1 : // -*- mode: c++ -*-
2 :
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31 :
32 : // Original author: Jim Blandy <jimb@mozilla.com> <jimb@red-bean.com>
33 :
34 : // byte_cursor.h: Classes for parsing values from a buffer of bytes.
35 : // The ByteCursor class provides a convenient interface for reading
36 : // fixed-size integers of arbitrary endianness, being thorough about
37 : // checking for buffer overruns.
38 :
39 : #ifndef COMMON_BYTE_CURSOR_H_
40 : #define COMMON_BYTE_CURSOR_H_
41 :
42 : #include <assert.h>
43 : #include <stdint.h>
44 : #include <stdlib.h>
45 : #include <string.h>
46 : #include <string>
47 :
48 : namespace google_breakpad {
49 :
50 : // A buffer holding a series of bytes.
51 : struct ByteBuffer {
52 : ByteBuffer() : start(0), end(0) { }
53 0 : ByteBuffer(const uint8_t *set_start, size_t set_size)
54 0 : : start(set_start), end(set_start + set_size) { }
55 0 : ~ByteBuffer() { };
56 :
57 : // Equality operators. Useful in unit tests, and when we're using
58 : // ByteBuffers to refer to regions of a larger buffer.
59 : bool operator==(const ByteBuffer &that) const {
60 : return start == that.start && end == that.end;
61 : }
62 : bool operator!=(const ByteBuffer &that) const {
63 : return start != that.start || end != that.end;
64 : }
65 :
66 : // Not C++ style guide compliant, but this definitely belongs here.
67 0 : size_t Size() const {
68 0 : assert(start <= end);
69 0 : return end - start;
70 : }
71 :
72 : const uint8_t *start, *end;
73 : };
74 :
75 : // A cursor pointing into a ByteBuffer that can parse numbers of various
76 : // widths and representations, strings, and data blocks, advancing through
77 : // the buffer as it goes. All ByteCursor operations check that accesses
78 : // haven't gone beyond the end of the enclosing ByteBuffer.
79 : class ByteCursor {
80 : public:
81 : // Create a cursor reading bytes from the start of BUFFER. By default, the
82 : // cursor reads multi-byte values in little-endian form.
83 0 : ByteCursor(const ByteBuffer *buffer, bool big_endian = false)
84 : : buffer_(buffer), here_(buffer->start),
85 0 : big_endian_(big_endian), complete_(true) { }
86 :
87 : // Accessor and setter for this cursor's endianness flag.
88 : bool big_endian() const { return big_endian_; }
89 : void set_big_endian(bool big_endian) { big_endian_ = big_endian; }
90 :
91 : // Accessor and setter for this cursor's current position. The setter
92 : // returns a reference to this cursor.
93 : const uint8_t *here() const { return here_; }
94 : ByteCursor &set_here(const uint8_t *here) {
95 : assert(buffer_->start <= here && here <= buffer_->end);
96 : here_ = here;
97 : return *this;
98 : }
99 :
100 : // Return the number of bytes available to read at the cursor.
101 0 : size_t Available() const { return size_t(buffer_->end - here_); }
102 :
103 : // Return true if this cursor is at the end of its buffer.
104 : bool AtEnd() const { return Available() == 0; }
105 :
106 : // When used as a boolean value this cursor converts to true if all
107 : // prior reads have been completed, or false if we ran off the end
108 : // of the buffer.
109 0 : operator bool() const { return complete_; }
110 :
111 : // Read a SIZE-byte integer at this cursor, signed if IS_SIGNED is true,
112 : // unsigned otherwise, using the cursor's established endianness, and set
113 : // *RESULT to the number. If we read off the end of our buffer, clear
114 : // this cursor's complete_ flag, and store a dummy value in *RESULT.
115 : // Return a reference to this cursor.
116 : template<typename T>
117 0 : ByteCursor &Read(size_t size, bool is_signed, T *result) {
118 0 : if (CheckAvailable(size)) {
119 0 : T v = 0;
120 0 : if (big_endian_) {
121 0 : for (size_t i = 0; i < size; i++)
122 0 : v = (v << 8) + here_[i];
123 : } else {
124 : // This loop condition looks weird, but size_t is unsigned, so
125 : // decrementing i after it is zero yields the largest size_t value.
126 0 : for (size_t i = size - 1; i < size; i--)
127 0 : v = (v << 8) + here_[i];
128 : }
129 0 : if (is_signed && size < sizeof(T)) {
130 0 : size_t sign_bit = (T)1 << (size * 8 - 1);
131 0 : v = (v ^ sign_bit) - sign_bit;
132 : }
133 0 : here_ += size;
134 0 : *result = v;
135 : } else {
136 0 : *result = (T) 0xdeadbeef;
137 : }
138 0 : return *this;
139 : }
140 :
141 : // Read an integer, using the cursor's established endianness and
142 : // *RESULT's size and signedness, and set *RESULT to the number. If we
143 : // read off the end of our buffer, clear this cursor's complete_ flag.
144 : // Return a reference to this cursor.
145 : template<typename T>
146 : ByteCursor &operator>>(T &result) {
147 : bool T_is_signed = (T)-1 < 0;
148 : return Read(sizeof(T), T_is_signed, &result);
149 : }
150 :
151 : // Copy the SIZE bytes at the cursor to BUFFER, and advance this
152 : // cursor to the end of them. If we read off the end of our buffer,
153 : // clear this cursor's complete_ flag, and set *POINTER to NULL.
154 : // Return a reference to this cursor.
155 : ByteCursor &Read(uint8_t *buffer, size_t size) {
156 : if (CheckAvailable(size)) {
157 : memcpy(buffer, here_, size);
158 : here_ += size;
159 : }
160 : return *this;
161 : }
162 :
163 : // Set STR to a copy of the '\0'-terminated string at the cursor. If the
164 : // byte buffer does not contain a terminating zero, clear this cursor's
165 : // complete_ flag, and set STR to the empty string. Return a reference to
166 : // this cursor.
167 : ByteCursor &CString(std::string *str) {
168 : const uint8_t *end
169 : = static_cast<const uint8_t *>(memchr(here_, '\0', Available()));
170 : if (end) {
171 : str->assign(reinterpret_cast<const char *>(here_), end - here_);
172 : here_ = end + 1;
173 : } else {
174 : str->clear();
175 : here_ = buffer_->end;
176 : complete_ = false;
177 : }
178 : return *this;
179 : }
180 :
181 : // Like CString(STR), but extract the string from a fixed-width buffer
182 : // LIMIT bytes long, which may or may not contain a terminating '\0'
183 : // byte. Specifically:
184 : //
185 : // - If there are not LIMIT bytes available at the cursor, clear the
186 : // cursor's complete_ flag and set STR to the empty string.
187 : //
188 : // - Otherwise, if the LIMIT bytes at the cursor contain any '\0'
189 : // characters, set *STR to a copy of the bytes before the first '\0',
190 : // and advance the cursor by LIMIT bytes.
191 : //
192 : // - Otherwise, set *STR to a copy of those LIMIT bytes, and advance the
193 : // cursor by LIMIT bytes.
194 : ByteCursor &CString(std::string *str, size_t limit) {
195 : if (CheckAvailable(limit)) {
196 : const uint8_t *end
197 : = static_cast<const uint8_t *>(memchr(here_, '\0', limit));
198 : if (end)
199 : str->assign(reinterpret_cast<const char *>(here_), end - here_);
200 : else
201 : str->assign(reinterpret_cast<const char *>(here_), limit);
202 : here_ += limit;
203 : } else {
204 : str->clear();
205 : }
206 : return *this;
207 : }
208 :
209 : // Set *POINTER to point to the SIZE bytes at the cursor, and advance
210 : // this cursor to the end of them. If SIZE is omitted, don't move the
211 : // cursor. If we read off the end of our buffer, clear this cursor's
212 : // complete_ flag, and set *POINTER to NULL. Return a reference to this
213 : // cursor.
214 : ByteCursor &PointTo(const uint8_t **pointer, size_t size = 0) {
215 : if (CheckAvailable(size)) {
216 : *pointer = here_;
217 : here_ += size;
218 : } else {
219 : *pointer = NULL;
220 : }
221 : return *this;
222 : }
223 :
224 : // Skip SIZE bytes at the cursor. If doing so would advance us off
225 : // the end of our buffer, clear this cursor's complete_ flag, and
226 : // set *POINTER to NULL. Return a reference to this cursor.
227 : ByteCursor &Skip(size_t size) {
228 : if (CheckAvailable(size))
229 : here_ += size;
230 : return *this;
231 : }
232 :
233 : private:
234 : // If there are at least SIZE bytes available to read from the buffer,
235 : // return true. Otherwise, set here_ to the end of the buffer, set
236 : // complete_ to false, and return false.
237 0 : bool CheckAvailable(size_t size) {
238 0 : if (Available() >= size) {
239 0 : return true;
240 : } else {
241 0 : here_ = buffer_->end;
242 0 : complete_ = false;
243 0 : return false;
244 : }
245 : }
246 :
247 : // The buffer we're reading bytes from.
248 : const ByteBuffer *buffer_;
249 :
250 : // The next byte within buffer_ that we'll read.
251 : const uint8_t *here_;
252 :
253 : // True if we should read numbers in big-endian form; false if we
254 : // should read in little-endian form.
255 : bool big_endian_;
256 :
257 : // True if we've been able to read all we've been asked to.
258 : bool complete_;
259 : };
260 :
261 : } // namespace google_breakpad
262 :
263 : #endif // COMMON_BYTE_CURSOR_H_
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