a custom signal to you. – dirkt Sep 28 '19 at 20:01
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It turns out that it's best not to listen for USB device attach events, but mount point attach events instead. In other words, instead of filtering on "usb" subsystem and "usb_device" devtype, use "block" subsystem and "partition" devtype, then check that the event contains ID_BUS="usb".

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