Add callback interface to re-direct console output

CMakeLists.txt

@@ -53,6 +53,8 @@ if(USE_AVX)
   add_definitions(-DXGBOOST_USE_AVX)
 endif()
 
+# enable custom logging
+add_definitions(-DDMLC_LOG_CUSTOMIZE=1)
 
 # compiled code customizations for R package
 if(R_LIB)

Makefile

@@ -68,7 +68,7 @@ endif
 endif
 
 export LDFLAGS= -pthread -lm $(ADD_LDFLAGS) $(DMLC_LDFLAGS) $(PLUGIN_LDFLAGS)
-export CFLAGS=  -std=c++11 -Wall -Wno-unknown-pragmas -Iinclude $(ADD_CFLAGS) $(PLUGIN_CFLAGS)
+export CFLAGS= -DDMLC_LOG_CUSTOMIZE=1 -std=c++11 -Wall -Wno-unknown-pragmas -Iinclude $(ADD_CFLAGS) $(PLUGIN_CFLAGS)
 CFLAGS += -I$(DMLC_CORE)/include -I$(RABIT)/include -I$(GTEST_PATH)/include
 #java include path
 export JAVAINCFLAGS = -I${JAVA_HOME}/include -I./java

include/xgboost/c_api.h

@@ -96,6 +96,15 @@ XGB_EXTERN_C typedef int XGBCallbackDataIterNext(  // NOLINT(*)
  */
 XGB_DLL const char *XGBGetLastError(void);
 
+/*!
+ * \brief register callback function for LOG(INFO) messages -- helpful messages
+ *        that are not errors.
+ * Note: this function can be called by multiple threads. The callback function
+ *       will run on the thread that registered it
+ * \return 0 for success, -1 for failure
+ */
+XGB_DLL int XGBRegisterLogCallback(void (*callback)(const char*));
+
 /*!
  * \brief load a data matrix
  * \param fname the name of the file

include/xgboost/logging.h

@@ -9,6 +9,7 @@
 #define XGBOOST_LOGGING_H_
 
 #include <dmlc/logging.h>
+#include <dmlc/thread_local.h>
 #include <sstream>
 #include "./base.h"
 
@@ -37,6 +38,23 @@ class TrackerLogger : public BaseLogger {
   ~TrackerLogger();
 };
 
+class LogCallbackRegistry {
+ public:
+  using Callback = void (*)(const char*);
+  LogCallbackRegistry()
+    : log_callback_([] (const char* msg) { std::cerr << msg << std::endl; }) {}
+  inline void Register(Callback log_callback) {
+    this->log_callback_ = log_callback;
+  }
+  inline Callback Get() const {
+    return log_callback_;
+  }
+ private:
+  Callback log_callback_;
+};
+
+using LogCallbackRegistryStore = dmlc::ThreadLocalStore<LogCallbackRegistry>;
+
 // redefines the logging macro if not existed
 #ifndef LOG
 #define LOG(severity) LOG_##severity.stream()

python-package/xgboost/core.py

@@ -100,13 +100,28 @@ def from_cstr_to_pystr(data, length):
     return res
 
 
+def _log_callback(msg):
+    """Redirect logs from native library into Python console"""
+    print("{0:s}".format(py_str(msg)))
+
+
+def _get_log_callback_func():
+    """Wrap log_callback() method in ctypes callback type"""
+    # pylint: disable=invalid-name
+    CALLBACK = ctypes.CFUNCTYPE(None, ctypes.c_char_p)
+    return CALLBACK(_log_callback)
+
+
 def _load_lib():
     """Load xgboost Library."""
     lib_path = find_lib_path()
     if len(lib_path) == 0:
         return None
     lib = ctypes.cdll.LoadLibrary(lib_path[0])
     lib.XGBGetLastError.restype = ctypes.c_char_p
+    lib.callback = _get_log_callback_func()
+    if lib.XGBRegisterLogCallback(lib.callback) != 0:
+        raise XGBoostError(lib.XGBGetLastError())
     return lib
 
 

src/c_api/c_api.cc

@@ -202,6 +202,13 @@ struct XGBAPIThreadLocalEntry {
 // define the threadlocal store.
 using XGBAPIThreadLocalStore = dmlc::ThreadLocalStore<XGBAPIThreadLocalEntry>;
 
+int XGBRegisterLogCallback(void (*callback)(const char*)) {
+  API_BEGIN();
+  LogCallbackRegistry* registry = LogCallbackRegistryStore::Get();
+  registry->Register(callback);
+  API_END();
+}
+
 int XGDMatrixCreateFromFile(const char *fname,
                             int silent,
                             DMatrixHandle *out) {

src/logging.cc

@@ -8,16 +8,24 @@
 #include <iostream>
 #include "./common/sync.h"
 
-namespace xgboost {
+#if !defined(XGBOOST_STRICT_R_MODE) || XGBOOST_STRICT_R_MODE == 0
+// Override logging mechanism for non-R interfaces
+void dmlc::CustomLogMessage::Log(const std::string& msg) {
+  const xgboost::LogCallbackRegistry* registry
+    = xgboost::LogCallbackRegistryStore::Get();
+  auto callback = registry->Get();
+  callback(msg.c_str());
+}
 
-#if XGBOOST_CUSTOMIZE_LOGGER == 0
+namespace xgboost {
 ConsoleLogger::~ConsoleLogger() {
-  std::cerr << log_stream_.str() << std::endl;
+  dmlc::CustomLogMessage::Log(log_stream_.str());
 }
 
 TrackerLogger::~TrackerLogger() {
   log_stream_ << '\n';
   rabit::TrackerPrint(log_stream_.str());
 }
-#endif
+
 }  // namespace xgboost
+#endif