android log机制——输出log

android log系统。

在android Java code中输出log

android系统有4种类型、6个优先级的log，有一些常量用于标识这些信息，相关的定义在frameworks/base/core/java/android/util/Log.java中可以看到：

    /**     * Priority constant for the println method; use Log.v.     */    public static final int VERBOSE = 2;    /**     * Priority constant for the println method; use Log.d.     */    public static final int DEBUG = 3;    /**     * Priority constant for the println method; use Log.i.     */    public static final int INFO = 4;    /**     * Priority constant for the println method; use Log.w.     */    public static final int WARN = 5;    /**     * Priority constant for the println method; use Log.e.     */    public static final int ERROR = 6;    /**     * Priority constant for the println method.     */    public static final int ASSERT = 7;    /** @hide */ public static final int LOG_ID_MAIN = 0;    /** @hide */ public static final int LOG_ID_RADIO = 1;    /** @hide */ public static final int LOG_ID_EVENTS = 2;    /** @hide */ public static final int LOG_ID_SYSTEM = 3;

Java层可以通过三个class来输出其中三种类型的log，三种类型分别为MAIN、RADIO和SYSTEM，三个class分别为Log、Rlog和Slog，其package则分别为android.util、android.telephony和 android.util。这些用于打印log的classes，其构造函数都为private，因而都不能创建其对象，但它们都提供了静态方法来给用户打印log。各个log打印class的实现都大同小异，可以看一下Log这个class中的一些：

    public static int v(String tag, String msg, Throwable tr) {        return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '\n' + getStackTraceString(tr));    }    /**     * Send a {@link #DEBUG} log message.     * @param tag Used to identify the source of a log message.  It usually identifies     *        the class or activity where the log call occurs.     * @param msg The message you would like logged.     */    public static int d(String tag, String msg) {        return println_native(LOG_ID_MAIN, DEBUG, tag, msg);    }

最终都会是调用Log.println_native()静态native方法来打印log，各个类中各个方法的不同之处也仅在于参数的差异。

Log.println_native()方法

这个方法的code在/frameworks/base/core/jni/android_util_Log.cpp，为：

static jint android_util_Log_println_native(JNIEnv* env, jobject clazz,        jint bufID, jint priority, jstring tagObj, jstring msgObj){    const char* tag = NULL;    const char* msg = NULL;    if (msgObj == NULL) {        jniThrowNullPointerException(env, "println needs a message");        return -1;    }    if (bufID < 0 || bufID >= LOG_ID_MAX) {        jniThrowNullPointerException(env, "bad bufID");        return -1;    }    if (tagObj != NULL)        tag = env->GetStringUTFChars(tagObj, NULL);    msg = env->GetStringUTFChars(msgObj, NULL);    int res = __android_log_buf_write(bufID, (android_LogPriority)priority, tag, msg);    if (tag != NULL)        env->ReleaseStringUTFChars(tagObj, tag);    env->ReleaseStringUTFChars(msgObj, msg);    return res;}/* * JNI registration. */static JNINativeMethod gMethods[] = {    /* name, signature, funcPtr */    { "isLoggable",      "(Ljava/lang/String;I)Z", (void*) android_util_Log_isLoggable },    { "println_native",  "(IILjava/lang/String;Ljava/lang/String;)I", (void*) android_util_Log_println_native },};

可以看到，干的都是转换参数的事情，最终再call到__android_log_buf_write()函数，这个函数的定义在system/core/liblog/logd_write.c，为：

int __android_log_buf_write(int bufID, int prio, const char *tag, const char *msg){    struct iovec vec[3];    char tmp_tag[32];    if (!tag)        tag = "";    /* XXX: This needs to go! */    if ((bufID != LOG_ID_RADIO) &&         (!strcmp(tag, "HTC_RIL") ||        !strncmp(tag, "RIL", 3) || /* Any log tag with "RIL" as the prefix */        !strncmp(tag, "IMS", 3) || /* Any log tag with "IMS" as the prefix */        !strcmp(tag, "AT") ||        !strcmp(tag, "GSM") ||        !strcmp(tag, "STK") ||        !strcmp(tag, "CDMA") ||        !strcmp(tag, "PHONE") ||        !strcmp(tag, "SMS"))) {            bufID = LOG_ID_RADIO;            // Inform third party apps/ril/radio.. to use Rlog or RLOG            snprintf(tmp_tag, sizeof(tmp_tag), "use-Rlog/RLOG-%s", tag);            tag = tmp_tag;    }    vec[0].iov_base   = (unsigned char *) &prio;    vec[0].iov_len    = 1;    vec[1].iov_base   = (void *) tag;    vec[1].iov_len    = strlen(tag) + 1;    vec[2].iov_base   = (void *) msg;    vec[2].iov_len    = strlen(msg) + 1;    return write_to_log(bufID, vec, 3);}

做了三件事情，一是根据log的tag，转换bufID，二是用传进来的参数构造一个struct iovec数组，三是将前一步构造的数组作为参数调用write_to_log()。write_to_log()是一个函数指针，在开始时，它指向了__write_to_log_init()：

static int (*write_to_log)(log_id_t, struct iovec *vec, size_t nr) = __write_to_log_init;

__write_to_log_init()的实现如下：

static int __write_to_log_init(log_id_t log_id, struct iovec *vec, size_t nr){#ifdef HAVE_PTHREADS    pthread_mutex_lock(&log_init_lock);#endif    if (write_to_log == __write_to_log_init) {        log_fds[LOG_ID_MAIN] = log_open("/dev/"LOGGER_LOG_MAIN, O_WRONLY);        log_fds[LOG_ID_RADIO] = log_open("/dev/"LOGGER_LOG_RADIO, O_WRONLY);        log_fds[LOG_ID_EVENTS] = log_open("/dev/"LOGGER_LOG_EVENTS, O_WRONLY);        log_fds[LOG_ID_SYSTEM] = log_open("/dev/"LOGGER_LOG_SYSTEM, O_WRONLY);        write_to_log = __write_to_log_kernel;        if (log_fds[LOG_ID_MAIN] < 0 || log_fds[LOG_ID_RADIO] < 0 ||                log_fds[LOG_ID_EVENTS] < 0) {            log_close(log_fds[LOG_ID_MAIN]);            log_close(log_fds[LOG_ID_RADIO]);            log_close(log_fds[LOG_ID_EVENTS]);            log_fds[LOG_ID_MAIN] = -1;            log_fds[LOG_ID_RADIO] = -1;            log_fds[LOG_ID_EVENTS] = -1;            write_to_log = __write_to_log_null;        }        if (log_fds[LOG_ID_SYSTEM] < 0) {            log_fds[LOG_ID_SYSTEM] = log_fds[LOG_ID_MAIN];        }    }#ifdef HAVE_PTHREADS    pthread_mutex_unlock(&log_init_lock);#endif    return write_to_log(log_id, vec, nr);}

这个地方，会检查write_to_log是否指向了__write_to_log_init，也就是是否是第一次打印log，如果是，则打开几个用于输出log的设备文件，然后使write_to_log函数指针指向__write_to_log_kernel，或者在打开输出log设备文件出现异常时，使write_to_log指向__write_to_log_null，最后再次调用经过了重定向的write_to_log，也就是__write_to_log_kernel或者__write_to_log_null函数。我们可以看一下那几个设备文件究竟是什麽(在system/core/include/cutils/logger.h)：

#define LOGGER_LOG_MAIN"log/main"#define LOGGER_LOG_RADIO"log/radio"#define LOGGER_LOG_EVENTS"log/events"#define LOGGER_LOG_SYSTEM"log/system"

接着继续来看__write_to_log_kernel或者__write_to_log_null函数：

static int __write_to_log_null(log_id_t log_fd, struct iovec *vec, size_t nr){    return -1;}static int __write_to_log_kernel(log_id_t log_id, struct iovec *vec, size_t nr){    ssize_t ret;    int log_fd;    if (/*(int)log_id >= 0 &&*/ (int)log_id < (int)LOG_ID_MAX) {        log_fd = log_fds[(int)log_id];    } else {        return EBADF;    }    do {        ret = log_writev(log_fd, vec, nr);    } while (ret < 0 && errno == EINTR);    return ret;}

由log_id获取到对应的log_fd，然后调用log_writev()打印log。可以看一下log_writev()的定义，它是一个宏：

#if FAKE_LOG_DEVICE// This will be defined when building for the host.#define log_open(pathname, flags) fakeLogOpen(pathname, flags)#define log_writev(filedes, vector, count) fakeLogWritev(filedes, vector, count)#define log_close(filedes) fakeLogClose(filedes)#else#define log_open(pathname, flags) open(pathname, (flags) | O_CLOEXEC)#define log_writev(filedes, vector, count) writev(filedes, vector, count)#define log_close(filedes) close(filedes)#endif

这些就都是标准的unix系统调用了。

本地层代码Log输出

以一些比较典型的native代码打印log的case为例。先来看一下，在JNI的code中打印log的方法。在JNI中，比较常见到用ALOGx这一组宏来打印log，比如在frameworks/base/core/jni/android/graphics/TextLayoutCache.cpp这个文件中的dumpCacheStats()函数：

void TextLayoutCache::dumpCacheStats() {    float remainingPercent = 100 * ((mMaxSize - mSize) / ((float)mMaxSize));    float timeRunningInSec = (systemTime(SYSTEM_TIME_MONOTONIC) - mCacheStartTime) / 1000000000;    size_t cacheSize = mCache.size();    ALOGD("------------------------------------------------");    ALOGD("Cache stats");    ALOGD("------------------------------------------------");    ALOGD("pid       : %d", getpid());    ALOGD("running   : %.0f seconds", timeRunningInSec);    ALOGD("entries   : %d", cacheSize);    ALOGD("max size  : %d bytes", mMaxSize);    ALOGD("used      : %d bytes according to mSize", mSize);    ALOGD("remaining : %d bytes or %2.2f percent", mMaxSize - mSize, remainingPercent);    ALOGD("hits      : %d", mCacheHitCount);    ALOGD("saved     : %0.6f ms", mNanosecondsSaved * 0.000001f);    ALOGD("------------------------------------------------");}

使用这组宏，需要定义另外一个宏来作为所打印log的tag：

#define LOG_TAG "TextLayoutCache"

此外，还要include头文件<cutils/log.h>。来看一下这些宏中的一些的定义：

/* * Simplified macro to send a debug log message using the current LOG_TAG. */#ifndef ALOGD#define ALOGD(...) ((void)ALOG(LOG_DEBUG, LOG_TAG, __VA_ARGS__))#endif/* * Simplified macro to send a warning log message using the current LOG_TAG. */#ifndef ALOGW#define ALOGW(...) ((void)ALOG(LOG_WARN, LOG_TAG, __VA_ARGS__))#endif/* * Basic log message macro. * * Example: *  ALOG(LOG_WARN, NULL, "Failed with error %d", errno); * * The second argument may be NULL or "" to indicate the "global" tag. */#ifndef ALOG#define ALOG(priority, tag, ...) \    LOG_PRI(ANDROID_##priority, tag, __VA_ARGS__)#endif/* * Log macro that allows you to specify a number for the priority. */#ifndef LOG_PRI#define LOG_PRI(priority, tag, ...) \    android_printLog(priority, tag, __VA_ARGS__)#endif#define android_printLog(prio, tag, fmt...) \    __android_log_print(prio, tag, fmt)

先来看一下，在native层中定义的priority(在system/core/include/android/log.h中)：

/* * Android log priority values, in ascending priority order. */typedef enum android_LogPriority {    ANDROID_LOG_UNKNOWN = 0,    ANDROID_LOG_DEFAULT,    /* only for SetMinPriority() */    ANDROID_LOG_VERBOSE,    ANDROID_LOG_DEBUG,    ANDROID_LOG_INFO,    ANDROID_LOG_WARN,    ANDROID_LOG_ERROR,    ANDROID_LOG_FATAL,    ANDROID_LOG_SILENT,     /* only for SetMinPriority(); must be last */} android_LogPriority;

另外，这些宏最终都会call到__android_log_print()，也是在system/core/liblog/logd_write.c中：

int __android_log_print(int prio, const char *tag, const char *fmt, ...){    va_list ap;    char buf[LOG_BUF_SIZE];    va_start(ap, fmt);    vsnprintf(buf, LOG_BUF_SIZE, fmt, ap);    va_end(ap);    return __android_log_write(prio, tag, buf);}

先是格式化参数，然后就是调用__android_log_write()函数。这个函数的code如下：

int __android_log_write(int prio, const char *tag, const char *msg){    struct iovec vec[3];    log_id_t log_id = LOG_ID_MAIN;    char tmp_tag[32];    if (!tag)        tag = "";    /* XXX: This needs to go! */    if (!strcmp(tag, "HTC_RIL") ||        !strncmp(tag, "RIL", 3) || /* Any log tag with "RIL" as the prefix */        !strncmp(tag, "IMS", 3) || /* Any log tag with "IMS" as the prefix */        !strcmp(tag, "AT") ||        !strcmp(tag, "GSM") ||        !strcmp(tag, "STK") ||        !strcmp(tag, "CDMA") ||        !strcmp(tag, "PHONE") ||        !strcmp(tag, "SMS")) {            log_id = LOG_ID_RADIO;            // Inform third party apps/ril/radio.. to use Rlog or RLOG            snprintf(tmp_tag, sizeof(tmp_tag), "use-Rlog/RLOG-%s", tag);            tag = tmp_tag;    }    vec[0].iov_base   = (unsigned char *) &prio;    vec[0].iov_len    = 1;    vec[1].iov_base   = (void *) tag;    vec[1].iov_len    = strlen(tag) + 1;    vec[2].iov_base   = (void *) msg;    vec[2].iov_len    = strlen(msg) + 1;    return write_to_log(log_id, vec, 3);}

这个函数与我们前面看到的__android_log_buf_write()非常相似。所不同的就是这个函数没有log_id参数，因而它默认是输出MAIN log，当log的TAG为某些特殊字串时，则输出RADIO log。最后同样是调用write_to_log这个函数指针来输出log。

我们再来看一个skia里面打log的SkDebugf()函数的实现：

#include <android/log.h>void SkDebugf(const char format[], ...) {    va_list args;    va_start(args, format);    __android_log_vprint(ANDROID_LOG_DEBUG, LOG_TAG, format, args);    va_end(args);}

call到了__android_log_vprint()来输出log，__android_log_vprint()的定义也在system/core/liblog/logd_write.c中：

int __android_log_vprint(int prio, const char *tag, const char *fmt, va_list ap){    char buf[LOG_BUF_SIZE];    vsnprintf(buf, LOG_BUF_SIZE, fmt, ap);    return __android_log_write(prio, tag, buf);}

一样是__android_log_write()函数。

Done.

在android Java code中输出log

Log.println_native()方法

本地层代码Log输出

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