From de7ad13b3c757cbcbfddc111fb8f94c31389e20f Mon Sep 17 00:00:00 2001
From: Michael Davidsaver <mdavidsaver@gmail.com>
Date: Sat, 13 Nov 2021 10:29:17 -0800
Subject: [PATCH] Com: posix warn of use of epicsThread from child after fork()

---
 modules/libcom/src/osi/os/posix/osdThread.c | 24 ++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/modules/libcom/src/osi/os/posix/osdThread.c b/modules/libcom/src/osi/os/posix/osdThread.c
index d20b7d123..e2e19b4b1 100644
--- a/modules/libcom/src/osi/os/posix/osdThread.c
+++ b/modules/libcom/src/osi/os/posix/osdThread.c
@@ -330,13 +330,30 @@ int          status;
     a_p->usePolicy = arg.ok;
 }
 #endif
-
+
+/* 0 - In the process which loads libCom.
+ * 1 - In a newly fork()'d child process
+ * 2 - In a child which has been warned
+ */
+static int childAfterFork;
+
+static void childHook(void)
+{
+    epicsAtomicSetIntT(&childAfterFork, 1);
+}
 
 static void once(void)
 {
     epicsThreadOSD *pthreadInfo;
     int status;
 
+#ifdef __rtems__
+    (void)childHook;
+#else
+    status = pthread_atfork(NULL, NULL, &childHook);
+    checkStatusOnce(status, "pthread_atfork");
+#endif
+
     pthread_key_create(&getpthreadInfo,0);
     status = osdPosixMutexInit(&onceLock,PTHREAD_MUTEX_DEFAULT);
     checkStatusOnceQuit(status,"osdPosixMutexInit","epicsThreadInit");
@@ -431,6 +448,11 @@ static void epicsThreadInit(void)
     static pthread_once_t once_control = PTHREAD_ONCE_INIT;
     int status = pthread_once(&once_control,once);
     checkStatusQuit(status,"pthread_once","epicsThreadInit");
+
+    if(epicsAtomicGetIntT(&childAfterFork)==1 &&  epicsAtomicCmpAndSwapIntT(&childAfterFork, 1, 2)==1) {
+        fprintf(stderr, "Warning: Undefined Behavior!\n"
+                        "         Detected use of epicsThread from child process after fork()\n");
+    }
 }
 
 LIBCOM_API