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
50 namespace android {
51 namespace wifilogd {
52 namespace 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.)
56 template <typename T>
CastEnumToInteger(T enum_value)57 constexpr 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.
75 template <typename T>
CopyFromBufferOrDie(NONNULL const void * buf,size_t buf_len)76 T 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.
87 template <typename T>
GetMaxVal()88 constexpr 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.
98 template <typename T>
GetMaxVal(const T &)99 constexpr T GetMaxVal(const T& /* t_instance */) {
100 return GetMaxVal<T>();
101 }
102
103 // Returns true if |c| is a printable character, for ASCII data.
IsAsciiPrintable(uint8_t c)104 inline bool IsAsciiPrintable(uint8_t c) {
105 return (c == '\t' || c == '\n' || (c >= ' ' && c <= '~'));
106 }
107
108 namespace internal {
109
110 // Implements the functionality documented for the SAFELY_CLAMP macro.
111 // This function should be used via the SAFELY_CLAMP macro.
112 template <typename SrcType, typename DstType, SrcType MinAsSrcType,
113 SrcType MaxAsSrcType, DstType MinAsDstType, DstType MaxAsDstType>
SafelyClamp(SrcType input)114 DstType 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