Fix SWIG methods that return char**

include/LightGBM/c_api.h

@@ -555,23 +555,39 @@ LIGHTGBM_C_EXPORT int LGBM_BoosterGetEvalCounts(BoosterHandle handle,
 /*!
  * \brief Get names of evaluation datasets.
  * \param handle Handle of booster
+ * \param len Number of ``char*`` pointers stored at ``out_strs``.
+ *            If smaller than the max size, only this many strings are copied
  * \param[out] out_len Total number of evaluation datasets
+ * \param buffer_len Size of pre-allocated strings. 
+ *                   Content is copied up to ``buffer_len - 1`` and null-terminated
+ * \param[out] out_buffer_len String sizes required to do the full string copies
  * \param[out] out_strs Names of evaluation datasets, should pre-allocate memory
  * \return 0 when succeed, -1 when failure happens
  */
 LIGHTGBM_C_EXPORT int LGBM_BoosterGetEvalNames(BoosterHandle handle,
+                                               const int len,
                                                int* out_len,
+                                               const size_t buffer_len,
+                                               size_t* out_buffer_len,
                                                char** out_strs);
 
 /*!
  * \brief Get names of features.
  * \param handle Handle of booster
+ * \param len Number of ``char*`` pointers stored at ``out_strs``.
+ *            If smaller than the max size, only this many strings are copied
  * \param[out] out_len Total number of features
+ * \param buffer_len Size of pre-allocated strings.
+ *                   Content is copied up to ``buffer_len - 1`` and null-terminated
+ * \param[out] out_buffer_len String sizes required to do the full string copies
  * \param[out] out_strs Names of features, should pre-allocate memory
  * \return 0 when succeed, -1 when failure happens
  */
 LIGHTGBM_C_EXPORT int LGBM_BoosterGetFeatureNames(BoosterHandle handle,
+                                                  const int len,
                                                   int* out_len,
+                                                  const size_t buffer_len,
+                                                  size_t* out_buffer_len,
                                                   char** out_strs);
 
 /*!

python-package/lightgbm/basic.py

@@ -2711,14 +2711,24 @@ def feature_name(self):
         num_feature = self.num_feature()
         # Get name of features
         tmp_out_len = ctypes.c_int(0)
-        string_buffers = [ctypes.create_string_buffer(255) for i in range_(num_feature)]
+        reserved_string_buffer_size = 255
+        required_string_buffer_size = ctypes.c_size_t(0)
+        string_buffers = [ctypes.create_string_buffer(reserved_string_buffer_size) for i in range_(num_feature)]
         ptr_string_buffers = (ctypes.c_char_p * num_feature)(*map(ctypes.addressof, string_buffers))
         _safe_call(_LIB.LGBM_BoosterGetFeatureNames(
             self.handle,
+            num_feature,
             ctypes.byref(tmp_out_len),
+            reserved_string_buffer_size,
+            ctypes.byref(required_string_buffer_size),
             ptr_string_buffers))
         if num_feature != tmp_out_len.value:
             raise ValueError("Length of feature names doesn't equal with num_feature")
+        if reserved_string_buffer_size < required_string_buffer_size.value:
+            raise BufferError(
+                "Allocated feature name buffer size ({}) was inferior to the needed size ({})."
+                .format(reserved_string_buffer_size, required_string_buffer_size.value)
+            )
         return [string_buffers[i].value.decode() for i in range_(num_feature)]
 
     def feature_importance(self, importance_type='split', iteration=None):
@@ -2898,14 +2908,26 @@ def __get_eval_info(self):
             if self.__num_inner_eval > 0:
                 # Get name of evals
                 tmp_out_len = ctypes.c_int(0)
-                string_buffers = [ctypes.create_string_buffer(255) for i in range_(self.__num_inner_eval)]
+                reserved_string_buffer_size = 255
+                required_string_buffer_size = ctypes.c_size_t(0)
+                string_buffers = [
+                    ctypes.create_string_buffer(reserved_string_buffer_size) for i in range_(self.__num_inner_eval)
+                ]
                 ptr_string_buffers = (ctypes.c_char_p * self.__num_inner_eval)(*map(ctypes.addressof, string_buffers))
                 _safe_call(_LIB.LGBM_BoosterGetEvalNames(
                     self.handle,
+                    self.__num_inner_eval,
                     ctypes.byref(tmp_out_len),
+                    reserved_string_buffer_size,
+                    ctypes.byref(required_string_buffer_size),
                     ptr_string_buffers))
                 if self.__num_inner_eval != tmp_out_len.value:
                     raise ValueError("Length of eval names doesn't equal with num_evals")
+                if reserved_string_buffer_size < required_string_buffer_size.value:
+                    raise BufferError(
+                        "Allocated eval name buffer size ({}) was inferior to the needed size ({})."
+                        .format(reserved_string_buffer_size, required_string_buffer_size.value)
+                    )
                 self.__name_inner_eval = \
                     [string_buffers[i].value.decode() for i in range_(self.__num_inner_eval)]
                 self.__higher_better_inner_eval = \

src/c_api.cpp

@@ -501,21 +501,31 @@ class Booster {
     return ret;
   }
 
-  int GetEvalNames(char** out_strs) const {
+  int GetEvalNames(char** out_strs, const int len, const size_t buffer_len, size_t *out_buffer_len) const {
+    *out_buffer_len = 0;
     int idx = 0;
     for (const auto& metric : train_metric_) {
       for (const auto& name : metric->GetName()) {
-        std::memcpy(out_strs[idx], name.c_str(), name.size() + 1);
+        if (idx < len) {
+          std::memcpy(out_strs[idx], name.c_str(), std::min(name.size() + 1, buffer_len));
+          out_strs[idx][buffer_len - 1] = '\0';
+        }
+        *out_buffer_len = std::max(name.size() + 1, *out_buffer_len);
         ++idx;
       }
     }
     return idx;
   }
 
-  int GetFeatureNames(char** out_strs) const {
+  int GetFeatureNames(char** out_strs, const int len, const size_t buffer_len, size_t *out_buffer_len) const {
+    *out_buffer_len = 0;
     int idx = 0;
     for (const auto& name : boosting_->FeatureNames()) {
-      std::memcpy(out_strs[idx], name.c_str(), name.size() + 1);
+      if (idx < len) {
+        std::memcpy(out_strs[idx], name.c_str(), std::min(name.size() + 1, buffer_len));
+        out_strs[idx][buffer_len - 1] = '\0';
+      }
+      *out_buffer_len = std::max(name.size() + 1, *out_buffer_len);
       ++idx;
     }
     return idx;
@@ -1356,17 +1366,27 @@ int LGBM_BoosterGetEvalCounts(BoosterHandle handle, int* out_len) {
   API_END();
 }
 
-int LGBM_BoosterGetEvalNames(BoosterHandle handle, int* out_len, char** out_strs) {
+int LGBM_BoosterGetEvalNames(BoosterHandle handle,
+                             const int len,
+                             int* out_len,
+                             const size_t buffer_len,
+                             size_t* out_buffer_len,
+                             char** out_strs) {
   API_BEGIN();
   Booster* ref_booster = reinterpret_cast<Booster*>(handle);
-  *out_len = ref_booster->GetEvalNames(out_strs);
+  *out_len = ref_booster->GetEvalNames(out_strs, len, buffer_len, out_buffer_len);
   API_END();
 }
 
-int LGBM_BoosterGetFeatureNames(BoosterHandle handle, int* out_len, char** out_strs) {
+int LGBM_BoosterGetFeatureNames(BoosterHandle handle,
+                                const int len,
+                                int* out_len,
+                                const size_t buffer_len,
+                                size_t* out_buffer_len,
+                                char** out_strs) {
   API_BEGIN();
   Booster* ref_booster = reinterpret_cast<Booster*>(handle);
-  *out_len = ref_booster->GetFeatureNames(out_strs);
+  *out_len = ref_booster->GetFeatureNames(out_strs, len, buffer_len, out_buffer_len);
   API_END();
 }
 

src/lightgbm_R.cpp

@@ -449,15 +449,25 @@ LGBM_SE LGBM_BoosterGetEvalNames_R(LGBM_SE handle,
   R_API_BEGIN();
   int len;
   CHECK_CALL(LGBM_BoosterGetEvalCounts(R_GET_PTR(handle), &len));
+
+  const size_t reserved_string_size = 128;
   std::vector<std::vector<char>> names(len);
   std::vector<char*> ptr_names(len);
   for (int i = 0; i < len; ++i) {
-    names[i].resize(128);
+    names[i].resize(reserved_string_size);
     ptr_names[i] = names[i].data();
   }
+
   int out_len;
-  CHECK_CALL(LGBM_BoosterGetEvalNames(R_GET_PTR(handle), &out_len, ptr_names.data()));
+  size_t required_string_size;
+  CHECK_CALL(
+    LGBM_BoosterGetEvalNames(
+      R_GET_PTR(handle),
+      len, &out_len,
+      reserved_string_size, &required_string_size,
+      ptr_names.data()));
   CHECK_EQ(out_len, len);
+  CHECK_GE(reserved_string_size, required_string_size);
   auto merge_names = Common::Join<char*>(ptr_names, "\t");
   EncodeChar(eval_names, merge_names.c_str(), buf_len, actual_len, merge_names.size() + 1);
   R_API_END();

swig/StringArray.hpp

@@ -0,0 +1,150 @@
+/*!
+ * Copyright (c) 2020 Microsoft Corporation. All rights reserved.
+ * Licensed under the MIT License. See LICENSE file in the project root for license information.
+ */
+#ifndef __STRING_ARRAY_H__
+#define __STRING_ARRAY_H__
+
+#include <new>
+#include <vector>
+#include <algorithm>
+
+/**
+ * Container that manages an array of fixed-length strings.
+ *
+ * To be compatible with SWIG's `various.i` extension module,
+ * the array of pointers to char* must be NULL-terminated:
+ *   [char*, char*, char*, ..., NULL]
+ * This implies that the length of this array is bigger
+ * by 1 element than the number of char* it stores.
+ * I.e., _num_elements == _array.size()-1
+ *
+ * The class also takes care of allocation of the underlying
+ * char* memory.
+ */
+class StringArray
+{
+  public:
+    StringArray(size_t num_elements, size_t string_size)
+      : _string_size(string_size),
+        _array(num_elements + 1, nullptr)
+    {
+        _allocate_strings(num_elements, string_size);
+    }
+
+    ~StringArray()
+    {
+        _release_strings();
+    }
+
+    /**
+     * Returns the pointer to the raw array.
+     * Notice its size is greater than the number of stored strings by 1.
+     *
+     * @return char** pointer to raw data (null-terminated).
+     */
+    char **data() noexcept
+    {
+        return _array.data();
+    }
+
+    /**
+     * Return char* from the array of size _string_size+1.
+     * Notice the last element in _array is already
+     * considered out of bounds.
+     *
+     * @param index Index of the element to retrieve.
+     * @return pointer or nullptr if index is out of bounds.
+     */
+    char *getitem(size_t index) noexcept
+    {
+        if (_in_bounds(index))
+            return _array[index];
+        else
+            return nullptr;
+    }
+
+    /**
+     * Safely copies the full content data
+     * into one of the strings in the array.
+     * If that is not possible, returns error (-1).
+     *
+     * @param index index of the string in the array.
+     * @param content content to store
+     *
+     * @return In case index results in out of bounds access,
+     * or content + 1 (null-terminator byte) doesn't fit
+     * into the target string (_string_size), it errors out
+     * and returns -1.
+     */
+    int setitem(size_t index, std::string content) noexcept
+    {
+        if (_in_bounds(index) && content.size() < _string_size)
+        {
+            std::strcpy(_array[index], content.c_str());
+            return 0;
+        } else {
+            return -1;
+        }
+    }
+
+    /**
+     * @return number of stored strings.
+     */
+    size_t get_num_elements() noexcept
+    {
+        return _array.size() - 1;
+    }
+
+  private:
+
+    /**
+     * Returns true if and only if within bounds.
+     * Notice that it excludes the last element of _array (NULL).
+     *
+     * @param index index of the element
+     * @return bool true if within bounds
+     */
+    bool _in_bounds(size_t index) noexcept
+    {
+        return index < get_num_elements();
+    }
+
+    /**
+     * Allocate an array of fixed-length strings.
+     *
+     * Since a NULL-terminated array is required by SWIG's `various.i`,
+     * the size of the array is actually `num_elements + 1` but only
+     * num_elements are filled.
+     *
+     * @param num_elements Number of strings to store in the array.
+     * @param string_size The size of each string in the array.
+     */
+    void _allocate_strings(int num_elements, int string_size)
+    {
+        for (int i = 0; i < num_elements; ++i)
+        {
+            // Leave space for \0 terminator:
+            _array[i] = new (std::nothrow) char[string_size + 1];
+
+            // Check memory allocation:
+            if (! _array[i]) {
+                _release_strings();
+                throw std::bad_alloc();
+            }
+        }
+    }
+
+    /**
+     * Deletes the allocated strings.
+     */
+    void _release_strings() noexcept
+    {
+        std::for_each(_array.begin(), _array.end(), [](char* c) { delete[] c; });
+    }
+
+    const size_t _string_size;
+    std::vector<char*> _array;
+};
+
+#endif // __STRING_ARRAY_H__