Android native开发：systemcorebase

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Android native开发：system/core/base

文章目录

• • 1. chrono_utils.h
  • 2. cmsg.h
  • 3. collections.h
  • 4. endian.h
  • 5. errno_restorer.h
  • 6. errors.h
  • 7. expected.h
  • 8. file.h
  • 9. format.h
  • 10. logging.h
  • 11. macros.h
  • 12. mapped_file.h
  • 13. memory.h
  • 14. no_destructor.h
  • 15. off64_t.h
  • 16. parsebool.h
  • 17. parsedouble.h
  • 18. parseint.h
  • 19. parsenetaddress.h
  • 20. process.h
  • 21. properties.h
  • 22. result.h
  • 23. scopeguard.h
  • 24. stringprintf.h
  • 25. strings.h
  • 26. test_utils.h
  • 27. thread_annotations.h
  • 28. threads.h
  • 29. unique_fd.h
  • 30. utf8.h

1. chrono_utils.h

跨平台，为Linux和Win/Darwin提供不同实现。

class boot_clock;
class Timer;

2. cmsg.h

Linux提供cmsg(3)系列宏管理local socket相关的control message。
对cmsg的封装，避免易发错误。

ssize_t SendFileDescriptorVector(borrowed_fd sock, const void* data, size_t len,const std::vector<int>& fds);
ssize_t ReceiveFileDescriptorVector(borrowed_fd sock, void* data, size_t len, size_t max_fds,std::vector<android::base::unique_fd>* fds);template <typename... Args>
ssize_t SendFileDescriptors(borrowed_fd sock, const void* data, size_t len, Args&&... sent_fds);template <typename... Args>
ssize_t ReceiveFileDescriptors(borrowed_fd sock, void* data, size_t len, Args&&... received_fds);

3. collections.h

辅助函数，把模板的变长参数放入集合容器中。

// Use as follows:template <typename... Args>std::vector<int> CreateVector(Args&&... args) {std::vector<int> result;Append(result, std::forward<Args>(args)...);return result;}template <typename CollectionType, typename T>
void Append(CollectionType& collection, T&& arg);template <typename CollectionType, typename T, typename... Args>
void Append(CollectionType& collection, T&& arg, Args&&... args);template <typename T, typename Arg, typename... Args>
void AssertType(Arg&&);template <typename T, typename Arg, typename... Args>
void AssertType(Arg&&, Args&&... args);

4. endian.h

一些宏定义，对函数__builtin_bswap16/32/64的封装。
和标准C库 <sys/endian.h>等价的跨平台实现。

5. errno_restorer.h

临时保存errno，作用域结束后恢复。

class ErrnoRestorer {public:ErrnoRestorer() : saved_errno_(errno) {}~ErrnoRestorer() { errno = saved_errno_; }private:const int saved_errno_;
};

6. errors.h

跨平台支持错误码转字符串，Linux上调用strerror。

std::string SystemErrorCodeToString(int error_code);

7. expected.h

实现std::expected。
当std::expected正式进入c++标准后，删除此实现，android::base::expected仅作为std::expected别名。

using android::base::expected;
using android::base::unexpected;expected<double,std::string> safe_divide(double i, double j) {if (j == 0) return unexpected("divide by zero");else return i / j;
}void test() {auto q = safe_divide(10, 0);if (q) { printf("%f\n", q.value()); }else { printf("%s\n", q.error().c_str()); }
}

8. file.h

文件操作的辅助函数。

class TemporaryFile;class TemporaryDir;bool ReadFdToString(borrowed_fd fd, std::string* content);
bool ReadFileToString(const std::string& path, std::string* content,bool follow_symlinks = false);bool WriteStringToFile(const std::string& content, const std::string& path,bool follow_symlinks = false);
bool WriteStringToFd(const std::string& content, borrowed_fd fd);bool WriteStringToFile(const std::string& content, const std::string& path,mode_t mode, uid_t owner, gid_t group,bool follow_symlinks = false);bool ReadFully(borrowed_fd fd, void* data, size_t byte_count);bool ReadFullyAtOffset(borrowed_fd fd, void* data, size_t byte_count, off64_t offset);
bool WriteFully(borrowed_fd fd, const void* data, size_t byte_count);bool RemoveFileIfExists(const std::string& path, std::string* err = nullptr);bool Realpath(const std::string& path, std::string* result);
bool Readlink(const std::string& path, std::string* result);std::string GetExecutablePath();
std::string GetExecutableDirectory();std::string Basename(const std::string& path);
std::string Dirname(const std::string& path);

9. format.h

通过这个头文件，可以引入fmtlib库。这个库是静态链接到libbase中的。
下面是一些使用的例子，在Android native代码中实测可用。

// We include fmtlib here as an alias, since libbase will have fmtlib statically linked already.
// It is accessed through its normal fmt:: namespace.#include <android-base/format.h>std::string msg = fmt::format("hello {} by fmt", "world");std::string message = fmt::sprintf("The answer is %d", 42);
fmt::fprintf(stderr, "Don't %s!", "panic");
fmt::printf("Elapsed time: %.2f seconds", 1.23);std::vector<int> v = {1, 2, 3};
fmt::print("{}", fmt::join(v, ", "));
// Output: "1, 2, 3"std::string answer = fmt::to_string(42);std::vector<char> out;
fmt::format_to(std::back_inserter(out), "{}", 42);// User-defined literal equivalent of :func:`fmt::format`.
using namespace fmt::literals;
std::string message = "The answer is {}"_format(42);
fmt::print("Elapsed time: {s:.2f} seconds", "s"_a=1.23);// Constructs a compile-time format string.
// A compile-time error because 'd' is an invalid specifier for strings.
std::string s = format(FMT_STRING("{:d}"), "foo");

10. logging.h

C++ stream接口封装的log。
Android输出到logcat，host上输出到stderr。可用SetLogger设置输出。
默认使用进程名作tag，也可定义宏LOG_TAG（需要在include这个头文件前）。

还提供一些assertion宏。CHECK, CHECK_EQ

// To log:
LOG(INFO) << "Some text; " << some_value;PLOG(ERROR) << "Write failed"; // 类似perror()
// `Write failed: I/O error`.CHECK(must_be_true);
CHECK_EQ(a, b) << z_is_interesting_too;// Log to the kernel log (dmesg).
void KernelLogger(LogId, LogSeverity, const char*, const char*, unsigned int, const char*);
// Log to stderr in the full logcat format (with pid/tid/time/tag details).
void StderrLogger(LogId, LogSeverity, const char*, const char*, unsigned int, const char*);
// Log just the message to stdout/stderr (without pid/tid/time/tag details).
// Useful for replacing printf(3)/perror(3)/err(3)/error(3) in command-line tools.
void StdioLogger(LogId, LogSeverity, const char*, const char*, unsigned int, const char*);void DefaultAborter(const char* abort_message);
void SetDefaultTag(const std::string& tag);void InitLogging(char* argv[],LogFunction&& logger = INIT_LOGGING_DEFAULT_LOGGER,AbortFunction&& aborter = DefaultAborter);// Replace the current logger.
void SetLogger(LogFunction&& logger);
// Replace the current aborter.
void SetAborter(AbortFunction&& aborter);

11. macros.h

几个宏定义。

#define TEMP_FAILURE_RETRY(exp)            
#define DISALLOW_COPY_AND_ASSIGN(TypeName)
#define DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName)#define arraysize(array) (sizeof(ArraySizeHelper(array)))
#define SIZEOF_MEMBER(t, f) sizeof(std::declval<t>().f)#define LIKELY( exp )       (__builtin_expect( (exp) != 0, true  ))
#define UNLIKELY( exp )     (__builtin_expect( (exp) != 0, false ))#define WARN_UNUSED __attribute__((warn_unused_result))#define ATTRIBUTE_UNUSED __attribute__((__unused__))
#define FALLTHROUGH_INTENDED [[clang::fallthrough]]  // NOLINT

12. mapped_file.h

提供filemapping跨平台实现。

class MappedFile {public:static std::unique_ptr<MappedFile> FromFd(borrowed_fd fd, off64_t offset, size_t length,static std::unique_ptr<MappedFile> FromOsHandle(os_handle h, off64_t offset, size_t length,int prot);MappedFile(MappedFile&& other);MappedFile& operator=(MappedFile&& other);char* data() const { return base_ + offset_; }size_t size() const { return size_; }private:MappedFile(char* base, size_t size, size_t offset) : base_(base), size_(size), offset_(offset) {}
};

13. memory.h

2个memcpy辅助函数。

// Use memcpy for access to unaligned data on targets with alignment
// restrictions.  The compiler will generate appropriate code to access these
// structures without generating alignment exceptions.
template <typename T>
static inline T get_unaligned(const void* address) {T result;memcpy(&result, address, sizeof(T));return result;
}template <typename T>
static inline void put_unaligned(void* address, T v) {memcpy(address, &v, sizeof(T));
}

14. no_destructor.h

TODO：没太理解这个类的目的是啥？？
使用placement new在栈空间上初始化类型T的对象。

template <typename T>
class NoDestructor;

15. off64_t.h

/** Mac OS has always had a 64-bit off_t, so it doesn't have off64_t. */
typedef off_t off64_t;

16. parsebool.h

把string解析成bool类型（ParseBoolResult封装）。

ParseBoolResult ParseBool(std::string_view s);

17. parsedouble.h

把string解析成double类型。
TODO：都是static函数？外部不能调用？

template <typename T, T (*strtox)(const char* str, char** endptr)>
static inline bool ParseFloatingPoint(const char* s, T* out, T min, T max);static inline bool ParseDouble(const char* s, double* out,double min = std::numeric_limits<double>::lowest(),double max = std::numeric_limits<double>::max());static inline bool ParseDouble(const std::string& s, double* out,double min = std::numeric_limits<double>::lowest(),double max = std::numeric_limits<double>::max());static inline bool ParseFloat(const char* s, float* out,float min = std::numeric_limits<float>::lowest(),float max = std::numeric_limits<float>::max());static inline bool ParseFloat(const std::string& s, float* out,float min = std::numeric_limits<float>::lowest(),float max = std::numeric_limits<float>::max());

18. parseint.h

string解析int辅助函数。

template <typename T>
bool ParseUint(const char* s, T* out, T max = std::numeric_limits<T>::max(),bool allow_suffixes = false);template <typename T>
bool ParseUint(const std::string& s, T* out, T max = std::numeric_limits<T>::max(),bool allow_suffixes = false);template <typename T>
bool ParseByteCount(const char* s, T* out, T max = std::numeric_limits<T>::max());template <typename T>
bool ParseByteCount(const std::string& s, T* out, T max = std::numeric_limits<T>::max());template <typename T>
bool ParseInt(const char* s, T* out,T min = std::numeric_limits<T>::min(),T max = std::numeric_limits<T>::max())template <typename T>
bool ParseInt(const std::string& s, T* out,T min = std::numeric_limits<T>::min(),T max = std::numeric_limits<T>::max());

19. parsenetaddress.h

string解析网络地址辅助函数。

bool ParseNetAddress(const std::string& address, std::string* host, int* port,std::string* canonical_address, std::string* error);

20. process.h

遍历/proc目录获取所有pid。

class AllPids {class PidIterator {public:PidIterator(DIR* dir) : dir_(dir, closedir) { Increment(); }PidIterator& operator++();};public:PidIterator begin() { return opendir("/proc"); }PidIterator end() { return nullptr; }
};

21. properties.h

封装读写property接口。

std::string GetProperty(const std::string& key, const std::string& default_value);bool GetBoolProperty(const std::string& key, bool default_value);template <typename T> T GetIntProperty(const std::string& key,T default_value,T min = std::numeric_limits<T>::min(),T max = std::numeric_limits<T>::max());template <typename T> T GetUintProperty(const std::string& key,T default_value,T max = std::numeric_limits<T>::max());bool SetProperty(const std::string& key, const std::string& value);bool WaitForProperty(const std::string& key, const std::string& expected_value,std::chrono::milliseconds relative_timeout = std::chrono::milliseconds::max());bool WaitForPropertyCreation(const std::string& key, std::chrono::milliseconds relative_timeout =std::chrono::milliseconds::max());// Cached system property lookup. For code that needs to read the same property multiple times,
// this class helps optimize those lookups.
class CachedProperty;

22. result.h

1）Result<T>包含一个类型T的值（类似optional<T>访问），或者包含类型ResultError的错误信息（通过Result<T>::error()访问）
2）ResultError包含一个string类型的信息和errno，且可直接用ostream输出。
3）ResultError<void>用于没有返回值的函数，函数成功返回，return { }。
4）返回的ResultError<T>，可以通过返回类型T对象，或者任何可以转换为T类型的对象，自动构建ResultError。
5）Error 和 ErrnoError用于创建出错时的Result<T>，Errorf 和 ErrnoErrorf格式化输出错误信息。
6）ResultError类型可以直接构建Result<T>, Result<U>.error() -> Result<T>

Result<U> CalculateResult(const T& input) {U output;if (!SomeOtherCppFunction(input, &output)) {return Errorf("SomeOtherCppFunction {} failed", input);}if (!c_api_function(output)) {return ErrnoErrorf("c_api_function {} failed", output);}return output;
}auto output = CalculateResult(input);
if (!output) return Error() << "CalculateResult failed: " << output.error();
UseOutput(*output);

23. scopeguard.h

提供一个RAII辅助类，确保在作用域结束时调用指定的函数（对象）。

// ScopeGuard ensures that the specified functor is executed no matter how the
// current scope exits.
template <typename F>
class ScopeGuard {public:ScopeGuard(F&& f) : f_(std::forward<F>(f)), active_(true) {}ScopeGuard(ScopeGuard&& that) noexcept : f_(std::move(that.f_)), active_(that.active_) {that.active_ = false;}template <typename Functor>ScopeGuard(ScopeGuard<Functor>&& that) : f_(std::move(that.f_)), active_(that.active_) {that.active_ = false;}~ScopeGuard() {if (active_) f_();}private:template <typename Functor>friend class ScopeGuard;F f_;bool active_;
};template <typename F>
ScopeGuard<F> make_scope_guard(F&& f) {return ScopeGuard<F>(std::forward<F>(f));
}

24. stringprintf.h

通过类似printf函数格式，拼接字符串的辅助函数。
可以用libfmt替代。

std::string StringPrintf(const char* fmt, ...);
void StringAppendF(std::string* dst, const char* fmt, ...);
void StringAppendV(std::string* dst, const char* format, va_list ap);

25. strings.h

string操作辅助函数，部分可以用std::string的标准库函数替代。

std::vector<std::string> Split(const std::string& s,const std::string& delimiters);std::string Trim(const std::string& s);template <typename ContainerT, typename SeparatorT>
std::string Join(const ContainerT& things, SeparatorT separator);// We instantiate the common cases in strings.cpp.
extern template std::string Join(const std::vector<std::string>&, char);
extern template std::string Join(const std::vector<const char*>&, char);
extern template std::string Join(const std::vector<std::string>&, const std::string&);
extern template std::string Join(const std::vector<const char*>&, const std::string&);bool StartsWith(std::string_view s, std::string_view prefix);
bool StartsWith(std::string_view s, char prefix);
bool StartsWithIgnoreCase(std::string_view s, std::string_view prefix);bool EndsWith(std::string_view s, std::string_view suffix);
bool EndsWith(std::string_view s, char suffix);
bool EndsWithIgnoreCase(std::string_view s, std::string_view suffix);bool EqualsIgnoreCase(std::string_view lhs, std::string_view rhs);// Removes `prefix` from the start of the given string and returns true (if
// it was present), false otherwise.
inline bool ConsumePrefix(std::string_view* s, std::string_view prefix);// Removes `suffix` from the end of the given string and returns true (if
// it was present), false otherwise.
inline bool ConsumeSuffix(std::string_view* s, std::string_view suffix);// Replaces `from` with `to` in `s`, once if `all == false`, or as many times as
// there are matches if `all == true`.
[[nodiscard]] std::string StringReplace(std::string_view s, std::string_view from,std::string_view to, bool all);

26. test_utils.h

gtest辅助类和宏。
assert_和expect_宏是对std::regex_search和gtest宏封装。

class CapturedStdFd；#define ASSERT_MATCH(str, pattern);
#define ASSERT_NOT_MATCH(str, pattern);
#define EXPECT_MATCH(str, pattern);
#define EXPECT_NOT_MATCH(str, pattern);

27. thread_annotations.h

#define THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))#define CAPABILITY(x) \THREAD_ANNOTATION_ATTRIBUTE__(capability(x))#define SCOPED_CAPABILITY \THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)#define SHARED_CAPABILITY(...) \THREAD_ANNOTATION_ATTRIBUTE__(shared_capability(__VA_ARGS__))#define GUARDED_BY(x) \THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))#define EXCLUSIVE_LOCKS_REQUIRED(...) \THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(__VA_ARGS__))#define SHARED_LOCKS_REQUIRED(...) \THREAD_ANNOTATION_ATTRIBUTE__(shared_locks_required(__VA_ARGS__))#define ACQUIRED_BEFORE(...) \THREAD_ANNOTATION_ATTRIBUTE__(acquired_before(__VA_ARGS__))
...

1）一些thread相关的__attribute__宏定义。
2）ScopedLockAssertion，为了Clang做线程安全分析的辅助类。

28. threads.h

跨平台支持获取thread id，在Linux上调用gettid()或syscall(__NR_gettid)

uint64_t GetThreadId();

29. unique_fd.h

封装fd的RAII类，提供unique_fd和borrowed_fd。

// 有bug的代码int print(int fd) {int rc;char buf[128];while ((rc = read(fd, buf, sizeof(buf))) > 0) {printf("%.*s", rc);}close(fd);}int bug() {FILE* f = fopen("foo", "r");print(fileno(f));fclose(f); // print中还在使用文件，外面把文件close了}

// Container for a file descriptor that automatically closes the descriptor as
// it goes out of scope.
//unique_fd ufd(open("/some/path", "r"));if (ufd.get() == -1) return error;// Do something useful, possibly including 'return'.return 0; // Descriptor is closed for you.inline FILE* Fdopen(unique_fd&& ufd, const char* mode) {
inline DIR* Fdopendir(unique_fd&& ufd) {
template <typename T>
int close(const android::base::unique_fd_impl<T>&);
template <typename T>
FILE* fdopen(const android::base::unique_fd_impl<T>&, const char* mode);
template <typename T>
DIR* fdopendir(const android::base::unique_fd_impl<T>&);

30. utf8.h

Windows平台上UTF和wchar相互转换的函数。

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