go-delve/delve · error

could not load swiss table group lengthMask: %v

Error message

could not load swiss table group lengthMask: %v

What it means

Set on a map Variable when the lengthMask field was found but field.asUint() failed to convert it to an unsigned integer. Delve expects lengthMask to be a plain unsigned integer type; any other DWARF type (or an unreadable value) makes the group-count computation impossible. Delve throws it because it must build a fake *[N]group type to walk the groups array.

Source

Thrown at pkg/proc/mapiter.go:640

	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
	}

	it.tab = r
}

// loadCurrentGroup loads the group at index it.groupIdx of it.tab into it.group
func (it *mapIteratorSwiss) loadCurrentGroup() {

View on GitHub (pinned to a23773e6c3)

Solutions

1. Update Delve to the version matching your Go toolchain
2. Rebuild the binary with a supported Go release and default debug flags
3. Check for post-processing of the binary (strip, objcopy) that damages DWARF and remove it
4. Use program-side logging to inspect the map instead

Example fix

// before: stripped binary
strip app && dlv exec app
// after: keep symbols
dlv exec app

if v.Unreadable != nil { return } // do not attempt to enumerate children of an unreadable map

func safeChildren(v *proc.Variable) ([]*proc.Variable, bool) { if v == nil || v.Unreadable != nil { return nil, false }; return v.Children, true }

if err := v.Unreadable; err != nil { log.Debug(err); return nil, err }

When it happens

Trigger: Iterating a swiss map where loadCurrentTable resolved lengthMask but its DWARF type is not a uint kind (e.g. changed type in a new Go runtime, or bitfield/typedef mismatch).

Common situations: Newer/older Go toolchain with altered internal/runtime/maps types; type info corrupted by aggressive DWARF compression or post-link tools; Delve version lagging behind the Go release used to build the binary.

Related errors

• could not load swiss table index: %v
• could not load swiss table groups field: %v
• could not load swiss table groups data: %v
• could not load swiss table groups lengthMask field: %v
• could not load swiss map group ctrl: %v

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/5deed04ef67da19a. Report an issue: GitHub.