What causes "invalid length: %d" in go-delve/delve?

Verify the expression actually addresses a string (print the parent struct/value).

invalid length: %d

Error message

invalid length: %d

What it means

After reading a string's `len` field from target memory, the value was negative. Go strings can never have negative lengths, so Delve rejects the header as corrupt rather than reading (or clamping) the string. This is a consistency check inside string header loading.

Source

Thrown at pkg/proc/variables.go:1542

func readStringInfo(mem MemoryReadWriter, arch *Arch, addr uint64, typ *godwarf.StringType) (uint64, int64, error) {
	// string data structure is always two ptrs in size. Addr, followed by len
	// https://research.swtch.com/godata

	mem = cacheMemory(mem, addr, arch.PtrSize()*2)

	var strlen int64
	var outaddr uint64
	var err error

	for _, field := range typ.StructType.Field {
		switch field.Name {
		case "len":
			strlen, err = readIntRaw(mem, addr+uint64(field.ByteOffset), int64(arch.PtrSize()))
			if err != nil {
				return 0, 0, fmt.Errorf("could not read string len %s", err)
			}
			if strlen < 0 {
				return 0, 0, fmt.Errorf("invalid length: %d", strlen)
			}
		case "str":
			outaddr, err = readUintRaw(mem, addr+uint64(field.ByteOffset), int64(arch.PtrSize()))
			if err != nil {
				return 0, 0, fmt.Errorf("could not read string pointer %s", err)
			}
			if addr == 0 {
				return 0, 0, nil
			}
		}
	}

	return outaddr, strlen, nil
}

func readStringValue(mem MemoryReadWriter, addr uint64, strlen int64, cfg LoadConfig) (string, error) {
	if strlen == 0 {
		return "", nil

View on GitHub (pinned to a23773e6c3)

Solutions

Verify the expression actually addresses a string (print the parent struct/value).
Inspect raw memory at the address to confirm header contents.
If memory is corrupted, find the code that overwrote it (this is a bug in the debugged program).

Defensive patterns

Strategy: validation

Validate before calling

// Verify the expression targets a real string:
// (dlv) print parent.strField   // then inspect len
// A negative len means the memory is not a valid string header.

Prevention

Confirm the evaluated address actually holds a string header (print the parent).
Treat negative len as a sign of memory corruption in the debugged program.
Avoid evaluating through stale or miscomputed pointer arithmetic.

When it happens

Trigger: Evaluating a string whose memory-resident `len` word is negative, i.e. the high bit of the pointer-size integer is set; typically means the header bytes were misread or the memory does not actually contain a string header.

Common situations: Reading misaligned or corrupted stack memory; evaluating through an incorrect expression that lands on non-string bytes; unsafe code that overwrote a string header.

Related errors

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/40820beffdf9278c. Report an issue: GitHub.