diff --git a/Makefile b/Makefile
index c5d052b7c..da9bb22fb 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,8 @@ export CC  = gcc
 export CXX = g++
 export MPICXX = mpicxx
 export LDFLAGS= -Llib
-export CFLAGS = -Wall -O3 -msse2  -Wno-unknown-pragmas -fPIC -Iinclude
+export WARNFLAGS= -Wall -Wextra -Wno-unused-parameter -Wno-unknown-pragmas -pedantic 
+export CFLAGS = -O3 -msse2 -fPIC -Iinclude $(WARNFLAGS) 
 
 # build path
 BPATH=.
@@ -15,7 +16,7 @@ ALIB= lib/librabit.a lib/librabit_mpi.a lib/librabit_empty.a lib/librabit_mock.a
 HEADERS=src/*.h include/*.h include/rabit/*.h
 .PHONY: clean all install mpi python
 
-all: lib/librabit.a lib/librabit_mock.a $(SLIB)
+all: lib/librabit.a lib/librabit_mock.a wrapper/librabit_wrapper.so wrapper/librabit_wrapper_mock.so
 mpi: lib/librabit_mpi.a wrapper/librabit_wrapper_mpi.so
 python: wrapper/librabit_wrapper.so wrapper/librabit_wrapper_mock.so
 
diff --git a/include/rabit/engine.h b/include/rabit/engine.h
index fbbdaa8f0..dd85b508b 100644
--- a/include/rabit/engine.h
+++ b/include/rabit/engine.h
@@ -242,8 +242,10 @@ class ReduceHandle {
   static int TypeSize(const MPI::Datatype &dtype);
 
  protected:
-  // handle data field
+  // handle function field
   void *handle_;
+  // reduce function of the reducer
+  IEngine::ReduceFunction *redfunc_;
   // handle to the type field
   void *htype_;
   // the created type in 4 bytes
diff --git a/include/rabit/rabit-inl.h b/include/rabit/rabit-inl.h
index 4ee1a42b5..4ffd812e7 100644
--- a/include/rabit/rabit-inl.h
+++ b/include/rabit/rabit-inl.h
@@ -201,7 +201,7 @@ inline void ReducerFunc_(const void *src_, void *dst_, int len_, const MPI::Data
   const char *psrc = reinterpret_cast<const char*>(src_);
   char *pdst = reinterpret_cast<char*>(dst_);
   DType tdst, tsrc;
-  for (size_t i = 0; i < len_; ++i) {
+  for (int i = 0; i < len_; ++i) {
     // use memcpy to avoid alignment issue
     std::memcpy(&tdst, pdst + i * kUnit, sizeof(tdst));
     std::memcpy(&tsrc, psrc + i * kUnit, sizeof(tsrc));
diff --git a/src/allreduce_base.cc b/src/allreduce_base.cc
index 590041f8d..b905c2217 100644
--- a/src/allreduce_base.cc
+++ b/src/allreduce_base.cc
@@ -27,6 +27,7 @@ AllreduceBase::AllreduceBase(void) {
   hadoop_mode = 0;
   version_number = 0;
   task_id = "NULL";
+  err_link = NULL;
   this->SetParam("rabit_reduce_buffer", "256MB");
 }
 
diff --git a/src/allreduce_base.h b/src/allreduce_base.h
index 78e69bd99..b1cc71ef1 100644
--- a/src/allreduce_base.h
+++ b/src/allreduce_base.h
@@ -249,7 +249,9 @@ class AllreduceBase : public IEngine {
     // buffer size, in bytes
     size_t buffer_size;
     // constructor
-    LinkRecord(void) {}
+    LinkRecord(void) 
+        : buffer_head(NULL), buffer_size(0) {
+    }
     // initialize buffer
     inline void InitBuffer(size_t type_nbytes, size_t count,
                            size_t reduce_buffer_size) {
@@ -276,6 +278,7 @@ class AllreduceBase : public IEngine {
      * \return the type of reading
      */
     inline ReturnType ReadToRingBuffer(size_t protect_start) {
+      utils::Assert(buffer_head != NULL, "ReadToRingBuffer: buffer not allocated");
       size_t ngap = size_read - protect_start;
       utils::Assert(ngap <= buffer_size, "Allreduce: boundary check");
       size_t offset = size_read % buffer_size;
diff --git a/src/allreduce_robust.cc b/src/allreduce_robust.cc
index 341ea558f..38cadbdb2 100644
--- a/src/allreduce_robust.cc
+++ b/src/allreduce_robust.cc
@@ -27,6 +27,7 @@ AllreduceRobust::AllreduceRobust(void) {
   result_buffer_round = 1;
   global_lazycheck = NULL;
   use_local_model = -1;
+  recover_counter = 0;
 }
 void AllreduceRobust::Init(void) {
   AllreduceBase::Init();
@@ -421,6 +422,8 @@ AllreduceRobust::ReturnType AllreduceRobust::TryResetLinks(void) {
  */
 bool AllreduceRobust::CheckAndRecover(ReturnType err_type) {
   if (err_type == kSuccess) return true;
+  utils::Assert(err_link != NULL, "must know the error source");
+  recover_counter += 1;
   {
     // simple way, shutdown all links
     for (size_t i = 0; i < all_links.size(); ++i) {
@@ -602,6 +605,9 @@ AllreduceRobust::TryRecoverData(RecoverType role,
     if (!req_data) return kSuccess;
   }
   utils::Assert(recv_link >= 0 || role == kHaveData, "recv_link must be active");
+  if (role == kPassData) {
+    links[recv_link].InitBuffer(1, size, reduce_buffer_size);
+  }
   for (int i = 0; i < nlink; ++i) {
     links[i].ResetSize();
   }
diff --git a/src/allreduce_robust.h b/src/allreduce_robust.h
index 078ff1598..23a3964d8 100644
--- a/src/allreduce_robust.h
+++ b/src/allreduce_robust.h
@@ -531,6 +531,8 @@ o   *  the input state must exactly one saved state(local state of current node)
   int use_local_model;
   // number of replica for global state/model
   int num_global_replica;
+  // number of times recovery happens
+  int recover_counter;
   // --- recovery data structure for local checkpoint
   // there is two version of the data structure,
   // at one time one version is valid and another is used as temp memory
diff --git a/src/engine.cc b/src/engine.cc
index 45bef329c..1aa220f0e 100644
--- a/src/engine.cc
+++ b/src/engine.cc
@@ -56,7 +56,8 @@ void Allreduce_(void *sendrecvbuf,
 }
 
 // code for reduce handle
-ReduceHandle::ReduceHandle(void) : handle_(NULL), htype_(NULL) {
+ReduceHandle::ReduceHandle(void) 
+  : handle_(NULL), redfunc_(NULL), htype_(NULL) {
 }
 ReduceHandle::~ReduceHandle(void) {}
 
@@ -64,17 +65,16 @@ int ReduceHandle::TypeSize(const MPI::Datatype &dtype) {
   return static_cast<int>(dtype.type_size);
 }
 void ReduceHandle::Init(IEngine::ReduceFunction redfunc, size_t type_nbytes) {
-  utils::Assert(handle_ == NULL, "cannot initialize reduce handle twice");
-  handle_ = reinterpret_cast<void*>(redfunc);
+  utils::Assert(redfunc_ == NULL, "cannot initialize reduce handle twice");
+  redfunc_ = redfunc;
 }
 void ReduceHandle::Allreduce(void *sendrecvbuf,
                              size_t type_nbytes, size_t count,
                              IEngine::PreprocFunction prepare_fun,
                              void *prepare_arg) {
-  utils::Assert(handle_ != NULL, "must intialize handle to call AllReduce");
+  utils::Assert(redfunc_ != NULL, "must intialize handle to call AllReduce");
   GetEngine()->Allreduce(sendrecvbuf, type_nbytes, count,
-                         reinterpret_cast<IEngine::ReduceFunction*>(handle_),
-                         prepare_fun, prepare_arg);
+                         redfunc_, prepare_fun, prepare_arg);
 }
 }  // namespace engine
 }  // namespace rabit
diff --git a/src/engine_mpi.cc b/src/engine_mpi.cc
index 9c6206ebf..006a91006 100644
--- a/src/engine_mpi.cc
+++ b/src/engine_mpi.cc
@@ -137,7 +137,8 @@ void Allreduce_(void *sendrecvbuf,
 }
 
 // code for reduce handle
-ReduceHandle::ReduceHandle(void) : handle_(NULL), htype_(NULL) {
+ReduceHandle::ReduceHandle(void) 
+    : handle_(NULL), redfunc_(NULL), htype_(NULL) {
 }
 ReduceHandle::~ReduceHandle(void) {
   if (handle_ != NULL) {