From eaad37911979ce7b91f3cac7215b034927e57d30 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Sun, 24 May 2026 21:25:57 +0300
Subject: [PATCH] cgen: guard C extern fn decls with #ifndef to coexist with
 macros (fix #27098) (#27236)

---
 vlib/v/gen/c/fn.v                             | 10 ++++++--
 .../c_extern_macro_name_guard_nix.c.must_have |  8 +++++++
 .../testdata/c_extern_macro_name_guard_nix.vv | 24 +++++++++++++++++++
 3 files changed, 40 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.vv

diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index dd412a588..c87e60048 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -1312,7 +1312,11 @@ fn (mut g Gen) gen_fn_decl(node &ast.FnDecl, skip bool) {
 			}
 		}
 	} else if g.inside_c_extern {
-		c_extern_fn_header := 'extern ${type_name} ${fn_attrs}${name.all_after_first('C__')}('
+		c_name := name.all_after_first('C__')
+		// Guard with #ifndef so the declaration is skipped when the C symbol
+		// is actually a preprocessor macro (e.g. WIFSTOPPED from <sys/wait.h>).
+		g.definitions.writeln('#ifndef ${c_name}')
+		c_extern_fn_header := 'extern ${type_name} ${fn_attrs}${c_name}('
 		g.definitions.write_string(c_extern_fn_header)
 	} else {
 		if !(node.is_pub || g.pref.is_debug) {
@@ -1360,7 +1364,9 @@ fn (mut g Gen) gen_fn_decl(node &ast.FnDecl, skip bool) {
 		&& !g.pref.is_test) || skip {
 		// Just a function header. Builtin function bodies are defined in builtin.o
 		g.definitions.writeln(');') // NO BODY')
-		if !g.inside_c_extern {
+		if g.inside_c_extern {
+			g.definitions.writeln('#endif')
+		} else {
 			g.writeln(');')
 		}
 		return
diff --git a/vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.c.must_have b/vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.c.must_have
new file mode 100644
index 000000000..4277015b2
--- /dev/null
+++ b/vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.c.must_have
@@ -0,0 +1,8 @@
+#ifndef WIFSTOPPED
+extern bool WIFSTOPPED(i32 _d1);
+#ifndef WCOREDUMP
+extern bool WCOREDUMP(i32 _d1);
+#ifndef WIFCONTINUED
+extern bool WIFCONTINUED(i32 _d1);
+#ifndef WSTOPSIG
+extern int WSTOPSIG(i32 _d1);
diff --git a/vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.vv b/vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.vv
new file mode 100644
index 000000000..5a47a3d6e
--- /dev/null
+++ b/vlib/v/gen/c/testdata/c_extern_macro_name_guard_nix.vv
@@ -0,0 +1,24 @@
+// vtest vflags: -no-parallel
+// Regression test for https://github.com/vlang/v/issues/27098 .
+// When a C function name (e.g. `WIFSTOPPED` from <sys/wait.h>) is actually
+// a preprocessor macro, V should still be able to emit an `extern` forward
+// declaration without breaking C compilation. The declaration is guarded by
+// `#ifndef`, so the C preprocessor skips it whenever the symbol is a macro.
+
+#flag -lc
+
+fn C.WIFSTOPPED(i32) bool
+
+fn C.WCOREDUMP(i32) bool
+
+fn C.WIFCONTINUED(i32) bool
+
+fn C.WSTOPSIG(i32) int
+
+fn main() {
+	x := i32(0)
+	_ = C.WIFSTOPPED(x)
+	_ = C.WCOREDUMP(x)
+	_ = C.WIFCONTINUED(x)
+	_ = C.WSTOPSIG(x)
+}
-- 
2.39.5