【Android】log系统初识

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本来准备采用写/dev/log/main的方式进行底层的写log测试，但是Android6后就没有/dev/log/main了，根据参考资料来查看分析，

得到一个大致的概念，Android中由于log量大，为了方便的处理log，采用进程logd来写log，我们调用Log.d（）方法写log的时候，会导致底层通过socket向logd进程写数据，logd将数据写到其缓存中，当使用logcat去获取数据的时候，也是通过socket通信去获取数据。

Log.d以及Slog的写log最终会调用到底层的方法

/system/core/liblog/logd_writer.c


117staticintlogdWrite(log_id_tlogId, structtimespec *ts,118structiovec *vec, size_tnr)119{120ssize_tret;121staticconstunsignedheaderLength = 1;122structiovecnewVec[nr + headerLength];123android_log_header_theader;124size_t i, payloadSize;125staticatomic_int_fast32_tdropped;126staticatomic_int_fast32_tdroppedSecurity;127128if (logdLoggerWrite.context.sock < 0) {129return -EBADF;130    }131132/* logd, after initialization and priv drop */133if (__android_log_uid() == AID_LOGD) {134/*135         * ignore log messages we send to ourself (logd).136         * Such log messages are often generated by libraries we depend on137         * which use standard Android logging.138         */139return 0;140    }141142/*143     *  struct {144     *      // what we provide to socket145     *      android_log_header_t header;146     *      // caller provides147     *      union {148     *          struct {149     *              char     prio;150     *              char     payload[];151     *          } string;152     *          struct {153     *              uint32_t tag154     *              char     payload[];155     *          } binary;156     *      };157     *  };158     */159160header.tid = gettid();161header.realtime.tv_sec = ts->tv_sec;162header.realtime.tv_nsec = ts->tv_nsec;163164newVec[0].iov_base = (unsignedchar *)&header;165newVec[0].iov_len  = sizeof(header);166167if (logdLoggerWrite.context.sock > 0) {168int32_tsnapshot = atomic_exchange_explicit(&droppedSecurity, 0,169memory_order_relaxed);170if (snapshot) {171android_log_event_int_tbuffer;172173header.id = LOG_ID_SECURITY;174buffer.header.tag = htole32(LIBLOG_LOG_TAG);175buffer.payload.type = EVENT_TYPE_INT;176buffer.payload.data = htole32(snapshot);177178newVec[headerLength].iov_base = &buffer;179newVec[headerLength].iov_len  = sizeof(buffer);180181ret = TEMP_FAILURE_RETRY(writev(logdLoggerWrite.context.sock, newVec, 2));182if (ret != (ssize_t)(sizeof(header) + sizeof(buffer))) {183atomic_fetch_add_explicit(&droppedSecurity, snapshot,184memory_order_relaxed);185            }186        }

查看writev的定义

/system/core/liblog/uio.c


52LIBLOG_ABI_PUBLICintwritev(intfd, conststructiovec *vecs, intcount)53{54inttotal = 0;5556for ( ; count > 0; count--, vecs++ ) {57constchar*  buf = vecs->iov_base;58intlen = vecs->iov_len;5960while (len > 0) {61intret = write( fd, buf, len );62if (ret < 0) {63if (total == 0)64total = -1;65gotoExit;66            }67if (ret == 0)68gotoExit;6970total += ret;71buf   += ret;72len   -= ret;73        }74    }75Exit:76returntotal;77}78

本质上还是调用的系统write方法，向logd进行写操作

也就是说，如果给入合适的输入格式，我们是可以直接通过write操作进行写log

如：

int main(int argc, char* const argv[])

{

int fd = open("/dev/socket/logdw", O_RDWR);

if (fd < 0)

aerr << "fd < 0" << endl;

else

aout << "fd = " << fd << endl;

write(fd, "=====00 log write===============", 22);

}

直接这样写是有问题的，数据格式不对，

Unknown:/data/local/tmp # ./wtLog

fd < 0

这个测试需要继续研究

如果要正确的处理，需要再查看logd的处理方式进行分析

logd启动后的入口是/system/core/logd/main.cpp中的main方法

logd对log进行处理，读log是从/dev/socket/logdr来获取数据

log处理流程

Log.d() -> liblog ---socket--->logd ----socket---->logcat

参考资料

本来准备采用写/dev/log/main的方式进行底层的写log测试，但是Android6后就没有/dev/log/main了，根据参考资料来查看

Log.d以及Slog的写log最终会调用到底层的方法

/system/core/liblog/logd_writer.c


117staticintlogdWrite(log_id_tlogId, structtimespec *ts,118structiovec *vec, size_tnr)119{120ssize_tret;121staticconstunsignedheaderLength = 1;122structiovecnewVec[nr + headerLength];123android_log_header_theader;124size_t i, payloadSize;125staticatomic_int_fast32_tdropped;126staticatomic_int_fast32_tdroppedSecurity;127128if (logdLoggerWrite.context.sock < 0) {129return -EBADF;130    }131132/* logd, after initialization and priv drop */133if (__android_log_uid() == AID_LOGD) {134/*135         * ignore log messages we send to ourself (logd).136         * Such log messages are often generated by libraries we depend on137         * which use standard Android logging.138         */139return 0;140    }141142/*143     *  struct {144     *      // what we provide to socket145     *      android_log_header_t header;146     *      // caller provides147     *      union {148     *          struct {149     *              char     prio;150     *              char     payload[];151     *          } string;152     *          struct {153     *              uint32_t tag154     *              char     payload[];155     *          } binary;156     *      };157     *  };158     */159160header.tid = gettid();161header.realtime.tv_sec = ts->tv_sec;162header.realtime.tv_nsec = ts->tv_nsec;163164newVec[0].iov_base = (unsignedchar *)&header;165newVec[0].iov_len  = sizeof(header);166167if (logdLoggerWrite.context.sock > 0) {168int32_tsnapshot = atomic_exchange_explicit(&droppedSecurity, 0,169memory_order_relaxed);170if (snapshot) {171android_log_event_int_tbuffer;172173header.id = LOG_ID_SECURITY;174buffer.header.tag = htole32(LIBLOG_LOG_TAG);175buffer.payload.type = EVENT_TYPE_INT;176buffer.payload.data = htole32(snapshot);177178newVec[headerLength].iov_base = &buffer;179newVec[headerLength].iov_len  = sizeof(buffer);180181ret = TEMP_FAILURE_RETRY(writev(logdLoggerWrite.context.sock, newVec, 2));182if (ret != (ssize_t)(sizeof(header) + sizeof(buffer))) {183atomic_fetch_add_explicit(&droppedSecurity, snapshot,184memory_order_relaxed);185            }186        }

查看writev的定义

/system/core/liblog/uio.c


52LIBLOG_ABI_PUBLICintwritev(intfd, conststructiovec *vecs, intcount)53{54inttotal = 0;5556for ( ; count > 0; count--, vecs++ ) {57constchar*  buf = vecs->iov_base;58intlen = vecs->iov_len;5960while (len > 0) {61intret = write( fd, buf, len );62if (ret < 0) {63if (total == 0)64total = -1;65gotoExit;66            }67if (ret == 0)68gotoExit;6970total += ret;71buf   += ret;72len   -= ret;73        }74    }75Exit:76returntotal;77}78

本质上还是调用的系统write方法，向logd进行写操作

也就是说，如果给入合适的输入格式，我们是可以直接通过write操作进行写log

如：

int main(int argc, char* const argv[])

{

int fd = open("/dev/socket/logdw", O_RDWR);

if (fd < 0)

aerr << "fd < 0" << endl;

else

aout << "fd = " << fd << endl;

write(fd, "=====00 log write===============", 22);

}

直接这样写是有问题的，数据格式不对，

Unknown:/data/local/tmp # ./wtLog

fd < 0

这个测试需要继续研究

如果要正确的处理，需要再查看logd的处理方式进行分析

logd启动后的入口是/system/core/logd/main.cpp中的main方法

logd对log进行处理，读log是从/dev/socket/logdr来获取数据

log处理流程

Log.d() -> liblog ---socket--->logd ----socket---->logcat

http://blog.csdn.net/KM_Cat/article/details/61320398

基于Android N的Log机制浅析

Android6的Logger日志系统

本来准备采用写/dev/log/main的方式进行底层的写log测试，但是Android6后就没有/dev/log/main了，根据参考资料来查看

Log.d以及Slog的写log最终会调用到底层的方法

/system/core/liblog/logd_writer.c


117staticintlogdWrite(log_id_tlogId, structtimespec *ts,118structiovec *vec, size_tnr)119{120ssize_tret;121staticconstunsignedheaderLength = 1;122structiovecnewVec[nr + headerLength];123android_log_header_theader;124size_t i, payloadSize;125staticatomic_int_fast32_tdropped;126staticatomic_int_fast32_tdroppedSecurity;127128if (logdLoggerWrite.context.sock < 0) {129return -EBADF;130    }131132/* logd, after initialization and priv drop */133if (__android_log_uid() == AID_LOGD) {134/*135         * ignore log messages we send to ourself (logd).136         * Such log messages are often generated by libraries we depend on137         * which use standard Android logging.138         */139return 0;140    }141142/*143     *  struct {144     *      // what we provide to socket145     *      android_log_header_t header;146     *      // caller provides147     *      union {148     *          struct {149     *              char     prio;150     *              char     payload[];151     *          } string;152     *          struct {153     *              uint32_t tag154     *              char     payload[];155     *          } binary;156     *      };157     *  };158     */159160header.tid = gettid();161header.realtime.tv_sec = ts->tv_sec;162header.realtime.tv_nsec = ts->tv_nsec;163164newVec[0].iov_base = (unsignedchar *)&header;165newVec[0].iov_len  = sizeof(header);166167if (logdLoggerWrite.context.sock > 0) {168int32_tsnapshot = atomic_exchange_explicit(&droppedSecurity, 0,169memory_order_relaxed);170if (snapshot) {171android_log_event_int_tbuffer;172173header.id = LOG_ID_SECURITY;174buffer.header.tag = htole32(LIBLOG_LOG_TAG);175buffer.payload.type = EVENT_TYPE_INT;176buffer.payload.data = htole32(snapshot);177178newVec[headerLength].iov_base = &buffer;179newVec[headerLength].iov_len  = sizeof(buffer);180181ret = TEMP_FAILURE_RETRY(writev(logdLoggerWrite.context.sock, newVec, 2));182if (ret != (ssize_t)(sizeof(header) + sizeof(buffer))) {183atomic_fetch_add_explicit(&droppedSecurity, snapshot,184memory_order_relaxed);185            }186        }

查看writev的定义

/system/core/liblog/uio.c


52LIBLOG_ABI_PUBLICintwritev(intfd, conststructiovec *vecs, intcount)53{54inttotal = 0;5556for ( ; count > 0; count--, vecs++ ) {57constchar*  buf = vecs->iov_base;58intlen = vecs->iov_len;5960while (len > 0) {61intret = write( fd, buf, len );62if (ret < 0) {63if (total == 0)64total = -1;65gotoExit;66            }67if (ret == 0)68gotoExit;6970total += ret;71buf   += ret;72len   -= ret;73        }74    }75Exit:76returntotal;77}78

本质上还是调用的系统write方法，向logd进行写操作

也就是说，如果给入合适的输入格式，我们是可以直接通过write操作进行写log

如：

int main(int argc, char* const argv[])

{

int fd = open("/dev/socket/logdw", O_RDWR);

if (fd < 0)

aerr << "fd < 0" << endl;

else

aout << "fd = " << fd << endl;

write(fd, "=====00 log write===============", 22);

}

直接这样写是有问题的，数据格式不对，

Unknown:/data/local/tmp # ./wtLog

fd < 0

这个测试需要继续研究

如果要正确的处理，需要再查看logd的处理方式进行分析

logd启动后的入口是/system/core/logd/main.cpp中的main方法

logd对log进行处理，读log是从/dev/socket/logdr来获取数据

log处理流程

Log.d() -> liblog ---socket--->logd ----socket---->logcat

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