在Android L之前的日志系统是Kernel层实现了若干个环形Buffer实现的。系统各个日志读写操作都是针对这几个RingBuffer来实现的。那就来一窥Kernel是怎么做的。相关源码是位于driver/staging/android/下面的logger.c和logger.h两个文件
1，在整个Android日志系统的位置
2，在logger.c中，入口函数
static int __init logger_init(void){...ret = create_log(LOGGER_LOG_MAIN, 256*1024);ret = create_log(LOGGER_LOG_EVENTS, 256*1024);ret = create_log(LOGGER_LOG_RADIO, 256*1024);ret = create_log(LOGGER_LOG_SYSTEM, 256*1024);...}
分别来看create_log的实现和LOGGER_LOG_*的相关定义
static int __init create_log(char *log_name, int size){...log->buffer = buffer;/* finally, initialize the misc device for this log */ret = misc_register(&log->misc);...}
#define LOGGER_LOG_RADIO "log_radio" /* radio-related messages */#define LOGGER_LOG_EVENTS "log_events" /* system/hardware events */#define LOGGER_LOG_SYSTEM "log_system" /* system/framework messages */#define LOGGER_LOG_MAIN "log_main" /* everything else */...
从上可以知道，create_log函数就是把创建4个misc设备，并注册上。在创建的过程中会初始化各个设备的logger_log结构体。也就是说有4个logger_log结构体的实例。这4个MISC设备（即对应4种Logger）的Buffer大小为256Kb，即RING BUFFER的大小为256Kb。
再来看这个十分重要的结构体logger_log的其它成员
/*** struct logger_log - represents a specific log, such as 'main' or 'radio'* @buffer: The actual ring buffer* @misc: The "misc" device representing the log* @wq: The wait queue for @readers* @readers: This log's readers* @mutex: The mutex that protects the @buffer* @w_off: The current write head offset* @head: The head, or location that readers start reading at.* @size: The size of the log* @logs: The list of log channels** This structure lives from module insertion until module removal, so it does* not need additional reference counting. The structure is protected by the* mutex 'mutex'.*/struct logger_log {unsigned char *buffer;struct miscdevice misc;wait_queue_head_t wq;struct list_head readers;struct mutex mutex;size_t w_off;size_t head;size_t size;struct list_head logs;};
不复杂，注释的解析也十分清楚。
用户层对于这4个MISC设备的操作，是通过/dev/log/*下面的几个设备实现的，它同样也有file_operations结构体中的读写函数指针，如下：
static const struct file_operations logger_fops = {.owner = THIS_MODULE,.read = logger_read,.aio_write = logger_aio_write,.poll = logger_poll,.unlocked_ioctl = logger_ioctl,.compat_ioctl = logger_ioctl,.open = logger_open,.release = logger_release,};
3，MISC设备节点是怎么生成？
在运行的系统的/dev/log目录下有如下节点，
root@U:/dev/log # lseventsmainradiosystem
这是因为在上面提到的misc_register(&log->misc)函数，在前面的Kernel设备驱动相关文章可以知道，最终是调用device_create() -> device_add()->kobject_uevent(&dev->kobj, KOBJ_ADD);就会发送一个KOBJ_ADD的UEVENT给到上层，由《Android的根文件系统》可以知道，这是由init进程来处理的。在init项目中devices.cpp会来处理这个ADD事件
...} else if(!strncmp(uevent->subsystem, "misc", 4) &&!strncmp(name, "log_", 4)) {INFO("kernel logger is deprecated\n");base = "/dev/log/";make_dir(base, 0755);name += 4;...
如果设备的名字前缀是log_的话，就会创建/dev/log/目录。所以就出现dev/log/下面的4个文件。
4，写操作分析
写操作是写一条消息，封装在logger_entry为头的消息。一次写操作大概构成如下：
struct logger_entry | priority | tag | msg
比如logcat打印出来如下：
12-06 16:12:23.849 3700 3788 | D | WifiStateMachine | : handleMessage: X
logger_aio_write()函数如下：
static ssize_t logger_aio_write(struct kiocb *iocb, const struct iovec *iov,unsigned long nr_segs, loff_t ppos){- //1，填充header头的结构体
- header.pid = current->tgid;
header.tid = current->pid;header.sec = now.tv_sec;header.nsec = now.tv_nsec;header.euid = current_euid();header.len = min_t(size_t, iocb->ki_nbytes, LOGGER_ENTRY_MAX_PAYLOAD);header.hdr_size = sizeof(struct logger_entry);/* null writes succeed, return zero */if (unlikely(!header.len))return 0;- //如果写的一条信息，会覆盖正在进程读的信息，那么就丢弃这条读，把读的偏移指到下一条安全的信息。
fix_up_readers(log, sizeof(struct logger_entry) + header.len);- //2，把header信息定写入
do_write_log(log, &header, sizeof(struct logger_entry));- //3，把用户传过来的Message信息写入
while (nr_segs-- > 0) {size_t len;ssize_t nr;/* figure out how much of this vector we can keep *///这里计算剩余的信息是否超过了MAX_PAYLOAD的长度，即4K，超过部分将会丢弃。- //一条信息基本上也不会超过4K，基本是取实际信息长度
len = min_t(size_t, iov->iov_len, header.len - ret);/* write out this segment's payload */nr = do_write_log_from_user(log, iov->iov_base, len);if (unlikely(nr < 0)) {log->w_off = orig;mutex_unlock(&log->mutex);return nr;}iov++;ret += nr;}- //4，唤醒读的队列
- /* wake up any blocked readers */
wake_up_interruptible(&log->wq);return ret;}
一条log日志的头部信息(12-06 16:12:23.849 3700 3788)是在kernel中写入的。do_write_log()
static void do_write_log(struct logger_log *log, const void *buf, size_t count){size_t len;//Buffer剩余的长度和消息长度取较小的值- //如果buffer剩余长度不够写完一条信息，就进入下面的if，把剩余的部分，从头部开始写进去。
len = min(count, log->size - log->w_off);memcpy(log->buffer + log->w_off, buf, len);//如果一条Log，不够剩余的buffer的话，上面memcpy就是写了前半部分，- //下面的memcpy就把剩下的后半部分（cout-len），写到log->buffer的头。
- //这里体现了循环Buffer。
if (count != len)memcpy(log->buffer, buf + len, count - len);- //移动w_off
log->w_off = logger_offset(log, log->w_off + count);}
写完头部，再写用户层传过来的信息
static ssize_t do_write_log_from_user(struct logger_log *log,const void __user *buf, size_t count){- //同理会作判断，是否剩余buffer能够写完，否则就把剩余的部分从头开始写
- len = min(count, log->size - log->w_off);
if (len && copy_from_user(log->buffer + log->w_off, buf, len))return -EFAULT;if (count != len)if (copy_from_user(log->buffer, buf + len, count - len))- log->w_off = logger_offset(log, log->w_off + count);
return count;}
5，写操作基本上完成了，接下来看读操作。
static ssize_t logger_read(struct file *file, char __user *buf,size_t count, loff_t *pos){//1，如果读的偏移r_off与写的偏移w_off相等，则循环阻塞，直到write最后唤醒while (1) {prepare_to_wait(&log->wq, &wait, TASK_INTERRUPTIBLE);//write函数的wake_up_interruptible(&log->wq);会唤醒//Write的off存在Logger_log即内存buffer中，而r_off存在reder，即读的进程中。所以执行两次不同的logcat，都是从头开始读的。ret = (log->w_off == reader->r_off);mutex_unlock(&log->mutex);if (!ret)break;//如果不相等就跳出去处理。schedule(); //把cpu调试出去，等待队列的唤醒}- if (!reader->r_all)
reader->r_off = get_next_entry_by_uid(log,reader->r_off, current_euid());/* get the size of the next entry *///2，获取要读的整个一条日志的长度，同样由两部分组成，一个head长度+信息的长度。ret = get_user_hdr_len(reader->r_ver) +get_entry_msg_len(log, reader->r_off);- /* get exactly one entry from the log */
//3，把log读ret长度的信息到buf中。ret = do_read_log_to_user(log, reader, buf, ret);out:mutex_unlock(&log->mutex);return ret;}
来看看第3步do_read_log_to_user的实现
static ssize_t do_read_log_to_user(struct logger_log *log,struct logger_reader *reader,char __user *buf,size_t count){//先把header copy到用户空间entry = get_entry_header(log, reader->r_off, &scratch);if (copy_header_to_user(reader->r_ver, entry, buf))return -EFAULT;count -= get_user_hdr_len(reader->r_ver);buf += get_user_hdr_len(reader->r_ver);msg_start = logger_offset(log,reader->r_off + sizeof(struct logger_entry));/** We read from the msg in two disjoint operations. First, we read from* the current msg head offset up to 'count' bytes or to the end of* the log, whichever comes first.*///同样做是否到log buffer尾部的判断，否则就要读两次len = min(count, log->size - msg_start);if (copy_to_user(buf, log->buffer + msg_start, len))return -EFAULT;- ...
if (count != len)if (copy_to_user(buf + len, log->buffer, count - len))return -EFAULT;//移动reader的off偏移reader->r_off = logger_offset(log, reader->r_off +sizeof(struct logger_entry) + count);return count + get_user_hdr_len(reader->r_ver);}
这样Logger的实现基本就完成了，循环Buffer的实现，也就是在读与写的操作过程，是否已经到了buffer尾的判断。
len = min(count, log->size - msg_start);if (copy_to_user(buf, log->buffer + msg_start, len))if (count != len)if (copy_to_user(buf + len, log->buffer, count - len))return -EFAULT;
RingBuffer是Kernel一个常用的数据结构，从Logger系统中看它的实现，看到其它模块使用，心里也有就数了。