一、设置保存log文件的路径
在手机刚开机的时候，会有类似如下命令执行
/system/bin/logcat -r 5120 -v threadtime -v usec -v printable -n 5 -f /data/local/log/logcat.log
/system/bin/logcat -r 5120 -v threadtime -v usec -v printable -n 5 -b radio -f /data/local/log/logcat-radio.log
/system/bin/logcat -r 5120 -v threadtime -v usec -v printable -n 5 -b events -f /data/local/log/logcat-events.log
我们先看下logcat的如何对这个命令的实现的，在其main函数中，对f命令的实现如下：
case 'f':
if ((tail_time == log_time::EPOCH) && (tail_lines != 0)) {
tail_time = lastLogTime(optarg);
}
// redirect output to a file
g_outputFileName = optarg;
把文件名保存在g_outputFileName了，然后在main函数后面会调用setupOutput函数，我们来看下这个函数：
static void setupOutput()
{
if (g_outputFileName == NULL) {
g_outFD = STDOUT_FILENO;
} else {
if (set_sched_policy(0, SP_BACKGROUND) < 0) {
fprintf(stderr, "failed to set background scheduling policyn");
}
struct sched_param param;
memset(¶m, 0, sizeof(param));
if (sched_setscheduler((pid_t) 0, SCHED_BATCH, ¶m) < 0) {
fprintf(stderr, "failed to set to batch schedulern");
}
if (setpriority(PRIO_PROCESS, 0, ANDROID_PRIORITY_BACKGROUND) < 0) {
fprintf(stderr, "failed set to priorityn");
}
g_outFD = openLogFile (g_outputFileName);//得到了fd
if (g_outFD < 0) {
logcat_panic(false, "couldn't open output file");
}
struct stat statbuf;
if (fstat(g_outFD, &statbuf) == -1) {
close(g_outFD);
logcat_panic(false, "couldn't get output file statn");
}
if ((size_t) statbuf.st_size > SIZE_MAX || statbuf.st_size < 0) {
close(g_outFD);
logcat_panic(false, "invalid output file statn");
}
g_outByteCount = statbuf.st_size;
}
}
在这个函数中把文件的fd获取到了，是g_outFD。
最后我们可以在printBinary函数中往这个文件中写值。
void printBinary(struct log_msg *buf)
{
size_t size = buf->len();
TEMP_FAILURE_RETRY(write(g_outFD, buf, size));
}
也可以通过processBuffer来往文件写log。我们最后应该是通过processBuffer来写log的。
也就是上面的命令最终会把log保存在/data/local/log/logcat-radio.log文件下，当然这只是radio的log。
二、logcat获取logd中的log
而我们再看logcat的main最后是一个死循环，一直调用android_logger_list_read来从logd中获取log，然后再打印。
while (1) {
struct log_msg log_msg;
log_device_t* d;
int ret = android_logger_list_read(logger_list, &log_msg);//调用android_logger_list_read获取log
if (ret == 0) {
logcat_panic(false, "read: unexpected EOF!n");
}
if (ret < 0) {
if (ret == -EAGAIN) {
break;
}
if (ret == -EIO) {
logcat_panic(false, "read: unexpected EOF!n");
}
if (ret == -EINVAL) {
logcat_panic(false, "read: unexpected length.n");
}
logcat_panic(false, "logcat read failure");
}
for(d = devices; d; d = d->next) {
if (android_name_to_log_id(d->device) == log_msg.id()) {
break;
}
}
if (!d) {
g_devCount = 2; // set to Multiple
d = &unexpected;
d->binary = log_msg.id() == LOG_ID_EVENTS;
}
if (dev != d) {
dev = d;
maybePrintStart(dev, printDividers);
}
if (g_printBinary) {
printBinary(&log_msg);
} else {
processBuffer(dev, &log_msg);
}
}
android_logger_list_free(logger_list);
return EXIT_SUCCESS;
打印的话就是通过之前传进来的文件，写log到该文件的fd。
android_logger_list_read函数就是通过socket连接logd获取log。
int android_logger_list_read(struct logger_list *logger_list,
struct log_msg *log_msg)
{
int ret, e;
struct logger *logger;
struct sigaction ignore;
struct sigaction old_sigaction;
unsigned int old_alarm = 0;
if (!logger_list) {
return -EINVAL;
}
if (logger_list->mode & ANDROID_LOG_PSTORE) {
return android_logger_list_read_pstore(logger_list, log_msg);
}
if (logger_list->mode & ANDROID_LOG_NONBLOCK) {
memset(&ignore, 0, sizeof(ignore));
ignore.sa_handler = caught_signal;
sigemptyset(&ignore.sa_mask);
}
if (logger_list->sock < 0) {
char buffer[256], *cp, c;
int sock = socket_local_client("logdr",
ANDROID_SOCKET_NAMESPACE_RESERVED,
SOCK_SEQPACKET);
if (sock < 0) {
if ((sock == -1) && errno) {
return -errno;
}
return sock;
}上面logdr就是logcat到logd的socket。
三、总结
3.1 开3个进程保存不同log
我们手机上会开3个logcat进程来保存log，这3个进程会一直开着就是上面的死循环来不断保存log。
/system/bin/logcat -r 5120 -v threadtime -v usec -v printable -n 5 -f /data/local/log/logcat.log
/system/bin/logcat -r 5120 -v threadtime -v usec -v printable -n 5 -b radio -f /data/local/log/logcat-radio.log
/system/bin/logcat -r 5120 -v threadtime -v usec -v printable -n 5 -b events -f /data/local/log/logcat-events.log
3.2 kernel相关log
另外kernel的log是通过log_read_kern.c中的函数来实现的，而写的话通过logd_write_kern.c来实现的。
是通过节点来实现，而不是通过socket到logd实现的
节点：
dev/log/main
dev/log/radio
dev/log/system
dev/log/events
下篇博客我们主要说下logd是如何处理logcat的请求读log的。