From 145369149620591f7205faaa7f5ee44bdd5ce15e Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <steinar+git@gunderson.no>
Date: Wed, 31 Dec 2025 11:20:25 +0100
Subject: [PATCH] Fix feature check

Use _POSIX_C_SOURCE to check for PTHREAD_MUTEX_ROBUST support. The latter is a enum, not a defined variable.

closes https://github.com/official-stockfish/Stockfish/pull/6510

No functional change
---
 src/shm_linux.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/shm_linux.h b/src/shm_linux.h
index a8b5404b2..52abe8ec4 100644
--- a/src/shm_linux.h
+++ b/src/shm_linux.h
@@ -502,7 +502,7 @@ class SharedMemory: public detail::SharedMemoryBase {
             return false;
 
         bool success = pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED) == 0;
-#ifdef PTHREAD_MUTEX_ROBUST
+#if _POSIX_C_SOURCE >= 200809L
         if (success)
             success = pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST) == 0;
 #endif
@@ -524,7 +524,7 @@ class SharedMemory: public detail::SharedMemoryBase {
             if (rc == 0)
                 return true;
 
-#ifdef PTHREAD_MUTEX_ROBUST
+#if _POSIX_C_SOURCE >= 200809L
             if (rc == EOWNERDEAD)
             {
                 if (pthread_mutex_consistent(&header_ptr_->mutex) == 0)