1/*
2 * Copyright (C) 2016 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// Local utilities (macros and free-standing functions).
18
19#ifndef LOCAL_UTILS_H_
20#define LOCAL_UTILS_H_
21
22#include <limits>
23#include <type_traits>
24
25#include "android-base/logging.h"
26
27// Converts the value SRC to a value of DST_TYPE, in the range of [MIN, MAX].
28// Values less than MIN are clamped to MIN, and values greater than MAX are
29// clamped to MAX. Conversions are safe in the sense that the range is checked
30// to be valid for both SRC and DST_TYPE, at compile-time.
31//
32// As compared to static_cast<>, SAFELY_CLAMP is a) more explicit, b) more
33// flexible, and c) less prone to surprising conversions (e.g. -1 becoming
34// UINT_MAX).
35#define SAFELY_CLAMP(SRC, DST_TYPE, MIN, MAX)                                \
36  local_utils::internal::SafelyClamp<decltype(SRC), DST_TYPE, MIN, MAX, MIN, \
37                                     MAX>(SRC)
38
39// While attributes are standard in C++11, these attributes are not part of
40// the standard. We use macros to abstract these attributes, to allow
41// the code to compile with compilers that don't recognize these attributes.
42#if defined(__clang__)
43#define NONNULL [[gnu::nonnull]]                 /* NOLINT(whitespace/braces) */
44#define RETURNS_NONNULL [[gnu::returns_nonnull]] /* NOLINT ... */
45#else
46#define NONNULL
47#define RETURNS_NONNULL
48#endif
49
50namespace android {
51namespace wifilogd {
52namespace local_utils {
53
54// Returns the value in |enum_value|, as the integral type underlying the
55// enum. (E.g. uint8_t, int32_t, etc.)
56template <typename T>
57constexpr auto CastEnumToInteger(T enum_value) {
58  static_assert(std::is_enum<T>::value, "argument must be of an enum type");
59  return static_cast<typename std::underlying_type<T>::type>(enum_value);
60}
61
62// Copies a |T| out of |buf|, aborting if |buf| is too short to hold a |T|.
63//
64// As compared to accessing the underlying data using reinterpret_cast<>,
65// CopyFromBufferOrDie() provides three benefits:
66// 1. Guarantees that the returned header is properly aligned. While
67//    many processors support some unaligned reads, there are some
68//    exceptions. E.g, a 64-bit unaligned read on 32-bit ARM may cause
69//    a program to abort.
70// 2. Removes the potential for bugs due to compiler optimizations based
71//    on type-based alias analysis. (These are the kinds of bugs that
72//    "strict-aliasing" warnings try to call out.)
73// 3. Verifies that the source buffer is large enough to contain the
74//    data we're trying to read out.
75template <typename T>
76T CopyFromBufferOrDie(NONNULL const void* buf, size_t buf_len) {
77  static_assert(std::is_trivially_copyable<T>::value,
78                "CopyFromBufferOrDie can only copy trivially copyable types");
79  T out;
80  CHECK(buf_len >= sizeof(out));
81  std::memcpy(&out, buf, sizeof(out));
82  return out;
83}
84
85// Returns the maximal value representable by T. Generates a compile-time
86// error if T is not an integral type.
87template <typename T>
88constexpr T GetMaxVal() {
89  // Give a useful error for non-numeric types, and avoid returning zero for
90  // pointers and C-style enums (http://stackoverflow.com/a/9201960).
91  static_assert(std::is_integral<T>::value,
92                "GetMaxVal requires an integral type");
93  return std::numeric_limits<T>::max();
94}
95
96// Returns the maximal value representable by |t_instance|. Generates a
97// compile-time error if |t_instance| is not an instance of an integral type.
98template <typename T>
99constexpr T GetMaxVal(const T& /* t_instance */) {
100  return GetMaxVal<T>();
101}
102
103// Returns true if |c| is a printable character, for ASCII data.
104inline bool IsAsciiPrintable(uint8_t c) {
105  return (c == '\t' || c == '\n' || (c >= ' ' && c <= '~'));
106}
107
108namespace internal {
109
110// Implements the functionality documented for the SAFELY_CLAMP macro.
111// This function should be used via the SAFELY_CLAMP macro.
112template <typename SrcType, typename DstType, SrcType MinAsSrcType,
113          SrcType MaxAsSrcType, DstType MinAsDstType, DstType MaxAsDstType>
114DstType SafelyClamp(SrcType input) {
115  static_assert(std::is_integral<SrcType>::value,
116                "source type must be integral");
117  static_assert(std::is_integral<DstType>::value,
118                "destination type must be integral");
119  static_assert(MinAsSrcType < MaxAsSrcType, "invalid source range");
120  static_assert(MinAsDstType < MaxAsDstType, "invalid destination range");
121  // Clients should use the SAFELY_CLAMP macro, in which case this should never
122  // happen. (When the SAFELY_CLAMP macro is used, the values can only be
123  // unequal if there was a narrowing conversion. But, in that case, the value
124  // should have failed to match the template, since narrowing-conversions are
125  // not allowed for non-type template arguments.
126  // http://stackoverflow.com/a/24346350)
127  //
128  // Anyway, these checks provide a fail-safe, in case clients use the template
129  // function directly, and pass in inconsistent values for the range
130  // definition.
131  static_assert(MinAsSrcType == MinAsDstType, "inconsistent range min");
132  static_assert(MaxAsSrcType == MaxAsDstType, "inconsistent range max");
133
134  if (input < MinAsSrcType) {
135    return MinAsDstType;
136  } else if (input > MaxAsSrcType) {
137    return MaxAsDstType;
138  } else {
139    // - Given that the template has matched, we know that MinAsSrcType,
140    //   MaxAsSrcType, MinAsDstType, and MaxAsDstType are valid for their
141    //   respective types. (See narrowing-conversion comment above.)
142    // - Given the static_assert()s above, we know that a) the ranges are
143    //   well-formed, and that the b) source range is identical to the
144    //   destination range.
145    // - Given the range checks above, we know that |input| is within the range.
146    //
147    // Hence, the value to be returned must be valid for DstType, and the
148    // expression below has the same value as |input|.
149    return static_cast<DstType>(input);
150  }
151}
152
153}  // namespace internal
154
155}  // namespace local_utils
156}  // namespace wifilogd
157}  // namespace android
158
159#endif  // LOCAL_UTILS_H_
160