1. Overview

ALOGV 的配置信息和使用方法如下：

#ifdef LOG_NDEBUG
#undef LOG_NDEBUG
#endif
#define LOG_NDEBUG 0

#define LOG_TAG "TAG_NAME"

#include "utils/Log.h"

static void method(int param) {
  ALOGV("%s: param is: %d", __func__, param);
}

ALOGD、ALOGI、ALOGW、ALOGE 默认开启，无需单独配置。

2. Detail

2.1 ALOGV 使用

packages/apps/Bluetooth/jni/com_android_bluetooth_btservice_AdapterService.cpp

#define LOG_NDEBUG 0

#define LOG_TAG "BluetoothServiceJni"

.......
#include "utils/Log.h"
......


static void adapter_state_change_callback(bt_state_t status) {
......
  ALOGV("%s: Status is: %d", __func__, status);
......
}

2.2 ALOGV 原理

2.2.1 #include "utils/Log.h"

system/core/libutils/include/utils/Log.h

// DO NOT INCLUDE ANYTHING NEW IN THIS FILE.

// <log/log.h> has replaced this file and all changes should go there instead.
// This path remains strictly to include that header as there are thousands of
// references to <utils/Log.h> in the tree.

#include <log/log.h>

2.2.2 #include <log/log.h>

system/logging/liblog/include/log/log.h

#pragma once

/* Too many in the ecosystem assume these are included */
#if !defined(_WIN32)
#include <pthread.h>
#endif
#include <stdint.h> /* uint16_t, int32_t */
#include <stdio.h>
#include <time.h>
#include <unistd.h>

#include <android/log.h>
#include <log/log_id.h>
#include <log/log_main.h>
#include <log/log_radio.h>
#include <log/log_safetynet.h>
#include <log/log_system.h>
#include <log/log_time.h>

#ifdef __cplusplus
extern "C" {
#endif

/*
 * LOG_TAG is the local tag used for the following simplified
 * logging macros.  You can change this preprocessor definition
 * before using the other macros to change the tag.
 */

#ifndef LOG_TAG
#define LOG_TAG NULL
#endif

/*
 * Normally we strip the effects of ALOGV (VERBOSE messages),
 * LOG_FATAL and LOG_FATAL_IF (FATAL assert messages) from the
 * release builds be defining NDEBUG.  You can modify this (for
 * example with "#define LOG_NDEBUG 0" at the top of your source
 * file) to change that behavior.
 */

#ifndef LOG_NDEBUG
#ifdef NDEBUG
#define LOG_NDEBUG 1
#else
#define LOG_NDEBUG 0
#endif
#endif

LOG_TAG
从源码注释可知，若未定义 LOG_TAG 宏，那么打印的 log 没有 tag 信息；
若在引用头文件之前定义 LOG_TAG 宏，则使用该宏作为 tag，如 2.1 ALOGV 使用
LOG_DEBUG
从源码注释可知，若想开启 ALOGV，则需要在源文件顶部定义 #define LOG_NDEBUG 0，如 2.1 ALOGV 使用

2.2.3 #include <log/log_main.h>

system/logging/liblog/include/log/log_main.h

#pragma once

#include <stdbool.h>
#include <sys/cdefs.h>
#include <sys/types.h>

#include <android/log.h>

......

#ifndef ALOGV

#define __ALOGV(...) ((void)ALOG(LOG_VERBOSE, LOG_TAG, __VA_ARGS__))   // 2

#if LOG_NDEBUG 
#define ALOGV(...)                   \
  do {                               \
    __FAKE_USE_VA_ARGS(__VA_ARGS__); \
    if (false) {                     \
      __ALOGV(__VA_ARGS__);          \
    }                                \
  } while (false)
#else
#define ALOGV(...) __ALOGV(__VA_ARGS__)   // 1
#endif

#endif

// ALOGD 不受此控制，默认开启
#ifndef ALOGD
#define ALOGD(...) ((void)ALOG(LOG_DEBUG, LOG_TAG, __VA_ARGS__))
#endif

由以上源码可知：若 LOG_NDEBUG 宏为0，则对应的是 // 1、// 2，相当于开启 ALOGV()；否则 ALOGV() 对应的是 do-while(false) 循环体，其内部是 if(false) 语句，相当于无实现。

继续分析 ((void)ALOG(LOG_VERBOSE, LOG_TAG, __VA_ARGS__))

#ifndef ALOG
#define ALOG(priority, tag, ...) LOG_PRI(ANDROID_##priority, tag, __VA_ARGS__)
#endif

ANDROID_##priority 表示拼接，根据之前传递的参数 LOG_VERBOSE，那么，此时相当于 LOG_PRI(ANDROID_LOG_VERBOSE, tag, __VA_ARGS__)，继续分析该函数

#ifndef LOG_PRI
#define LOG_PRI(priority, tag, ...) android_printLog(priority, tag, __VA_ARGS__)
#endif

#define android_printLog(prio, tag, ...) \
  __android_log_print(prio, tag, __VA_ARGS__)

2.2.4 #include <android/log.h>

system/logging/liblog/include/android/log.h

int __android_log_print(int prio, const char* tag, const char* fmt, ...)
    __attribute__((__format__(printf, 3, 4)));

Log 优先级定义如下：

/**
 * Android log priority values, in increasing order of priority.
 */
typedef enum android_LogPriority {
  /** For internal use only.  */
  ANDROID_LOG_UNKNOWN = 0,
  /** The default priority, for internal use only.  */
  ANDROID_LOG_DEFAULT, /* only for SetMinPriority() */
  /** Verbose logging. Should typically be disabled for a release apk. */
  ANDROID_LOG_VERBOSE,
  /** Debug logging. Should typically be disabled for a release apk. */
  ANDROID_LOG_DEBUG,
  /** Informational logging. Should typically be disabled for a release apk. */
  ANDROID_LOG_INFO,
  /** Warning logging. For use with recoverable failures. */
  ANDROID_LOG_WARN,
  /** Error logging. For use with unrecoverable failures. */
  ANDROID_LOG_ERROR,
  /** Fatal logging. For use when aborting. */
  ANDROID_LOG_FATAL,
  /** For internal use only.  */
  ANDROID_LOG_SILENT, /* only for SetMinPriority(); must be last */
} android_LogPriority;

system/logging/liblog/logger_write.cpp

int __android_log_print(int prio, const char* tag, const char* fmt, ...) {
  ErrnoRestorer errno_restorer;

  if (!__android_log_is_loggable(prio, tag, ANDROID_LOG_VERBOSE)) {
    return -EPERM;
  }

  va_list ap;
  __attribute__((uninitialized)) char buf[LOG_BUF_SIZE];

  va_start(ap, fmt);
  vsnprintf(buf, LOG_BUF_SIZE, fmt, ap);
  va_end(ap);

  __android_log_message log_message = {
      sizeof(__android_log_message), LOG_ID_MAIN, prio, tag, nullptr, 0, buf};
  __android_log_write_log_message(&log_message);
  return 1;
}