From d96ea18b8fc41f6a3a4a842d81fa75da7e794f64 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Fri, 5 Jun 2026 04:30:40 +0300
Subject: [PATCH] cgen: use union tag for C union keepalive helpers (#27351)

---
 vlib/v/gen/c/cgen.v                           |  3 ++-
 .../boehm_keepalive_c_union_tag.c.must_have   |  2 ++
 .../c/testdata/boehm_keepalive_c_union_tag.h  | 13 ++++++++++
 .../c/testdata/boehm_keepalive_c_union_tag.vv | 25 +++++++++++++++++++
 4 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.h
 create mode 100644 vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.vv

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 29ad8a704..15f47b0d5 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -2624,7 +2624,8 @@ fn (mut g Gen) cc_type(typ ast.Type, is_prefix_struct bool) string {
 			if info.is_anon {
 				styp = 'C__' + styp
 			} else if !info.is_typedef {
-				styp = 'struct ${styp}'
+				tag := if info.is_union { 'union' } else { 'struct' }
+				styp = '${tag} ${styp}'
 			}
 		}
 	}
diff --git a/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.c.must_have b/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.c.must_have
new file mode 100644
index 000000000..70505b96e
--- /dev/null
+++ b/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.c.must_have
@@ -0,0 +1,2 @@
+union my_data* it, voidptr* out, int idx)
+struct my_event* it, voidptr* out, int idx)
diff --git a/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.h b/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.h
new file mode 100644
index 000000000..a0a1e70e3
--- /dev/null
+++ b/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.h
@@ -0,0 +1,13 @@
+#include <stdint.h>
+
+typedef union my_data {
+	void *ptr;
+	int fd;
+	uint32_t u32;
+	uint64_t u64;
+} my_data_t;
+
+struct my_event {
+	uint32_t events;
+	union my_data data;
+};
diff --git a/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.vv b/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.vv
new file mode 100644
index 000000000..9f0f742ad
--- /dev/null
+++ b/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.vv
@@ -0,0 +1,25 @@
+// vtest vflags: -prod -gc boehm_full_opt
+#include "@VMODROOT/vlib/v/gen/c/testdata/boehm_keepalive_c_union_tag.h"
+
+union C.my_data {
+mut:
+	ptr voidptr
+	fd  int
+}
+
+struct C.my_event {
+mut:
+	events u32
+	data   C.my_data
+}
+
+fn consume(n int) {
+	_ = n
+}
+
+fn main() {
+	mut ev := C.my_event{}
+	ev.data.fd = 42
+	fd := unsafe { ev.data.fd }
+	consume(fd)
+}
-- 
2.39.5