Android Log日志系统

0. 前言

1. Native的Log写过程解析

2. Socket的另一端Logd

0. 前言

Android中 logd 详解_私房菜的博客-CSDN博客_logd

里面讲了Java层Logd的框架和代码和Native的一些代码

1. Native的Log写过程解析

普通LogI会调用到ALOG -->位于logger_write.cpp

__android_log_print --> __android_log_logd_logger --> write_to_log

调用__android_log_logd_logger:

void __android_log_logd_logger(const struct __android_log_message* log_message) {int buffer_id = log_message->buffer_id == LOG_ID_DEFAULT ? LOG_ID_MAIN : log_message->buffer_id;struct iovec vec[3];vec[0].iov_base =const_cast<unsigned char*>(reinterpret_cast<const unsigned char*>(&log_message->priority));vec[0].iov_len = 1;vec[1].iov_base = const_cast<void*>(static_cast<const void*>(log_message->tag));vec[1].iov_len = strlen(log_message->tag) + 1;vec[2].iov_base = const_cast<void*>(static_cast<const void*>(log_message->message));vec[2].iov_len = strlen(log_message->message) + 1;write_to_log(static_cast<log_id_t>(buffer_id), vec, 3);
}

调用write_to_log做了一些参数检查，最终又调到了LogdWrite

static int write_to_log(log_id_t log_id, struct iovec* vec, size_t nr) {int ret;struct timespec ts;
...clock_gettime(CLOCK_REALTIME, &ts);if (log_id == LOG_ID_SECURITY)if (vec[0].iov_len < 4) ret = check_log_uid_permissions();if (!__android_log_security()) if (logger_ratelimit(log_id, ts) == LOGGER_RATELIMIT_SHIELD)
...ret = LogdWrite(log_id, &ts, vec, nr);PmsgWrite(log_id, &ts, vec, nr);return ret;
}

LogdWrite是往logd的socket中/dev/socket/logdw 这节点中写

int LogdWrite(log_id_t logId, struct timespec* ts, struct iovec* vec, size_t nr) {LogdSocket& logd_socket = logId == LOG_ID_SECURITY ? LogdSocket::BlockingSocket() : LogdSocket::NonBlockingSocket();if (__android_log_is_loggable_len(ANDROID_LOG_INFO, "liblog", strlen("liblog"))) {ret = TEMP_FAILURE_RETRY(writev(logd_socket.sock(), newVec, 2));if (ret != (ssize_t)(sizeof(header) + sizeof(buffer))) {if (WriteDropLog(newVec, 2) != static_cast<int>(iov_length(newVec, 2)))atomic_fetch_add_explicit(&dropped, snapshot, memory_order_relaxed);}}if (RedirectLogWrite(logId)) {ret = WriteAppLog(logId, newVec, i);}ret = TEMP_FAILURE_RETRY(writev(logd_socket.sock(), newVec, i));if (ret < 0 && errno != EAGAIN) {logd_socket.Reconnect();ret = TEMP_FAILURE_RETRY(writev(logd_socket.sock(), newVec, i));}
}

PmsgWrite

是往/dev/pmsg0节点中写，这个节点是pstore 内核日志的通路

2. Socket的另一端Logd

system\logging\logd\main.cpp中规定了logd初始化的组件

SerializedLogBuffer TODO后面看看 socket on Android

LogListener->StartListener

int LogListener::GetLogSocket() {static const char socketName[] = "logdw";int sock = android_get_control_socket(socketName);if (sock < 0) {  // logd started up in init.shsock = socket_local_server(socketName, ANDROID_SOCKET_NAMESPACE_RESERVED, SOCK_DGRAM);int on = 1;if (setsockopt(sock, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on))) {return -1;}}return sock;
}

LogListener开始监听

void LogListener::HandleData() {// + 1 to ensure null terminator if MAX_PAYLOAD buffer is received__attribute__((uninitialized)) charbuffer[sizeof(android_log_header_t) + LOGGER_ENTRY_MAX_PAYLOAD + 1];struct iovec iov = {buffer, sizeof(buffer) - 1};alignas(4) char control[CMSG_SPACE(sizeof(struct ucred))];struct msghdr hdr = {nullptr, 0, &iov, 1, control, sizeof(control), 0,};// To clear the entire buffer is secure/safe, but this contributes to 1.68%// overhead under logging load. We are safe because we check counts, but// still need to clear null terminator// memset(buffer, 0, sizeof(buffer));ssize_t n = recvmsg(socket_, &hdr, 0);
...logbuf_->Log(logId, header->realtime, cred->uid, cred->pid, header->tid, msg,((size_t)n <= UINT16_MAX) ? (uint16_t)n : UINT16_MAX, secontext, secontext_len);
}

logbuf->log是刷写log的函数。

LogBuffer这个类可以研究一下，log的循环使用是怎么实现的？

3、Logcat

#system/core/logcat/logcat.cpp

main函数中直接调用logcat run，最终调用到函数

android_logger_list_read(logger_list.get(), &log_msg)

也是通过socket（logdr）来传递信息的