1 /* 2 * Copyright (C) 2019 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef UTIL_CHRE_NESTED_DATA_PTR_H_ 18 #define UTIL_CHRE_NESTED_DATA_PTR_H_ 19 20 #include <string.h> 21 #include <type_traits> 22 23 namespace chre { 24 25 /** 26 * Template which provides type punning capability between the template type and 27 * void*. Note that this void* representation must not be dereferenced - it is 28 * only safe as a temporary representation of the underlying type, for example 29 * passed through to a callback which accepts an opaque void* parameter. 30 */ 31 template <typename DataType> 32 union NestedDataPtr { 33 static_assert(sizeof(DataType) <= sizeof(void *), 34 "Requested data type must fit in a void* to use NestedDataPtr"); 35 // If the sizeof() check passes, then this is unlikely to be an issue, and in 36 // many usage scenarios this wouldn't be an issue (e.g. reinterpreting a value 37 // stored in a register), but it's included here just to be safe. 38 static_assert(alignof(DataType) <= alignof(void *), 39 "Additional alignment in NestedDataPtr can't be guaranteed"); 40 static_assert(std::is_trivially_copyable<DataType>::value, 41 "Only trivially copyable types may be used in NestedDataPtr"); 42 43 NestedDataPtr() = default; 44 NestedDataPtr(DataType nestedData)45 explicit NestedDataPtr(DataType nestedData) : data(nestedData) {} NestedDataPtr(void * ptr)46 explicit NestedDataPtr(void *ptr) { 47 // We use memcpy here and in the void* conversion operator, as the C++11 48 // language standard defines that accessing any field of a union other than 49 // most recently set value is undefined behavior, unless it's a structure 50 // with a common prefix to the active field. Most compilers (e.g. GCC, 51 // clang) allow for this anyways as it's permitted in C, but to avoid the UB 52 // we do the conversion via memcpy. Note that compilers will recognize this 53 // as a simple store operation and produce equivalent assembly as if we were 54 // assigning to mUnusedPtr. 55 memcpy(&data, &ptr, sizeof(ptr)); 56 } 57 58 // Implicit conversions DataType()59 operator DataType() const { 60 return data; 61 } 62 63 operator void *() const { 64 void *result; 65 static_assert(sizeof(data) <= sizeof(result), "Broken assumption"); 66 memcpy(&result, &data, sizeof(data)); 67 return result; 68 } 69 70 DataType data; 71 72 private: 73 // Here to force that this union has at least the alignment + size of a 74 // pointer, but we don't access it directly 75 void *mUnusedPtr; 76 }; 77 78 } // namespace chre 79 80 #endif // UTIL_CHRE_NESTED_DATA_PTR_H_