From 97e894747f27f4076891b66421460465b486c2b7 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 06:24:06 +0300
Subject: [PATCH] cgen: fix invalid memory access printing union containing
 interface member (fixes #25953)

---
 vlib/v/gen/c/cgen.v                           | 12 ++++---
 .../c/testdata/iface_method_order.c.must_have |  3 +-
 .../union_implementing_interface_test.v       | 33 +++++++++++++++++++
 3 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 281b49a97..710a1ecb9 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -11632,10 +11632,12 @@ fn (mut g Gen) interface_table() string {
 			// msvc can not process `static struct x[0] = {};`
 			methods_struct.writeln('${g.static_modifier}${methods_struct_name} ${interface_name}_name_table[1];')
 		} else {
+			name_table_len := iname_table_length + 1
 			if g.pref.build_mode != .build_module {
-				methods_struct.writeln('${g.static_modifier}${methods_struct_name} ${interface_name}_name_table[${iname_table_length}] = {')
+				methods_struct.writeln('${g.static_modifier}${methods_struct_name} ${interface_name}_name_table[${name_table_len}] = {')
+				methods_struct.writeln('\t{0},')
 			} else {
-				methods_struct.writeln('${g.static_modifier}${methods_struct_name} ${interface_name}_name_table[${iname_table_length}];')
+				methods_struct.writeln('${g.static_modifier}${methods_struct_name} ${interface_name}_name_table[${name_table_len}];')
 			}
 		}
 		mut cast_functions := strings.new_builder(100)
@@ -11683,7 +11685,9 @@ fn (mut g Gen) interface_table() string {
 			$if debug_interface_table ? {
 				eprintln('>> interface name: ${isym.name} | concrete type: ${st.debug()} | st symname: ${st_sym.name}')
 			}
-			// Speaker_Cat_index = 0
+			// Reserve interface type index 0 as an invalid/uninitialized sentinel.
+			// That keeps stray bytes from overlapping storage, like unions, from
+			// aliasing a valid concrete interface variant.
 			interface_index_name := '_${interface_name}_${cctype}_index'
 			if already_generated_mwrappers[interface_index_name] > 0 {
 				continue
@@ -12021,7 +12025,7 @@ return ${cast_shared_struct_str};
 			if g.pref.build_mode != .build_module {
 				methods_struct.writeln('\t},')
 			}
-			iin_idx := already_generated_mwrappers[interface_index_name] - iinidx_minimum_base
+			iin_idx := already_generated_mwrappers[interface_index_name] - iinidx_minimum_base + 1
 			if g.pref.build_mode != .build_module {
 				sb.writeln('${g.static_modifier}const u32 ${interface_index_name} = ${iin_idx};')
 			} else {
diff --git a/vlib/v/gen/c/testdata/iface_method_order.c.must_have b/vlib/v/gen/c/testdata/iface_method_order.c.must_have
index b0540a9ad..a50639861 100644
--- a/vlib/v/gen/c/testdata/iface_method_order.c.must_have
+++ b/vlib/v/gen/c/testdata/iface_method_order.c.must_have
@@ -2,7 +2,8 @@ struct _main__Foo_interface_methods {
 void (*_method_a)(void* _);
 void (*_method_b)(void* _);
 };
-struct _main__Foo_interface_methods main__Foo_name_table[3] = {
+struct _main__Foo_interface_methods main__Foo_name_table[4] = {
+{0},
 {
 ._method_a = main__Bar_a_Interface_main__Foo_method_adapter,
 ._method_b = main__Bar_b_Interface_main__Foo_method_adapter,
diff --git a/vlib/v/tests/unions/union_implementing_interface_test.v b/vlib/v/tests/unions/union_implementing_interface_test.v
index 00134d476..295b685bf 100644
--- a/vlib/v/tests/unions/union_implementing_interface_test.v
+++ b/vlib/v/tests/unions/union_implementing_interface_test.v
@@ -18,3 +18,36 @@ fn test_union_implementing_interface() {
 	})
 	assert s.speak() == 'hi, u.vint: 123'
 }
+
+interface Any {}
+
+struct Thing {}
+
+union Bad {
+	f f64
+	a Any
+}
+
+union BadBits {
+	f   f64
+	bad Bad
+}
+
+fn bad_from_float(x f64) Bad {
+	bits := BadBits{
+		f: x
+	}
+	return unsafe { bits.bad }
+}
+
+fn test_union_str_with_invalid_interface_member_does_not_crash() {
+	x := [bad_from_float(2.0)]
+	assert x.str() == '[Bad{\n    f: 2.0\n    a: unknown interface value\n}]'
+}
+
+fn test_union_str_with_valid_interface_member_still_works() {
+	x := Bad{
+		a: Thing{}
+	}
+	assert x.str().contains('a: Any(Thing{})')
+}
-- 
2.39.5