From 4b1c3d25b6b2d9c4f765b594594c88e174f625d0 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:25 +0300
Subject: [PATCH] checker: fix undeclared c constant not being checked (fixes
 #15893)

---
 vlib/builtin/cfns.c.v                                     | 8 ++++++++
 vlib/net/http/backend_vschannel_windows.c.v               | 2 ++
 vlib/sync/stdatomic/1.declarations.c.v                    | 4 ++++
 vlib/v/checker/checker.v                                  | 5 +++++
 vlib/v/checker/tests/undeclared_c_identifier_err.out      | 6 ++++++
 vlib/v/checker/tests/undeclared_c_identifier_err.vv       | 5 +++++
 vlib/v/pref/arch.c.v                                      | 2 ++
 .../array_ops_create_just_one_closure_test.c.v            | 2 ++
 vlib/v/tests/project_with_c_code_ct_ifs/ctimeifblock.c.v  | 2 ++
 9 files changed, 36 insertions(+)
 create mode 100644 vlib/v/checker/tests/undeclared_c_identifier_err.out
 create mode 100644 vlib/v/checker/tests/undeclared_c_identifier_err.vv

diff --git a/vlib/builtin/cfns.c.v b/vlib/builtin/cfns.c.v
index 6de56fdaa..d4e797b2a 100644
--- a/vlib/builtin/cfns.c.v
+++ b/vlib/builtin/cfns.c.v
@@ -3,6 +3,14 @@ module builtin
 @[typedef]
 pub struct C.FILE {}
 
+// Virtual C globals that are available through libc or generated C headers.
+__global C.errno int
+__global C.stdin &C.FILE
+__global C.stdout &C.FILE
+__global C.stderr &C.FILE
+__global C.environ &&char
+__global C._wyp &u64
+
 // <string.h>
 fn C.memcpy(dest voidptr, const_src voidptr, n usize) voidptr
 
diff --git a/vlib/net/http/backend_vschannel_windows.c.v b/vlib/net/http/backend_vschannel_windows.c.v
index 88e13b930..c6ea57b71 100644
--- a/vlib/net/http/backend_vschannel_windows.c.v
+++ b/vlib/net/http/backend_vschannel_windows.c.v
@@ -9,6 +9,8 @@ module http
 
 pub struct C.TlsContext {}
 
+const C.vsc_init_resp_buff_size int
+
 fn C.new_tls_context() C.TlsContext
 
 fn vschannel_ssl_do(req &Request, port int, method Method, host_name string, path string) !Response {
diff --git a/vlib/sync/stdatomic/1.declarations.c.v b/vlib/sync/stdatomic/1.declarations.c.v
index 293845e52..9a8760c21 100644
--- a/vlib/sync/stdatomic/1.declarations.c.v
+++ b/vlib/sync/stdatomic/1.declarations.c.v
@@ -112,6 +112,10 @@ fn C.atomic_fetch_sub_u64(voidptr, u64) u64
 fn C.atomic_thread_fence(i32)
 fn C.cpu_relax()
 
+pub const C.memory_order_relaxed i32
+pub const C.memory_order_acquire i32
+pub const C.memory_order_release i32
+
 fn C.ANNOTATE_RWLOCK_CREATE(voidptr)
 fn C.ANNOTATE_RWLOCK_ACQUIRED(voidptr, i32)
 fn C.ANNOTATE_RWLOCK_RELEASED(voidptr, i32)
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 2483d1b05..cf1184eef 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -5820,6 +5820,11 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 		if x := c.table.global_scope.find_const(node.name) {
 			return x.typ
 		}
+		c_name := node.name.all_after('C.')
+		if !c.pref.translated && !c.file.is_translated && c_name.to_upper() != c_name {
+			c.error('undefined C identifier: `${node.name}`', node.pos)
+			return ast.int_type
+		}
 		return ast.int_type
 	}
 	if c.inside_sql {
diff --git a/vlib/v/checker/tests/undeclared_c_identifier_err.out b/vlib/v/checker/tests/undeclared_c_identifier_err.out
new file mode 100644
index 000000000..e8ce73d37
--- /dev/null
+++ b/vlib/v/checker/tests/undeclared_c_identifier_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/undeclared_c_identifier_err.vv:4:12: error: undefined C identifier: `C.errono`
+    2 | 
+    3 | fn main() {
+    4 |     println(C.errono)
+      |               ~~~~~~
+    5 | }
diff --git a/vlib/v/checker/tests/undeclared_c_identifier_err.vv b/vlib/v/checker/tests/undeclared_c_identifier_err.vv
new file mode 100644
index 000000000..e17f21819
--- /dev/null
+++ b/vlib/v/checker/tests/undeclared_c_identifier_err.vv
@@ -0,0 +1,5 @@
+#include <errno.h>
+
+fn main() {
+	println(C.errono)
+}
diff --git a/vlib/v/pref/arch.c.v b/vlib/v/pref/arch.c.v
index 19610d28c..38f2ee7bd 100644
--- a/vlib/v/pref/arch.c.v
+++ b/vlib/v/pref/arch.c.v
@@ -1,5 +1,7 @@
 module pref
 
+const C.__V_architecture int
+
 pub enum Arch {
 	_auto
 	amd64 // aka x86_64
diff --git a/vlib/v/tests/builtin_arrays/array_ops_create_just_one_closure_test.c.v b/vlib/v/tests/builtin_arrays/array_ops_create_just_one_closure_test.c.v
index 470377605..aa3a84109 100644
--- a/vlib/v/tests/builtin_arrays/array_ops_create_just_one_closure_test.c.v
+++ b/vlib/v/tests/builtin_arrays/array_ops_create_just_one_closure_test.c.v
@@ -2,6 +2,8 @@ struct C.builtin__closure__Closure {
 	closure_cap int
 }
 
+__global C.g_closure voidptr
+
 fn setup(fname string) (int, int, []int) {
 	println(fname)
 	return unsafe { &C.builtin__closure__Closure(voidptr(&C.g_closure)).closure_cap }, 42, []int{len: 5, init: index * 5}
diff --git a/vlib/v/tests/project_with_c_code_ct_ifs/ctimeifblock.c.v b/vlib/v/tests/project_with_c_code_ct_ifs/ctimeifblock.c.v
index e29362e80..cfe964af4 100644
--- a/vlib/v/tests/project_with_c_code_ct_ifs/ctimeifblock.c.v
+++ b/vlib/v/tests/project_with_c_code_ct_ifs/ctimeifblock.c.v
@@ -6,6 +6,8 @@ $if !linux {
 	#include "@VMODROOT/a_nonlinux.h"
 }
 
+__global C.version &char
+
 fn main() {
 	C.printf(c'a: %s\n', C.version)
 }
-- 
2.39.5