go-delve/delve · error

can not set %s

Error message

can not set %s

What it means

After handling general-purpose registers, the amd64 SetReg only supports writing XMM0-XMM15 floating-point/SIMD registers. Any other register number (e.g. x87 st registers, segment descriptor bases, debug registers reachable only through other paths) is rejected with this error.

Source

Thrown at pkg/proc/linutil/regs_amd64_arch.go:218

	if p != nil {
		if reg.Bytes != nil && len(reg.Bytes) != 8 {
			return false, fmt.Errorf("wrong number of bytes for register %s (%d)", regnum.AMD64ToName(regNum), len(reg.Bytes))
		}
		*p = reg.Uint64Val
		return false, nil
	}

	if r.loadFpRegs != nil {
		err := r.loadFpRegs(r)
		if err != nil {
			return false, err
		}
		r.loadFpRegs = nil
	}

	if regNum < regnum.AMD64_XMM0 || regNum > regnum.AMD64_XMM0+15 {
		return false, fmt.Errorf("can not set %s", regnum.AMD64ToName(regNum))
	}

	reg.FillBytes()

	err := r.Fpregset.SetXmmRegister(int(regNum-regnum.AMD64_XMM0), reg.Bytes)
	if err != nil {
		return false, err
	}
	return true, nil
}

View on GitHub (pinned to a23773e6c3)

Solutions

1. Only call SetReg for registers it supports (GPRs and XMM0-XMM15); reject or no-op unsupported registers in the caller
2. Use the appropriate backend API if the register must be modified (e.g. ptrace POKEUSER on a specific offset, or a debugger feature that supports it)
3. Verify the register number constant belongs to regnum.AMD64_* and not another architecture

Example fix

// before
thread.SetReg(regnum.AMD64_ST0, reg) // unsupported
// after
if regnum.AMD64_XMM0 <= regNum && regNum <= regnum.AMD64_XMM0+15 {
    thread.SetReg(regNum, reg)
} else {
    return fmt.Errorf("register %s is not settable via SetReg", regnum.AMD64ToName(regNum))
}

if regNum < regnum.AMD64_XMM0 || regNum > regnum.AMD64_XMM0+15 {
    isGPR := false // check against the supported GPR set first
    if !isGPR {
        return fmt.Errorf("register %s is not settable", regnum.AMD64ToName(regNum))
    }
}

func isSettableAMD64(regNum regnum.AMD64) bool {
    return regNum <= regnum.AMD64_Rflags || (regNum >= regnum.AMD64_XMM0 && regNum <= regnum.AMD64_XMM0+15)
}

if err := thread.SetReg(regNum, reg); err != nil {
    var unsupported *regUnsupportedError
    if strings.Contains(err.Error(), "can not set") {
        return fmt.Errorf("skipping non-settable register %s", regnum.AMD64ToName(regNum))
    }
    return err
}

When it happens

Trigger: Calling SetReg on Linux/amd64 with a register number outside the XMM0..XMM0+15 range and outside the general-purpose switch handled earlier (where p was nil and no fallback exists).

Common situations: Attempting to modify AVX YMM/ZMM registers, x87 control registers, or debug registers through the ptrace-backed SetReg path; using a register-number constant from a different architecture by mistake.

Related errors

• could not restore RFLAGS register: %v
• wrong number of bytes for register %s (%d)
• can not change register values of core process
• read out of bounds
• write out of bounds

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/73bac9bfdd5b58e6. Report an issue: GitHub.