diff --git a/docs/C11_SOURCE_COMPATIBILITY.md b/docs/C11_SOURCE_COMPATIBILITY.md
new file mode 100644
index 00000000..1a0c4119
--- /dev/null
+++ b/docs/C11_SOURCE_COMPATIBILITY.md
@@ -0,0 +1,8 @@
+# C11 source compatibility
+
+A C11 label must precede a statement. The empty statement after retry_candidates keeps the following local declaration valid without changing control flow.
+main.cpp uses fntrace.h as the single owner of fntrace_is_game_started and its C linkage. Redundant block declarations can disagree under GCC 9.
+
+Run `python runtime/tests/test_overlay_retry_c11.py --compiler gcc` and `python runtime/tests/test_fntrace_c_linkage.py`.
+The first compiles the actual label/declaration excerpt in strict C11 mode. The second checks declaration ownership.
+These focused checks do not replace a full GCC 9 compile/link of the current framework.
diff --git a/runtime/CMakeLists.txt b/runtime/CMakeLists.txt
index 21ae85a5..829d345a 100644
--- a/runtime/CMakeLists.txt
+++ b/runtime/CMakeLists.txt
@@ -562,6 +562,10 @@ if(BUILD_TESTING)
 
     find_package(Python3 COMPONENTS Interpreter)
     if(Python3_Interpreter_FOUND)
+        add_test(NAME fntrace_c_linkage_test COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/tests/test_fntrace_c_linkage.py)
+        if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+            add_test(NAME overlay_retry_c11_test COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/tests/test_overlay_retry_c11.py --compiler ${CMAKE_C_COMPILER})
+        endif()
         add_test(NAME disc_companion_test COMMAND ${Python3_EXECUTABLE} ${PSXRECOMP_ROOT}/tools/tests/test_disc_companion.py)
         add_test(NAME sbi_registry_test COMMAND ${Python3_EXECUTABLE} ${PSXRECOMP_ROOT}/tools/tests/test_sbi_registry.py)
         add_test(NAME frame_interpolation_context_ownership_test
diff --git a/runtime/src/main.cpp b/runtime/src/main.cpp
index 5ab2d000..121dfbfa 100644
--- a/runtime/src/main.cpp
+++ b/runtime/src/main.cpp
@@ -6740,7 +6740,6 @@ static NetplayVblankEpilogue sdl_vblank_present_body(void) {
     ep.override = override;
 
     /* Turbo-active / multitap arming share game-started detection. */
-    extern int fntrace_is_game_started(void);
 
     if (psx_netplay_active()) {
         psx_netplay_finish_frame();
@@ -7070,7 +7069,6 @@ static NetplayVblankEpilogue sdl_vblank_present_body(void) {
         return ep;
     /* Engage widescreen at game entry: BIOS boot stays authentic 4:3. */
     if (!g_ws_engaged) {
-        extern int fntrace_is_game_started(void);
         if (fntrace_is_game_started()) {
             g_ws_engaged = true;
             g_ws_projection_mode = -1;
diff --git a/runtime/src/overlay_loader.c b/runtime/src/overlay_loader.c
index e0b4bc27..c2c7ea89 100644
--- a/runtime/src/overlay_loader.c
+++ b/runtime/src/overlay_loader.c
@@ -3597,6 +3597,7 @@ int overlay_loader_dispatch(CPUState *cpu, uint32_t addr) {
     }
     int lazy_loaded = 0;
 retry_candidates:
+    ; /* C11 labels must precede a statement. */
     int head = idx_head(phys);
     int loaded_range_ci = -1;
     int lazy_exact = 0;
diff --git a/runtime/tests/test_fntrace_c_linkage.py b/runtime/tests/test_fntrace_c_linkage.py
new file mode 100644
index 00000000..330ff900
--- /dev/null
+++ b/runtime/tests/test_fntrace_c_linkage.py
@@ -0,0 +1,6 @@
+from pathlib import Path
+p=Path(__file__).resolve().parents[2]/"runtime/src/main.cpp"
+s=p.read_text(encoding="utf-8")
+assert '#include "fntrace.h"' in s
+assert 'extern int fntrace_is_game_started' not in s
+print("fntrace C header ownership: PASS")
diff --git a/runtime/tests/test_overlay_retry_c11.py b/runtime/tests/test_overlay_retry_c11.py
new file mode 100644
index 00000000..3d2c345f
--- /dev/null
+++ b/runtime/tests/test_overlay_retry_c11.py
@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+"""Compile the production retry label and following declaration as strict C11."""
+import argparse
+from pathlib import Path
+import shutil
+import subprocess
+import tempfile
+parser = argparse.ArgumentParser()
+parser.add_argument('--compiler', default='cc')
+args = parser.parse_args()
+root = Path(__file__).resolve().parents[1]
+text = (root/'src/overlay_loader.c').read_text(encoding='utf-8')
+start = text.index('retry_candidates:')
+end = text.index('int loaded_range_ci', start)
+fragment = text[start:end]
+with tempfile.TemporaryDirectory() as tmp:
+    source = Path(tmp)/'label.c'
+    source.write_text('static int idx_head(int p) { return p; }\nint main(void) { int phys=0; if (phys) goto retry_candidates;\n'+fragment+'return head; }\n')
+    subprocess.run([shutil.which(args.compiler) or args.compiler,'-std=c11','-pedantic-errors','-fsyntax-only',str(source)],check=True)
+print('production retry label: strict C11 passes')