In this blog, we will analyze the functions provided by logging.h. by referencing this file, you can include < Android base / logging.h >, which is implemented in logging.cpp.

First of all, let's take a look at its comments: it provides an interface of c++ stream, and PLOG will print out specific errors, and it also supports the printing of logcat, stderr and dmesg.

//
// Google-style C++ logging.
//

// This header provides a C++ stream interface to logging.
//
// To log:
//
//   LOG(INFO) << "Some text; " << some_value;
//
// Replace `INFO` with any severity from `enum LogSeverity`.
//
// To log the result of a failed function and include the string
// representation of `errno` at the end:
//
//   PLOG(ERROR) << "Write failed";
//
// The output will be something like `Write failed: I/O error`.
// Remember this as 'P' as in perror(3).
//
// To output your own types, simply implement operator<< as normal.
//
// By default, output goes to logcat on Android and stderr on the host.
// A process can use `SetLogger` to decide where all logging goes.
// Implementations are provided for logcat, stderr, and dmesg.

// This header also provides assertions:
//
//   CHECK(must_be_true);
//   CHECK_EQ(a, b) << z_is_interesting_too;

// NOTE: For Windows, you must include logging.h after windows.h to allow the
// following code to suppress the evil ERROR macro:

Let's take a look at this header file. Before using it, we must call the InitLogging function. Here are three loggers to choose from: kernelllogger, StderrLogger, and LogdLogger. In android, the default is LogdLogger.

void KernelLogger(LogId, LogSeverity, const char*, const char*, unsigned int, const char*);
void StderrLogger(LogId, LogSeverity, const char*, const char*, unsigned int, const char*);

void DefaultAborter(const char* abort_message);

#ifdef __ANDROID__
// We expose this even though it is the default because a user that wants to
// override the default log buffer will have to construct this themselves.
class LogdLogger {
 public:
  explicit LogdLogger(LogId default_log_id = android::base::MAIN);

  void operator()(LogId, LogSeverity, const char* tag, const char* file,
                  unsigned int line, const char* message);

 private:
  LogId default_log_id_;
};
#endif

// Configure logging based on ANDROID_LOG_TAGS environment variable.
// We need to parse a string that looks like
//
//      *:v jdwp:d dalvikvm:d dalvikvm-gc:i dalvikvmi:i
//
// The tag (or '*' for the global level) comes first, followed by a colon and a
// letter indicating the minimum priority level we're expected to log.  This can
// be used to reveal or conceal logs with specific tags.
#ifdef __ANDROID__
#define INIT_LOGGING_DEFAULT_LOGGER LogdLogger()
#else
#define INIT_LOGGING_DEFAULT_LOGGER StderrLogger
#endif
void InitLogging(char* argv[],
                 LogFunction&& logger = INIT_LOGGING_DEFAULT_LOGGER,
                 AbortFunction&& aborter = DefaultAborter);
#undef INIT_LOGGING_DEFAULT_LOGGER

Let's take a look at the implementation of these three loggers.

The kernel is to write to dev/kmsg

void KernelLogger(android::base::LogId, android::base::LogSeverity severity,
                  const char* tag, const char*, unsigned int, const char* msg) {
  // clang-format off
  static constexpr int kLogSeverityToKernelLogLevel[] = {
      [android::base::VERBOSE] = 7,              // KERN_DEBUG (there is no verbose kernel log
                                                 //             level)
      [android::base::DEBUG] = 7,                // KERN_DEBUG
      [android::base::INFO] = 6,                 // KERN_INFO
      [android::base::WARNING] = 4,              // KERN_WARNING
      [android::base::ERROR] = 3,                // KERN_ERROR
      [android::base::FATAL_WITHOUT_ABORT] = 2,  // KERN_CRIT
      [android::base::FATAL] = 2,                // KERN_CRIT
  };
  // clang-format on
  static_assert(arraysize(kLogSeverityToKernelLogLevel) == android::base::FATAL + 1,
                "Mismatch in size of kLogSeverityToKernelLogLevel and values in LogSeverity");

  static int klog_fd = TEMP_FAILURE_RETRY(open("/dev/kmsg", O_WRONLY | O_CLOEXEC));
  if (klog_fd == -1) return;

  int level = kLogSeverityToKernelLogLevel[severity];

  // The kernel's printk buffer is only 1024 bytes.
  // TODO: should we automatically break up long lines into multiple lines?
  // Or we could log but with something like "..." at the end?
  char buf[1024];
  size_t size = snprintf(buf, sizeof(buf), "<%d>%s: %s\n", level, tag, msg);
  if (size > sizeof(buf)) {
    size = snprintf(buf, sizeof(buf), "<%d>%s: %zu-byte message too long for printk\n",
                    level, tag, size);
  }

  iovec iov[1];
  iov[0].iov_base = buf;
  iov[0].iov_len = size;
  TEMP_FAILURE_RETRY(writev(klog_fd, iov, 1));
}

Here's stderr

void StderrLogger(LogId, LogSeverity severity, const char*, const char* file,
                  unsigned int line, const char* message) {
  struct tm now;
  time_t t = time(nullptr);

#if defined(_WIN32)
  localtime_s(&now, &t);
#else
  localtime_r(&t, &now);
#endif

  char timestamp[32];
  strftime(timestamp, sizeof(timestamp), "%m-%d %H:%M:%S", &now);

  static const char log_characters[] = "VDIWEFF";
  static_assert(arraysize(log_characters) - 1 == FATAL + 1,
                "Mismatch in size of log_characters and values in LogSeverity");
  char severity_char = log_characters[severity];
  fprintf(stderr, "%s %c %s %5d %5d %s:%u] %s\n", ProgramInvocationName().c_str(),
          severity_char, timestamp, getpid(), GetThreadId(), file, line, message);
}

Logd is the last call to the __android_log_buf_print function to write to logd. This function calls log/log.h

void LogdLogger::operator()(LogId id, LogSeverity severity, const char* tag,
                            const char* file, unsigned int line,
                            const char* message) {
  static constexpr android_LogPriority kLogSeverityToAndroidLogPriority[] = {
      ANDROID_LOG_VERBOSE, ANDROID_LOG_DEBUG, ANDROID_LOG_INFO,
      ANDROID_LOG_WARN,    ANDROID_LOG_ERROR, ANDROID_LOG_FATAL,
      ANDROID_LOG_FATAL,
  };
  static_assert(arraysize(kLogSeverityToAndroidLogPriority) == FATAL + 1,
                "Mismatch in size of kLogSeverityToAndroidLogPriority and values in LogSeverity");

  int priority = kLogSeverityToAndroidLogPriority[severity];
  if (id == DEFAULT) {
    id = default_log_id_;
  }

  static constexpr log_id kLogIdToAndroidLogId[] = {
    LOG_ID_MAX, LOG_ID_MAIN, LOG_ID_SYSTEM,
  };
  static_assert(arraysize(kLogIdToAndroidLogId) == SYSTEM + 1,
                "Mismatch in size of kLogIdToAndroidLogId and values in LogId");
  log_id lg_id = kLogIdToAndroidLogId[id];

  if (priority == ANDROID_LOG_FATAL) {
    __android_log_buf_print(lg_id, priority, tag, "%s:%u] %s", file, line,
                            message);
  } else {
    __android_log_buf_print(lg_id, priority, tag, "%s", message);
  }

This file provides a very convenient function. It can not only use the c + + stream, but also switch between various logs, as long as you specify different logger s in InitLogging.

Let's take an example of init. Init calls the initkernelllogging function at the beginning. This function writes all the standard input and output and standard errors to / sys/fs/selinux/null, and there will be no more. Then I set an InitLogging as kernel logger, which is written in ksmg.

void InitKernelLogging(char* argv[]) {
    // Make stdin/stdout/stderr all point to /dev/null.
    int fd = open("/sys/fs/selinux/null", O_RDWR);
    if (fd == -1) {
        int saved_errno = errno;
        android::base::InitLogging(argv, &android::base::KernelLogger);
        errno = saved_errno;
        PLOG(FATAL) << "Couldn't open /sys/fs/selinux/null";
    }
    dup2(fd, 0);
    dup2(fd, 1);
    dup2(fd, 2);
    if (fd > 2) close(fd);

    android::base::InitLogging(argv, &android::base::KernelLogger);
}