firecracker-microvm/firecracker · error

TimerFd read failed: {err:#}

Error message

TimerFd read failed: {err:#}

What it means

Error "TimerFd read failed: {err:#}" thrown in firecracker-microvm/firecracker.

Source

Thrown at src/utils/src/time.rs:252

            },
        };
        // SAFETY: Safe because this doesn't modify any memory and we check the return value.
        let ret = unsafe { libc::timerfd_settime(self.as_raw_fd(), 0, &spec, ptr::null_mut()) };
        assert!(
            0 <= ret,
            "TimerFd arm failed: {:#}",
            std::io::Error::last_os_error()
        );
    }

    /// Read the value from the timerfd. Since it is always created with NONBLOCK flag,
    /// this function does not block.
    pub fn read(&mut self) -> u64 {
        let mut buf = [0u8; size_of::<u64>()];
        match self.0.read(buf.as_mut_slice()) {
            Ok(_) => u64::from_ne_bytes(buf),
            Err(inner) if inner.kind() == ErrorKind::WouldBlock => 0,
            Err(err) => panic!("TimerFd read failed: {err:#}"),
        }
    }

    /// Tell if the timer is currently armed.
    pub fn is_armed(&self) -> bool {
        // SAFETY: Zero init of a PDO type.
        let mut spec: libc::itimerspec = unsafe { std::mem::zeroed() };
        // SAFETY: Safe because timerfd_gettime is trusted to only modify `spec`.
        let ret = unsafe { libc::timerfd_gettime(self.as_raw_fd(), &mut spec) };
        assert!(
            0 <= ret,
            "TimerFd arm failed: {:#}",
            std::io::Error::last_os_error()
        );
        spec.it_value.tv_sec != 0 || spec.it_value.tv_nsec != 0
    }
}

View on GitHub (pinned to 0a745def42)

Solutions

1. Check that the timerfd was created successfully and not closed before read.
2. Handle EAGAIN on non-blocking timerfd reads and re-arm the timer correctly.

When it happens

Trigger: Thrown at src/utils/src/time.rs:252 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of firecracker-microvm/firecracker@0a745def42 (2026-08-19). Data as JSON: /api/errors/33da26fcb90905a1. Report an issue: GitHub.