go-delve/delve · error

could not read string len %s

Error message

could not read string len %s

What it means

While reading a Go string header (the `stringStructDWARF` layout), Delve could not read the `len` field from target memory via `readIntRaw`. The underlying memory-read error is embedded in the message. Without the length, the string value cannot be loaded.

Source

Thrown at pkg/proc/variables.go:1539

	return dstv.writeEmptyInterface(typeAddr, srcv)
}

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
}

View on GitHub (pinned to a23773e6c3)

Solutions

1. Read the embedded cause (%s) to see the actual memory error.
2. Re-evaluate the variable once the target is fully stopped and live.
3. For core dumps, ensure the memory containing the string header was dumped.
4. Verify the variable address is valid by printing the enclosing struct.

// (dlv) print str                 // see if the header itself is readable
// (dlv) print uintptr(str.str)    // verify data pointer

// Client-side: retry evaluation once the target is fully stopped
if strings.Contains(err.Error(), "could not read string len") {
    time.Sleep(50 * time.Millisecond)
    return evalVariable(expr) // retry after state settles
}

When it happens

Trigger: Evaluating any string variable or string field when the read of the `len` word at `addr+ByteOffset` fails; e.g. the string header address is invalid or the memory region is inaccessible.

Common situations: Inspecting strings in core dumps where the containing page was not captured; reading strings through a partially-connected remote session; evaluating variables whose addresses come from stale/unreadable stack frames.

Related errors

• could not read string pointer %s
• could not read string at %#v due to %s
• parent pointer unreadable: %w
• invalid length: %d
• short read

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/4a72a1a734ea4898. Report an issue: GitHub.