{"record":{"id":"33da26fcb90905a1","repo":"firecracker-microvm/firecracker","slug":"timerfd-read-failed-err","errorCode":null,"errorMessage":"TimerFd read failed: {err:#}","messagePattern":"TimerFd read failed: (.+?)","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"error","filePath":"src/utils/src/time.rs","lineNumber":299,"sourceCode":"            std::io::Error::last_os_error()\n        );\n    }\n\n    /// Read the value from the timer. Since a real timer is always created with the NONBLOCK\n    /// flag, this function does not block and returns `0` if the timer has not fired. For a mock\n    /// timer it returns the number of expirations that have occurred (per the virtual clock)\n    /// since the previous read.\n    pub fn read(&mut self) -> u64 {\n        let file = match self {\n            TimerFd::Real(file) => file,\n            #[cfg(any(test, feature = \"test-utils\"))]\n            TimerFd::Mock(mock) => return mock.read(),\n        };\n        let mut buf = [0u8; size_of::<u64>()];\n        match file.read(buf.as_mut_slice()) {\n            Ok(_) => u64::from_ne_bytes(buf),\n            Err(inner) if inner.kind() == ErrorKind::WouldBlock => 0,\n            Err(err) => panic!(\"TimerFd read failed: {err:#}\"),\n        }\n    }\n\n    /// Tell if the timer is currently armed.\n    pub fn is_armed(&self) -> bool {\n        let file = match self {\n            TimerFd::Real(file) => file,\n            #[cfg(any(test, feature = \"test-utils\"))]\n            TimerFd::Mock(mock) => return mock.is_armed(),\n        };\n        // SAFETY: Zero init of a PDO type.\n        let mut spec: libc::itimerspec = unsafe { std::mem::zeroed() };\n        // SAFETY: Safe because timerfd_gettime is trusted to only modify `spec`.\n        let ret = unsafe { libc::timerfd_gettime(file.as_raw_fd(), &mut spec) };\n        assert!(\n            0 <= ret,\n            \"TimerFd arm failed: {:#}\",\n            std::io::Error::last_os_error()","sourceCodeStart":281,"sourceCodeEnd":317,"githubUrl":"https://github.com/firecracker-microvm/firecracker/blob/23b09b943fa3cd4a04004fe55724f0b5c2410f45/src/utils/src/time.rs#L281-L317","documentation":"Reading the underlying timerfd file descriptor inside TimerFd::read failed with an I/O error other than EAGAIN/WouldBlock (which is normally swallowed and reported as 0 expirations, since the fd is opened NONBLOCK). This means the kernel refused or failed the read — typically because the fd is invalid/closed, or an unexpected OS-level condition occurred — so the number of expirations could not be retrieved. It fires as a panic at src/utils/src/time.rs:299 whenever file.read() on the real timerfd returns any error other than WouldBlock.","triggerScenarios":"Thrown at src/utils/src/time.rs:252 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":["Check that the timerfd was created successfully and not closed before read.","Handle EAGAIN on non-blocking timerfd reads and re-arm the timer correctly."],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"23b09b943fa3cd4a04004fe55724f0b5c2410f45","analyzedAt":"2026-09-19T00:01:22.326Z","contentChangedAt":"2026-09-19T00:01:22.326Z","schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}