liblog module

概述

源码分析

1. properties模块

1.1 __android_log_is_loggable

// INFO, 如果定义了LOG_TAG则使用LOG_TAG，否则为空，default是INFO
int __android_log_is_loggable(int prio, const char* tag, int default_prio) {
  auto len = tag ? strlen(tag) : 0;
  return __android_log_is_loggable_len(prio, tag, len, default_prio);
}

1.2 __android_log_is_loggable_len

int __android_log_is_loggable_len(int prio, const char* tag, size_t len, int default_prio) {
    // 默认是比verbose还小的
  int minimum_log_priority = __android_log_get_minimum_priority();
    // first_stage_init阶段，这里返回的为-1
  int property_log_level = __android_log_level(tag, len);

  if (property_log_level >= 0 && minimum_log_priority != ANDROID_LOG_DEFAULT) {
    return prio >= std::min(property_log_level, minimum_log_priority);
  } else if (property_log_level >= 0) {
    return prio >= property_log_level;
  } else if (minimum_log_priority != ANDROID_LOG_DEFAULT) {
    return prio >= minimum_log_priority;
  } else {
      // 所以最终走这里，要大于等于INFO，所以first_stage_init阶段，VERBOSE和DEBUG的log可能不会打印出来
    return prio >= default_prio;
  }
}

1.3 __android_log_level

// tag为null，len为0
static int __android_log_level(const char* tag, size_t len) {
  /* sizeof() is used on this array below */
  static const char log_namespace[] = "persist.log.tag.";
  static const size_t base_offset = 8; /* skip "persist." */

  if (tag == nullptr || len == 0) {
      // 默认是进程名字
    auto& tag_string = GetDefaultTag();
    tag = tag_string.c_str();
    len = tag_string.size();
  }

  /* sizeof(log_namespace) = strlen(log_namespace) + 1 */
  char key[sizeof(log_namespace) + len];
  char* kp;
  size_t i;
  char c = 0;
  /*
   * Single layer cache of four properties. Priorities are:
   *    log.tag.<tag>
   *    persist.log.tag.<tag>
   *    log.tag
   *    persist.log.tag
   * Where the missing tag matches all tags and becomes the
   * system global default. We do not support ro.log.tag* .
   */
  static char* last_tag;
  static size_t last_tag_len;
  static uint32_t global_serial;
  /* some compilers erroneously see uninitialized use. !not_locked */
  uint32_t current_global_serial = 0;
  static struct cache_char tag_cache[2];
  static struct cache_char global_cache[2];
  int change_detected;
  int global_change_detected;
  int not_locked;

  strcpy(key, log_namespace);
// 为0，则获取到锁了
  global_change_detected = change_detected = not_locked = lock();
// 获取到锁了
  if (!not_locked) {
    /*
     *  check all known serial numbers to changes.
     */
    for (i = 0; i < (sizeof(tag_cache) / sizeof(tag_cache[0])); ++i) {
        // 第一次的时候，cache->pinfo为null，所以为false，change_detected为0
      if (check_cache(&tag_cache[i].cache)) {
        change_detected = 1;
      }
    }
    for (i = 0; i < (sizeof(global_cache) / sizeof(global_cache[0])); ++i) {
        // 第一次的时候，其为false，global_change_detected还是0
      if (check_cache(&global_cache[i].cache)) {
        global_change_detected = 1;
      }
    }
// 在first_stage_init阶段，还没有初始化property，所以这里为-1
    current_global_serial = __system_property_area_serial();
      // 所以第一次的时候，这里是不相等的，所以为1
    if (current_global_serial != global_serial) {
      change_detected = 1;
      global_change_detected = 1;
    }
  }

  if (len) {
    int local_change_detected = change_detected;
    if (!not_locked) {
        // 第一次last_tag为null
      if (!last_tag || !last_tag[0] || (last_tag[0] != tag[0]) ||
          strncmp(last_tag + 1, tag + 1, last_tag_len - 1)) {
        /* invalidate log.tag.<tag> cache */
        for (i = 0; i < (sizeof(tag_cache) / sizeof(tag_cache[0])); ++i) {
            // 初始化tag_cache
          tag_cache[i].cache.pinfo = NULL;
          tag_cache[i].c = ‘\0‘;
        }
        if (last_tag) last_tag[0] = ‘\0‘;
        local_change_detected = 1;
      }
      if (!last_tag || !last_tag[0]) {
        if (!last_tag) {
            // 第一次就进行分配了
          last_tag = static_cast<char*>(calloc(1, len + 1));
          last_tag_len = 0;
          if (last_tag) last_tag_len = len + 1;
        } else if (len >= last_tag_len) {
          last_tag = static_cast<char*>(realloc(last_tag, len + 1));
          last_tag_len = 0;
          if (last_tag) last_tag_len = len + 1;
        }
        if (last_tag) {
            // 将tag拷贝到last_tag上
          strncpy(last_tag, tag, len);
          last_tag[len] = ‘\0‘;
        }
      }
    }
      // 组合persist.log.tag.TAGNAME
    strncpy(key + sizeof(log_namespace) - 1, tag, len);
    key[sizeof(log_namespace) - 1 + len] = ‘\0‘;

    kp = key;
    for (i = 0; i < (sizeof(tag_cache) / sizeof(tag_cache[0])); ++i) {
      struct cache_char* cache = &tag_cache[i];
      struct cache_char temp_cache;
// 第一次不走这里
      if (not_locked) {
        temp_cache.cache.pinfo = NULL;
        temp_cache.c = ‘\0‘;
        cache = &temp_cache;
      }
        // 走这里，local_change_detected为1了
      if (local_change_detected) {
        refresh_cache(cache, kp);
      }
// ‘\0‘为false的，不会走这里
      if (cache->c) {
        c = cache->c;
        break;
      }
// 去掉persist，再来一次
      kp = key + base_offset;
    }
  }

  switch (toupper(c)) { /* if invalid, resort to global */
    case ‘V‘:
    case ‘D‘:
    case ‘I‘:
    case ‘W‘:
    case ‘E‘:
    case ‘F‘: /* Not officially supported */
    case ‘A‘:
    case ‘S‘:
    case BOOLEAN_FALSE: /* Not officially supported */
      break;
    default:
      /* clear ‘.‘ after log.tag */
      key[sizeof(log_namespace) - 2] = ‘\0‘;

      kp = key;
          // 和上面是差不多的
      for (i = 0; i < (sizeof(global_cache) / sizeof(global_cache[0])); ++i) {
        struct cache_char* cache = &global_cache[i];
        struct cache_char temp_cache;

        if (not_locked) {
          temp_cache = *cache;
          if (temp_cache.cache.pinfo != cache->cache.pinfo) { /* check atomic */
            temp_cache.cache.pinfo = NULL;
            temp_cache.c = ‘\0‘;
          }
          cache = &temp_cache;
        }
        if (global_change_detected) {
          refresh_cache(cache, kp);
        }

        if (cache->c) {
          c = cache->c;
          break;
        }

        kp = key + base_offset;
      }
      break;
  }

  if (!not_locked) {
    global_serial = current_global_serial;
    unlock();
  }

  switch (toupper(c)) {
    /* clang-format off */
    case ‘V‘: return ANDROID_LOG_VERBOSE;
    case ‘D‘: return ANDROID_LOG_DEBUG;
    case ‘I‘: return ANDROID_LOG_INFO;
    case ‘W‘: return ANDROID_LOG_WARN;
    case ‘E‘: return ANDROID_LOG_ERROR;
    case ‘F‘: /* FALLTHRU */ /* Not officially supported */
    case ‘A‘: return ANDROID_LOG_FATAL;
    case BOOLEAN_FALSE: /* FALLTHRU */ /* Not Officially supported */
    case ‘S‘: return ANDROID_LOG_SILENT;
      /* clang-format on */
  }
    // 所以在fist_stage_init阶段，这里返回的是-1
  return -1;
}

1.4 check_cache

static int check_cache(struct cache* cache) {
  return cache->pinfo && __system_property_serial(cache->pinfo) != cache->serial;
}

1.5 refresh_cache

static void refresh_cache(struct cache_char* cache, const char* key) {
  char buf[PROP_VALUE_MAX];

  if (!cache->cache.pinfo) {
    cache->cache.pinfo = __system_property_find(key);
    if (!cache->cache.pinfo) {
        // first_stag_init的时候，cache都是为NULL的，所以，直接返回了
      return;
    }
  }
  cache->cache.serial = __system_property_serial(cache->cache.pinfo);
  __system_property_read(cache->cache.pinfo, 0, buf);
  switch (buf[0]) {
    case ‘t‘:
    case ‘T‘:
      cache->c = strcasecmp(buf + 1, "rue") ? buf[0] : BOOLEAN_TRUE;
      break;
    case ‘f‘:
    case ‘F‘:
      cache->c = strcasecmp(buf + 1, "alse") ? buf[0] : BOOLEAN_FALSE;
      break;
    default:
      cache->c = buf[0];
  }
}

问题

补充

参考

liblog module

原文：https://www.cnblogs.com/pyjetson/p/14879076.html