valyala/fasthttp · error

cannot mark listening socket nonblocking

Error message

cannot mark listening socket nonblocking: %w

What it means

On z/OS s390x, socket creation and the FD_CLOEXEC fcntl succeeded, but unix.FcntlInt(fd, F_SETFL, O_NONBLOCK) failed. The library requires a non-blocking listening socket and therefore closes the fd and returns this wrapped error, aborting NewListener.

Solutions

1. Inspect the wrapped errno to identify EBADF vs EPERM vs EINVAL
2. Confirm security software (RACF/ACF2 profiles) permits F_SETFL on socket descriptors
3. Raise fd limits and avoid fd exhaustion races
4. Pin a newer golang.org/x/sys and Go toolchain with improved z/OS support

null

null

ln, err := cfg.NewListener("tcp", addr)
if err != nil {
    if strings.Contains(err.Error(), "cannot mark listening socket nonblocking") {
        var errno syscall.Errno
        if errors.As(err, &errno) {
            log.Printf("F_SETFL O_NONBLOCK failed: %v — verify security policy permits F_SETFL", errno)
        }
        return err
    }
}

When it happens

Trigger: Calling NewListener on z/OS s390x when fcntl(F_SETFL, O_NONBLOCK) on the listening socket returns an error (EBADF, EINVAL, or EPERM under restrictive policies).

Common situations: z/OS systems with security software blocking F_SETFL on sockets; rare fd-table races after near-exhaustion; outdated Go z/OS ports with fcntl quirks.

Related errors

• cannot mark listening socket close-on-exec
• cannot create listening socket
• cannot make non-blocked listening socket
• cannot create listening unblocked socket
• cannot enable so_reuseaddr

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/4a62d36355bb5a13. Report an issue: GitHub.

Appendix: source

Thrown at tcplisten/socket_zos_s390x.go:24

	"fmt"

	"golang.org/x/sys/unix"
)

func newSocketCloexec(domain, typ, proto int) (int, error) {
	fd, err := unix.Socket(domain, typ, proto)
	if err != nil {
		return -1, fmt.Errorf("cannot create listening socket: %w", err)
	}
	_, err = unix.FcntlInt(uintptr(fd), unix.F_SETFD, unix.FD_CLOEXEC)
	if err != nil {
		unix.Close(fd) //nolint:errcheck
		return -1, fmt.Errorf("cannot mark listening socket close-on-exec: %w", err)
	}
	_, err = unix.FcntlInt(uintptr(fd), unix.F_SETFL, unix.O_NONBLOCK)
	if err != nil {
		unix.Close(fd) //nolint:errcheck
		return -1, fmt.Errorf("cannot mark listening socket nonblocking: %w", err)
	}
	return fd, nil
}

View on GitHub (pinned to c96f600972)