diff --git a/coregrind/m_scheduler/scheduler.c b/coregrind/m_scheduler/scheduler.c
index 4983ffe..c97042d 100644
--- a/coregrind/m_scheduler/scheduler.c
+++ b/coregrind/m_scheduler/scheduler.c
@@ -1424,15 +1424,27 @@ void do_client_request ( ThreadId tid )
          break;
 
       case VG_USERREQ__PRINTF: {
-         Int count = 
-            VG_(vmessage)( Vg_ClientMsg, (char *)arg[1], (void*)arg[2] );
+         union {
+            va_list vargs;
+            unsigned long t;
+         } args;
+         Int count;
+         args.t = (unsigned long)arg[2];
+         count =
+            VG_(vmessage)( Vg_ClientMsg, (char *)arg[1], args.vargs );
             VG_(message_flush)();
             SET_CLREQ_RETVAL( tid, count );
          break; }
 
       case VG_USERREQ__INTERNAL_PRINTF: {
-         Int count = 
-            VG_(vmessage)( Vg_DebugMsg, (char *)arg[1], (void*)arg[2] );
+         union {
+            va_list vargs;
+            unsigned long t;
+         } args;
+         Int count;
+         args.t = (unsigned long)arg[2];
+         count =
+            VG_(vmessage)( Vg_DebugMsg, (char *)arg[1], args.vargs );
             VG_(message_flush)();
             SET_CLREQ_RETVAL( tid, count );
          break; }
@@ -1443,8 +1455,14 @@ void do_client_request ( ThreadId tid )
          break; }
 
       case VG_USERREQ__PRINTF_BACKTRACE: {
-         Int count =
-            VG_(vmessage)( Vg_ClientMsg, (char *)arg[1], (void*)arg[2] );
+         union {
+            va_list vargs;
+            unsigned long t;
+         } args;
+         Int count;
+         args.t = (unsigned long)arg[2];
+         count =
+            VG_(vmessage)( Vg_ClientMsg, (char *)arg[1], args.vargs );
             VG_(message_flush)();
             VG_(get_and_pp_StackTrace)( tid, VG_(clo_backtrace_size) );
             SET_CLREQ_RETVAL( tid, count );
diff --git a/coregrind/pub_core_clreq.h b/coregrind/pub_core_clreq.h
index 412f89e..3059e70 100644
--- a/coregrind/pub_core_clreq.h
+++ b/coregrind/pub_core_clreq.h
@@ -64,13 +64,16 @@ static int VALGRIND_INTERNAL_PRINTF(const char *format, ...)
 static int VALGRIND_INTERNAL_PRINTF(const char *format, ...)
 {
    unsigned long _qzz_res = 0;
-   va_list vargs;
-   va_start(vargs, format);
+   union {
+      va_list vargs;
+      unsigned long t;
+   } args;
+   va_start(args.vargs, format);
    VALGRIND_DO_CLIENT_REQUEST(
       _qzz_res, 0, VG_USERREQ__INTERNAL_PRINTF,
-      (unsigned long)format, (unsigned long)vargs, 0, 0, 0
+      (unsigned long)format, (unsigned long)(args.t), 0, 0, 0
    );
-   va_end(vargs);
+   va_end(args.vargs);
    return _qzz_res;
 }
 
diff --git a/include/valgrind.h b/include/valgrind.h
index b178cda..9236092 100644
--- a/include/valgrind.h
+++ b/include/valgrind.h
@@ -4181,12 +4181,15 @@ static int
 VALGRIND_PRINTF(const char *format, ...)
 {
    unsigned long _qzz_res;
-   va_list vargs;
-   va_start(vargs, format);
+   union {
+      va_list vargs;
+      unsigned long t;
+   } args;
+   va_start(args.vargs, format);
    VALGRIND_DO_CLIENT_REQUEST(_qzz_res, 0, VG_USERREQ__PRINTF,
-                              (unsigned long)format, (unsigned long)vargs, 
+                              (unsigned long)format, (unsigned long)(args.t),
                               0, 0, 0);
-   va_end(vargs);
+   va_end(args.vargs);
    return (int)_qzz_res;
 }
 
@@ -4196,12 +4199,15 @@ static int
 VALGRIND_PRINTF_BACKTRACE(const char *format, ...)
 {
    unsigned long _qzz_res;
-   va_list vargs;
-   va_start(vargs, format);
+   union {
+      va_list vargs;
+      unsigned long t;
+   } args;
+   va_start(args.vargs, format);
    VALGRIND_DO_CLIENT_REQUEST(_qzz_res, 0, VG_USERREQ__PRINTF_BACKTRACE,
-                              (unsigned long)format, (unsigned long)vargs, 
+                              (unsigned long)format, (unsigned long)(args.t),
                               0, 0, 0);
-   va_end(vargs);
+   va_end(args.vargs);
    return (int)_qzz_res;
 }
 


