could not load swiss table groups lengthMask field: %v

Error message

could not load swiss table groups lengthMask field: %v

What it means

Set on a map Variable while Delve reads the 'lengthMask' field (group count mask) of the swiss map groups struct. groups.toField(it.groupsFieldLengthMask) failed to resolve or read the field. Delve throws it because it needs lengthMask to compute the number of groups ([lengthMask+1]group) for the fake array type used during iteration.

Source

Thrown at pkg/proc/mapiter.go:635

	if err != nil {
		it.v.Unreadable = fmt.Errorf("could not load swiss table index: %v", err)
		return
	}

	groups, err2 := tab.toField(it.tableFieldGroups)
	if err2 != nil {
		it.v.Unreadable = fmt.Errorf("could not load swiss table groups field: %v", err2)
		return
	}
	r.groups, err2 = groups.toField(it.groupsFieldData)
	if err2 != nil {
		it.v.Unreadable = fmt.Errorf("could not load swiss table groups data: %v", err2)
		return
	}

	field, err2 = groups.toField(it.groupsFieldLengthMask)
	if err2 != nil {
		it.v.Unreadable = fmt.Errorf("could not load swiss table groups lengthMask field: %v", err2)
		return
	}
	groupsLengthMask, err := field.asUint()
	if err != nil {
		it.v.Unreadable = fmt.Errorf("could not load swiss table group lengthMask: %v", err)
		return
	}

	// convert the type of groups from *group to *[len]group so that it's easier to use
	r.groups.DwarfType = pointerTo(fakeArrayType(groupsLengthMask+1, it.groupType), it.v.bi.Arch)
	r.groups.RealType = r.groups.DwarfType
	r.groups = r.groups.maybeDereference()

	if r.groups.Addr == 0 {
		it.v.Unreadable = errSwissTableNilGroups
		return
	}

View on GitHub (pinned to a23773e6c3)

Solutions

Upgrade Delve so its expected groups struct fields match your Go runtime
Rebuild without debug-info stripping flags (-w, -s)
Confirm the target binary's Go version with 'go version <binary>'
Inspect the map through alternate means (fmt.Println in the program, len())

Defensive patterns

Strategy: type-guard

Validate before calling

if v.Unreadable != nil { return v.Unreadable } // swiss map fields unresolvable; upgrade Delve/rebuild target

Type guard

func inspectable(v *proc.Variable) bool { return v != nil && v.Unreadable == nil && v.Kind == reflect.Map }

Try / catch

if v.Unreadable != nil { fallbackToLenOnlyView(v); return }

Prevention

Pin Go versions to those listed as supported by your Delve build
Never strip DWARF from binaries you intend to debug
Test map inspection on a trivial program after toolchain upgrades
Treat Unreadable as a first-class error in all variable-handling code paths

When it happens

Trigger: Iterating a swiss map when the maps.groups struct in the debuggee has no 'lengthMask' field resolvable via DWARF, or reading that field's memory fails.

Common situations: Go version where the groups struct field is named or laid out differently; DWARF truncated by build flags; attaching to a process built by a newer Go than Delve supports.

Related errors

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/be3fa0b709768a7d. Report an issue: GitHub.