firecracker-microvm/firecracker · error

Failed to clone API event FD

Error message

Failed to clone API event FD

What it means

Error "Failed to clone API event FD" thrown in firecracker-microvm/firecracker.

Source

Thrown at src/firecracker/src/api_server_adapter.rs:178

    pci_enabled: bool,
    api_payload_limit: usize,
    mmds_size_limit: usize,
    metadata_json: Option<&str>,
) -> Result<(), ApiServerError> {
    // FD to notify of API events. This is a blocking eventfd by design.
    // It is used in the config/pre-boot loop which is a simple blocking loop
    // which only consumes API events.
    let api_event_fd = EventFd::new(libc::EFD_SEMAPHORE).expect("Cannot create API Eventfd.");
    // FD used to signal API thread to stop/shutdown.
    let api_kill_switch = EventFd::new(libc::EFD_NONBLOCK).expect("Cannot create API kill switch.");

    // Channels for both directions between Vmm and Api threads.
    let (to_vmm, from_api) = channel();
    let (to_api, from_vmm) = channel();

    let to_vmm_event_fd = api_event_fd
        .try_clone()
        .expect("Failed to clone API event FD");
    let api_seccomp_filter = seccomp_filters
        .remove("api")
        .expect("Missing seccomp filter for API thread.");

    let mut server = match HttpServer::new(&bind_path) {
        Ok(s) => s,
        Err(ServerError::IOError(inner)) if inner.kind() == std::io::ErrorKind::AddrInUse => {
            let sock_path = bind_path.display().to_string();
            return Err(ApiServerError::FailedToBindSocket(sock_path));
        }
        Err(err) => {
            return Err(ApiServerError::FailedToBindAndRunHttpServer(err));
        }
    };
    info_unrestricted!("Listening on API socket ({bind_path:?}).");

    let api_kill_switch_clone = api_kill_switch
        .try_clone()

View on GitHub (pinned to 0a745def42)

Solutions

1. Check the process file-descriptor limit; cloning the event FD fails when no fds are available.
2. Verify the source eventfd is still open at the time of the clone.

When it happens

Trigger: Thrown at src/firecracker/src/api_server_adapter.rs:178 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/2ec2c44deb2455da. Report an issue: GitHub.