转自: http://my.oschina.net/wolfcs/blog/164624
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:
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* Priority constant for the println method; use Log.v. |
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public static final int VERBOSE = 2 ; |
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* Priority constant for the println method; use Log.d. |
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public static final int DEBUG = 3 ; |
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* Priority constant for the println method; use Log.i. |
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public static final int INFO = 4 ; |
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* Priority constant for the println method; use Log.w. |
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public static final int WARN = 5 ; |
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* Priority constant for the println method; use Log.e. |
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public static final int ERROR = 6 ; |
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* Priority constant for the println method. |
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public static final int ASSERT = 7 ; |
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/** @hide */ public static final int LOG_ID_MAIN = 0 ; |
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/** @hide */ public static final int LOG_ID_RADIO = 1 ; |
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/** @hide */ public static final int LOG_ID_EVENTS = 2 ; |
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/** @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:
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public static int v(String tag, String msg, Throwable tr) { |
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return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '\n' + getStackTraceString(tr)); |
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* Send a {@link #DEBUG} log message. |
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* @param tag Used to identify the source of a log message. It usually identifies |
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* the class or activity where the log call occurs. |
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* @param msg The message you would like logged. |
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public static int d(String tag, String msg) { |
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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:
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static jint android_util_Log_println_native(JNIEnv* env, jobject clazz, |
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jint bufID, jint priority, jstring tagObj, jstring msgObj) |
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const char * tag = NULL; |
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const char * msg = NULL; |
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jniThrowNullPointerException(env, "println needs a message" ); |
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if (bufID < 0 || bufID >= LOG_ID_MAX) { |
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jniThrowNullPointerException(env, "bad bufID" ); |
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tag = env->GetStringUTFChars(tagObj, NULL); |
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msg = env->GetStringUTFChars(msgObj, NULL); |
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int res = __android_log_buf_write(bufID, (android_LogPriority)priority, tag, msg); |
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env->ReleaseStringUTFChars(tagObj, tag); |
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env->ReleaseStringUTFChars(msgObj, msg); |
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static JNINativeMethod gMethods[] = { |
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{ "isLoggable" , "(Ljava/lang/String;I)Z" , ( void *) android_util_Log_isLoggable }, |
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{ "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:
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int __android_log_buf_write( int bufID, int prio, const char *tag, const char *msg) |
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if ((bufID != LOG_ID_RADIO) && |
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(! strcmp (tag, "HTC_RIL" ) || |
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! strncmp (tag, "RIL" , 3) || |
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! strncmp (tag, "IMS" , 3) || |
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! strcmp (tag, "GSM" ) || |
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! strcmp (tag, "STK" ) || |
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! strcmp (tag, "CDMA" ) || |
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! strcmp (tag, "PHONE" ) || |
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! strcmp (tag, "SMS" ))) { |
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snprintf(tmp_tag, sizeof (tmp_tag), "use-Rlog/RLOG-%s" , tag); |
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vec[0].iov_base = (unsigned char *) &prio; |
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vec[1].iov_base = ( void *) tag; |
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vec[1].iov_len = strlen (tag) + 1; |
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vec[2].iov_base = ( void *) msg; |
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vec[2].iov_len = strlen (msg) + 1; |
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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():
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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:
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static int __write_to_log_init(log_id_t log_id, struct iovec *vec, size_t nr) |
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pthread_mutex_lock(&log_init_lock); |
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if (write_to_log == __write_to_log_init) { |
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log_fds[LOG_ID_MAIN] = log_open( "/dev/" LOGGER_LOG_MAIN, O_WRONLY); |
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log_fds[LOG_ID_RADIO] = log_open( "/dev/" LOGGER_LOG_RADIO, O_WRONLY); |
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log_fds[LOG_ID_EVENTS] = log_open( "/dev/" LOGGER_LOG_EVENTS, O_WRONLY); |
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log_fds[LOG_ID_SYSTEM] = log_open( "/dev/" LOGGER_LOG_SYSTEM, O_WRONLY); |
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write_to_log = __write_to_log_kernel; |
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if (log_fds[LOG_ID_MAIN] < 0 || log_fds[LOG_ID_RADIO] < 0 || |
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log_fds[LOG_ID_EVENTS] < 0) { |
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log_close(log_fds[LOG_ID_MAIN]); |
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log_close(log_fds[LOG_ID_RADIO]); |
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log_close(log_fds[LOG_ID_EVENTS]); |
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log_fds[LOG_ID_MAIN] = -1; |
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log_fds[LOG_ID_RADIO] = -1; |
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log_fds[LOG_ID_EVENTS] = -1; |
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write_to_log = __write_to_log_null; |
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if (log_fds[LOG_ID_SYSTEM] < 0) { |
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log_fds[LOG_ID_SYSTEM] = log_fds[LOG_ID_MAIN]; |
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pthread_mutex_unlock(&log_init_lock); |
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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):
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#define LOGGER_LOG_MAIN "log/main" |
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#define LOGGER_LOG_RADIO "log/radio" |
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#define LOGGER_LOG_EVENTS "log/events" |
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#define LOGGER_LOG_SYSTEM "log/system" |
接着继续来看__write_to_log_kernel或者__write_to_log_null函数:
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static int __write_to_log_null(log_id_t log_fd, struct iovec *vec, size_t nr) |
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static int __write_to_log_kernel(log_id_t log_id, struct iovec *vec, size_t nr) |
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if ( ( int )log_id < ( int )LOG_ID_MAX) { |
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log_fd = log_fds[( int )log_id]; |
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ret = log_writev(log_fd, vec, nr); |
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} while (ret < 0 && errno == EINTR); |
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:
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#define log_open(pathname, flags) fakeLogOpen(pathname, flags) |
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#define log_writev(filedes, vector, count) fakeLogWritev(filedes, vector, count) |
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#define log_close(filedes) fakeLogClose(filedes) |
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#define log_open(pathname, flags) open(pathname, (flags) | O_CLOEXEC) |
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#define log_writev(filedes, vector, count) writev(filedes, vector, count) |
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#define log_close(filedes) close(filedes) |
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:
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void TextLayoutCache::dumpCacheStats() { |
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float remainingPercent = 100 * ((mMaxSize - mSize) / (( float )mMaxSize)); |
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float timeRunningInSec = (systemTime(SYSTEM_TIME_MONOTONIC) - mCacheStartTime) / 1000000000; |
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size_t cacheSize = mCache.size(); |
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ALOGD( "------------------------------------------------" ); |
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ALOGD( "------------------------------------------------" ); |
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ALOGD( "pid : %d" , getpid()); |
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ALOGD( "running : %.0f seconds" , timeRunningInSec); |
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ALOGD( "entries : %d" , cacheSize); |
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ALOGD( "max size : %d bytes" , mMaxSize); |
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ALOGD( "used : %d bytes according to mSize" , mSize); |
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ALOGD( "remaining : %d bytes or %2.2f percent" , mMaxSize - mSize, remainingPercent); |
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ALOGD( "hits : %d" , mCacheHitCount); |
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ALOGD( "saved : %0.6f ms" , mNanosecondsSaved * 0.000001f); |
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ALOGD( "------------------------------------------------" ); |
使用这组宏,需要定义另外一个宏来作为所打印log的tag:
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#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:
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#define ALOGD(...) ((void)ALOG(LOG_DEBUG, LOG_TAG, __VA_ARGS__)) |
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#define ALOGW(...) ((void)ALOG(LOG_WARN, LOG_TAG, __VA_ARGS__)) |
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#define ALOG(priority, tag, ...) \ |
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LOG_PRI(ANDROID_##priority, tag, __VA_ARGS__) |
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#define LOG_PRI(priority, tag, ...) \ |
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android_printLog(priority, tag, __VA_ARGS__) |
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#define android_printLog(prio, tag, fmt...) \ |
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__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):
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typedef enum android_LogPriority { |
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ANDROID_LOG_UNKNOWN = 0, |
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} 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:
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int __android_log_print( int prio, const char *tag, const char *fmt, ...) |
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char buf[LOG_BUF_SIZE]; |
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vsnprintf(buf, LOG_BUF_SIZE, fmt, ap); |
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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:
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int __android_log_write( int prio, const char *tag, const char *msg) |
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log_id_t log_id = LOG_ID_MAIN; |
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if (! strcmp (tag, "HTC_RIL" ) || |
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! strncmp (tag, "RIL" , 3) || |
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! strncmp (tag, "IMS" , 3) || |
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! strcmp (tag, "GSM" ) || |
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! strcmp (tag, "STK" ) || |
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! strcmp (tag, "CDMA" ) || |
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! strcmp (tag, "PHONE" ) || |
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! strcmp (tag, "SMS" )) { |
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log_id = LOG_ID_RADIO; |
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snprintf(tmp_tag, sizeof (tmp_tag), "use-Rlog/RLOG-%s" , tag); |
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vec[0].iov_base = (unsigned char *) &prio; |
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vec[1].iov_base = ( void *) tag; |
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vec[1].iov_len = strlen (tag) + 1; |
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vec[2].iov_base = ( void *) msg; |
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vec[2].iov_len = strlen (msg) + 1; |
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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:
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#include <android/log.h> |
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void SkDebugf( const char format[], ...) { |
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va_start (args, format); |
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__android_log_vprint(ANDROID_LOG_DEBUG, LOG_TAG, format, 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:
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int __android_log_vprint( int prio, const char *tag, const char *fmt, va_list ap) |
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char buf[LOG_BUF_SIZE]; |
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vsnprintf(buf, LOG_BUF_SIZE, fmt, ap); |
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return __android_log_write(prio, tag, buf); |
一样是__android_log_write()函数。
Done.