Use weak symbol for rtsConfig (support existence of TH in client programs)

2 files changed, 8 insertions, 1 deletions

diff --git a/cbits/hook.c b/cbits/hook.c
index b13576f..22aa259 100644
--- a/cbits/hook.c
+++ b/cbits/hook.c
@@ -9,7 +9,7 @@ static size_t min_sz(size_t a, size_t b) {
 	return a < b ? a : b;
 }
 
-extern RtsConfig rtsConfig;
+extern RtsConfig __attribute__((weak)) rtsConfig;
 
 // A copy of GCDetails_ with known structure that can be depended on by the Haskell code.
 struct ShadowDetails {
@@ -226,6 +226,11 @@ bool set_gchook(void) {
 		goto unlock_return_retval;
 	}
 
+	if (&rtsConfig == NULL) {
+		fprintf(stderr, "ghc-gc-hook: ERROR: rtsConfig not defined; the GC hook cannot be used from within a TemplateHaskell splice\n");
+		goto unlock_return_retval;
+	}
+
 	old_hook = rtsConfig.gcDoneHook;
 	rtsConfig.gcDoneHook = hook_callback;
 
diff --git a/test/Main.hs b/test/Main.hs
index aad5ebb..90e5f90 100644
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE ForeignFunctionInterface #-}
+{-# LANGUAGE TemplateHaskell #-}
 module Main where
 
 import Control.Monad (forM_, when)
@@ -10,6 +11,7 @@ import GHC.GC_Hook
 
 foreign import ccall "get_my_delegate_ptr" c_get_my_delegate_ptr :: IO (Ptr ())
 
+$(return [])
 
 {-# NOINLINE invokeGCsometimes #-}
 invokeGCsometimes :: IO ()