ยง Varargs in GHC: T7160.hs

A comment from this test case tells us why the function debugBelch2 exists: 
ghc/testsuite/tests/rts/T7160.hs
-- Don't use debugBelch() directly, because we cannot call varargs functions
-- using the FFI (doing so produces a segfault on 64-bit Linux, for example).
-- See Debug.Trace.traceIO, which also uses debugBelch2.
foreign import ccall "&debugBelch2" fun :: FunPtr (Ptr () -> Ptr () -> IO ())
The implementation is: 
ghc/libraries/base/cbits/PrelIOUtils.c

void debugBelch2(const char*s, char *t)
{
    debugBelch(s,t);
}

ghc/rts/RtsMessages.c

RtsMsgFunction *debugMsgFn  = rtsDebugMsgFn;
...

void
debugBelch(const char*s, ...)
{
  va_list ap;
  va_start(ap,s);
  (*debugMsgFn)(s,ap);
  va_end(ap);
}