prometheus/node_exporter · error

length of bytes received from sysctl

Error message

length of bytes received from sysctl (%d) does not match expected bytes (long: %d), (int: %d)

What it means

BSD sysctl validation guard in getCLong: unix.SysctlRaw returned a byte buffer whose length is neither sizeof(long) nor sizeof(int), so the raw value cannot be safely reinterpreted as a numeric sysctl. This indicates a kernel/platform returning an unexpected size for MIB entry b.mib, or a sysctl whose value is not scalar numeric at all.

Solutions

  1. Check the sysctl type: `sysctl -n <name>` / `sysctl -x` to see whether it is truly a long/int or a string/opaque value; if so, use the appropriate handler instead of the long getter.
  2. Verify OS/arch support — unusual architectures may return different sizes; report/fix the size handling for that platform.
  3. Update FreeBSD/OS version mappings if the kernel changed the value's representation.
Defensive patterns

Strategy: type-guard

When it happens

Trigger: Thrown at collector/sysctl_bsd.go:108 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of prometheus/node_exporter@17ddd77c59 (2026-09-07). Data as JSON: /api/errors/2f20656887e6b0a9. Report an issue: GitHub.

Appendix: source

Thrown at collector/sysctl_bsd.go:108

func (b bsdSysctl) getCLong() (float64, error) {
	raw, err := unix.SysctlRaw(b.mib)
	if err != nil {
		return 0, err
	}

	if len(raw) == C.sizeof_long {
		return float64(*(*C.long)(unsafe.Pointer(&raw[0]))), nil
	}

	if len(raw) == C.sizeof_int {
		// This is valid for at least vfs.bufspace, and the default
		// long handler - which can clamp longs to 32-bits:
		//   https://github.com/freebsd/freebsd/blob/releng/10.3/sys/kern/vfs_bio.c#L338
		//   https://github.com/freebsd/freebsd/blob/releng/10.3/sys/kern/kern_sysctl.c#L1062
		return float64(*(*C.int)(unsafe.Pointer(&raw[0]))), nil
	}

	return 0, fmt.Errorf(
		"length of bytes received from sysctl (%d) does not match expected bytes (long: %d), (int: %d)",
		len(raw),
		C.sizeof_long,
		C.sizeof_int,
	)

}

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