prevent undefined behaviour at a language level while probing getenv

Backport of a5823872283be23731f1bcde7e19a926c44b31a4

This test in Configure tries to probe for undefined behaviour in
getenv (), but provokes undefined behaviour in C/C++ by falling off
the end of a function with a non-void return type.

Without optimization clang++ generated a ud2 instruction here on
amd64 producing an illegal instruction exception.  With optimization
the test case fell off the end and started re-executing main (),
eventually producing a SIGBUS.

Simply dropping the value of getenv () here and returning NULL wasn't
useful, under -O2 the compiler optimized away the getenv () call,
voiding the whole point of the test.

diff --git a/U/threads/d_getenv_thread.U b/U/threads/d_getenv_thread.U
index da22f99..b3590c5 100644 (file)
--- a/U/threads/d_getenv_thread.U
+++ b/U/threads/d_getenv_thread.U
@@ -47,7 +47,7 @@ $cat >try.c <<EOCP
 void *
 thread_start(void * arg)
 {
 void *
 thread_start(void * arg)
 {

-    (void *) getenv("HOME");
+    return (void *) getenv("HOME");

 }
 
 int main() {
 }
 
 int main() {


@@ -78,7 +78,7 @@ int main() {
         exit(2);
     }
 
         exit(2);
     }
 

-    exit(! strcmp(main_buffer, save_main_buffer) == 0);
+    exit(! (strcmp(main_buffer, save_main_buffer) == 0));

 }
 EOCP
 val=
 }
 EOCP
 val=