Android应用程序框架层和系统运行库层日志系统源代码分析

转载自 http://blog.csdn.net/luoshengyang/article/details/6598703

 

在开发Android应用程序时，少不了使用Log来监控和调试程序的执行。在上一篇文章Android日志系统驱动程序Logger源代码分析中，我们分析了驱动程序Logger的源代码，在前面的文章浅谈Android系统开发中Log的使用一文，我们也简单介绍在应用程序中使Log的方法，在这篇文章中，我们将详细介绍Android应用程序框架层和系统运行库存层日志系统的源代码，使得我们可以更好地理解Android的日志系统的实现。

我们在Android应用程序，一般是调用应用程序框架层的Java接口（android.util.Log）来使用日志系统，这个Java接口通过JNI方法和系统运行库最终调用内核驱动程序Logger把Log写到内核空间中。按照这个调用过程，我们一步步介绍Android应用程序框架层日志系统的源代码。学习完这个过程之后，我们可以很好地理解Android系统的架构，即应用程序层（Application）的接口是如何一步一步地调用到内核空间的。

一. 应用程序框架层日志系统Java接口的实现。

在浅谈Android系统开发中Log的使用一文中，我们曾经介绍过Android应用程序框架层日志系统的源代码接口。这里，为了描述方便和文章的完整性，我们重新贴一下这部份的代码，在frameworks/base/core/java/android/util/Log.java文件中，实现日志系统的Java接口：

................................................

public final class Log {

................................................

/**
* 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;

.....................................................

public static int v(String tag, String msg) {
return println_native(LOG_ID_MAIN, VERBOSE, tag, msg);
}

public static int v(String tag, String msg, Throwable tr) {
return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '\n' + getStackTraceString(tr));
}

public static int d(String tag, String msg) {
return println_native(LOG_ID_MAIN, DEBUG, tag, msg);
}

public static int d(String tag, String msg, Throwable tr) {
return println_native(LOG_ID_MAIN, DEBUG, tag, msg + '\n' + getStackTraceString(tr));
}

public static int i(String tag, String msg) {
return println_native(LOG_ID_MAIN, INFO, tag, msg);
}

public static int i(String tag, String msg, Throwable tr) {
return println_native(LOG_ID_MAIN, INFO, tag, msg + '\n' + getStackTraceString(tr));
}

public static int w(String tag, String msg) {
return println_native(LOG_ID_MAIN, WARN, tag, msg);
}

public static int w(String tag, String msg, Throwable tr) {
return println_native(LOG_ID_MAIN, WARN, tag, msg + '\n' + getStackTraceString(tr));
}

public static int w(String tag, Throwable tr) {
return println_native(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr));
}

public static int e(String tag, String msg) {
return println_native(LOG_ID_MAIN, ERROR, tag, msg);
}

public static int e(String tag, String msg, Throwable tr) {
return println_native(LOG_ID_MAIN, ERROR, tag, msg + '\n' + getStackTraceString(tr));
}

..................................................................
/** @hide */ public static native int LOG_ID_MAIN = 0;
/** @hide */  public static native int LOG_ID_RADIO = 1;
/** @hide */ public static native int LOG_ID_EVENTS = 2;
/** @hide */  public static native int LOG_ID_SYSTEM = 3;

/** @hide */  public static native int println_native(int bufID,
int priority, String tag, String msg);
}

................................................ public final class Log { ................................................ /** * 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; ..................................................... public static int v(String tag, String msg) { return println_native(LOG_ID_MAIN, VERBOSE, tag, msg); } public static int v(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '\n' + getStackTraceString(tr)); } public static int d(String tag, String msg) { return println_native(LOG_ID_MAIN, DEBUG, tag, msg); } public static int d(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, DEBUG, tag, msg + '\n' + getStackTraceString(tr)); } public static int i(String tag, String msg) { return println_native(LOG_ID_MAIN, INFO, tag, msg); } public static int i(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, INFO, tag, msg + '\n' + getStackTraceString(tr)); } public static int w(String tag, String msg) { return println_native(LOG_ID_MAIN, WARN, tag, msg); } public static int w(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, WARN, tag, msg + '\n' + getStackTraceString(tr)); } public static int w(String tag, Throwable tr) { return println_native(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr)); } public static int e(String tag, String msg) { return println_native(LOG_ID_MAIN, ERROR, tag, msg); } public static int e(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, ERROR, tag, msg + '\n' + getStackTraceString(tr)); } .................................................................. /** @hide */ public static native int LOG_ID_MAIN = 0; /** @hide */ public static native int LOG_ID_RADIO = 1; /** @hide */ public static native int LOG_ID_EVENTS = 2; /** @hide */ public static native int LOG_ID_SYSTEM = 3; /** @hide */ public static native int println_native(int bufID, int priority, String tag, String msg); } 定义了2~7一共6个日志优先级别ID和4个日志缓冲区ID。回忆一下Android日志系统驱动程序Logger源代码分析一文，在Logger驱动程序模块中，定义了log_main、log_events和log_radio三个日志缓冲区，分别对应三个设备文件/dev/log/main、/dev/log/events和/dev/log/radio。这里的4个日志缓冲区的前面3个ID就是对应这三个设备文件的文件描述符了，在下面的章节中，我们将看到这三个文件描述符是如何创建的。在下载下来的Android内核源代码中，第4个日志缓冲区LOG_ID_SYSTEM并没有对应的设备文件，在这种情况下，它和LOG_ID_MAIN对应同一个缓冲区ID，在下面的章节中，我们同样可以看到这两个ID是如何对应到同一个设备文件的。

在整个Log接口中，最关键的地方声明了println_native本地方法，所有的Log接口都是通过调用这个本地方法来实现Log的定入。下面我们就继续分析这个本地方法println_native。

二. 应用程序框架层日志系统JNI方法的实现。

在frameworks/base/core/jni/android_util_Log.cpp文件中，实现JNI方法println_native：

/* //device/libs/android_runtime/android_util_Log.cpp
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
** http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

#define LOG_NAMESPACE "log.tag."
#define LOG_TAG "Log_println"

#include <assert.h>
#include <cutils/properties.h>
#include <utils/Log.h>
#include <utils/String8.h>

#include "jni.h"
#include "utils/misc.h"
#include "android_runtime/AndroidRuntime.h"

#define MIN(a,b) ((a<b)?a:b)

namespace android {

struct levels_t {
jint verbose;
jint debug;
jint info;
jint warn;
jint error;
jint assert;
};
static levels_t levels;

static int toLevel(const char* value)
{
switch (value[0]) {
case 'V': return levels.verbose;
case 'D': return levels.debug;
case 'I': return levels.info;
case 'W': return levels.warn;
case 'E': return levels.error;
case 'A': return levels.assert;
case 'S': return -1; // SUPPRESS
}
return levels.info;
}

static jboolean android_util_Log_isLoggable(JNIEnv* env, jobject clazz, jstring tag, jint level)
{
#ifndef HAVE_ANDROID_OS
return false;
#else /* HAVE_ANDROID_OS */
int len;
char key[PROPERTY_KEY_MAX];
char buf[PROPERTY_VALUE_MAX];

if (tag == NULL) {
return false;
}

jboolean result = false;

const char* chars = env->GetStringUTFChars(tag, NULL);

if ((strlen(chars)+sizeof(LOG_NAMESPACE)) > PROPERTY_KEY_MAX) {
jclass clazz = env->FindClass("java/lang/IllegalArgumentException");
char buf2[200];
snprintf(buf2, sizeof(buf2),  "Log tag \"%s\" exceeds limit of %d characters\n",
chars, PROPERTY_KEY_MAX - sizeof(LOG_NAMESPACE));

// release the chars!
env->ReleaseStringUTFChars(tag, chars);

env->ThrowNew(clazz, buf2);
return false;
} else {
strncpy(key, LOG_NAMESPACE, sizeof(LOG_NAMESPACE)-1);
strcpy(key + sizeof(LOG_NAMESPACE) - 1, chars);
}

env->ReleaseStringUTFChars(tag, chars);

len = property_get(key, buf, "");
int logLevel = toLevel(buf);
return (logLevel >= 0 && level >= logLevel) ? true : false;
#endif /* HAVE_ANDROID_OS */
}

/*
* In class android.util.Log:
* public static native int println_native(int buffer, int priority, String tag, String msg)
*/
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) {
jclass npeClazz;

npeClazz = env->FindClass("java/lang/NullPointerException");
assert(npeClazz != NULL);

env->ThrowNew(npeClazz, "println needs a message");
return -1;
}

if (bufID < 0 || bufID >= LOG_ID_MAX) {
jclass npeClazz;

npeClazz = env->FindClass("java/lang/NullPointerException");
assert(npeClazz != NULL);

env->ThrowNew(npeClazz, "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 },
};

int register_android_util_Log(JNIEnv* env)
{
jclass clazz = env->FindClass("android/util/Log");

if (clazz == NULL) {
LOGE("Can't find android/util/Log");
return -1;
}

levels.verbose = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "VERBOSE", "I"));
levels.debug = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "DEBUG", "I"));
levels.info = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "INFO", "I"));
levels.warn = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "WARN", "I"));
levels.error = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ERROR", "I"));
levels.assert = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ASSERT", "I"));

return AndroidRuntime::registerNativeMethods(env, "android/util/Log", gMethods, NELEM(gMethods));
}

}; // namespace android

/* //device/libs/android_runtime/android_util_Log.cpp ** ** Copyright 2006, The Android Open Source Project ** ** Licensed under the Apache License, Version 2.0 (the "License"); ** you may not use this file except in compliance with the License. ** You may obtain a copy of the License at ** ** http://www.apache.org/licenses/LICENSE-2.0 ** ** Unless required by applicable law or agreed to in writing, software ** distributed under the License is distributed on an "AS IS" BASIS, ** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. ** See the License for the specific language governing permissions and ** limitations under the License. */ #define LOG_NAMESPACE "log.tag." #define LOG_TAG "Log_println" #include <assert.h> #include <cutils/properties.h> #include <utils/Log.h> #include <utils/String8.h> #include "jni.h" #include "utils/misc.h" #include "android_runtime/AndroidRuntime.h" #define MIN(a,b) ((a<b)?a:b) namespace android { struct levels_t { jint verbose; jint debug; jint info; jint warn; jint error; jint assert; }; static levels_t levels; static int toLevel(const char* value) { switch (value[0]) { case 'V': return levels.verbose; case 'D': return levels.debug; case 'I': return levels.info; case 'W': return levels.warn; case 'E': return levels.error; case 'A': return levels.assert; case 'S': return -1; // SUPPRESS } return levels.info; } static jboolean android_util_Log_isLoggable(JNIEnv* env, jobject clazz, jstring tag, jint level) { #ifndef HAVE_ANDROID_OS return false; #else /* HAVE_ANDROID_OS */ int len; char key[PROPERTY_KEY_MAX]; char buf[PROPERTY_VALUE_MAX]; if (tag == NULL) { return false; } jboolean result = false; const char* chars = env->GetStringUTFChars(tag, NULL); if ((strlen(chars)+sizeof(LOG_NAMESPACE)) > PROPERTY_KEY_MAX) { jclass clazz = env->FindClass("java/lang/IllegalArgumentException"); char buf2[200]; snprintf(buf2, sizeof(buf2), "Log tag \"%s\" exceeds limit of %d characters\n", chars, PROPERTY_KEY_MAX - sizeof(LOG_NAMESPACE)); // release the chars! env->ReleaseStringUTFChars(tag, chars); env->ThrowNew(clazz, buf2); return false; } else { strncpy(key, LOG_NAMESPACE, sizeof(LOG_NAMESPACE)-1); strcpy(key + sizeof(LOG_NAMESPACE) - 1, chars); } env->ReleaseStringUTFChars(tag, chars); len = property_get(key, buf, ""); int logLevel = toLevel(buf); return (logLevel >= 0 && level >= logLevel) ? true : false; #endif /* HAVE_ANDROID_OS */ } /* * In class android.util.Log: * public static native int println_native(int buffer, int priority, String tag, String msg) */ 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) { jclass npeClazz; npeClazz = env->FindClass("java/lang/NullPointerException"); assert(npeClazz != NULL); env->ThrowNew(npeClazz, "println needs a message"); return -1; } if (bufID < 0 || bufID >= LOG_ID_MAX) { jclass npeClazz; npeClazz = env->FindClass("java/lang/NullPointerException"); assert(npeClazz != NULL); env->ThrowNew(npeClazz, "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 }, }; int register_android_util_Log(JNIEnv* env) { jclass clazz = env->FindClass("android/util/Log"); if (clazz == NULL) { LOGE("Can't find android/util/Log"); return -1; } levels.verbose = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "VERBOSE", "I")); levels.debug = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "DEBUG", "I")); levels.info = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "INFO", "I")); levels.warn = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "WARN", "I")); levels.error = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ERROR", "I")); levels.assert = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ASSERT", "I")); return AndroidRuntime::registerNativeMethods(env, "android/util/Log", gMethods, NELEM(gMethods)); } }; // namespace android 在gMethods变量中，定义了println_native本地方法对应的函数调用是android_util_Log_println_native。在android_util_Log_println_native函数中，通过了各项参数验证正确后，就调用运行时库函数__android_log_buf_write来实现Log的写入操作。__android_log_buf_write函实实现在liblog库中，它有4个参数，分别缓冲区ID、优先级别ID、Tag字符串和Msg字符串。下面运行时库liblog中的__android_log_buf_write的实现。

三. 系统运行库层日志系统的实现。

在系统运行库层liblog库的实现中，内容比较多，这里，我们只关注日志写入操作__android_log_buf_write的相关实现：

int __android_log_buf_write(int bufID, int prio, const char *tag, const char *msg)
{
struct iovec vec[3];

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 */
!strcmp(tag, "AT") ||
!strcmp(tag, "GSM") ||
!strcmp(tag, "STK") ||
!strcmp(tag, "CDMA") ||
!strcmp(tag, "PHONE") ||
!strcmp(tag, "SMS"))
bufID = LOG_ID_RADIO;

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);
}

int __android_log_buf_write(int bufID, int prio, const char *tag, const char *msg) { struct iovec vec[3]; 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 */ !strcmp(tag, "AT") || !strcmp(tag, "GSM") || !strcmp(tag, "STK") || !strcmp(tag, "CDMA") || !strcmp(tag, "PHONE") || !strcmp(tag, "SMS")) bufID = LOG_ID_RADIO; 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); }

函数首先是检查传进来的tag参数是否是为HTC_RIL、RIL、AT、GSM、STK、CDMA、PHONE和SMS中的一个，如果是，就无条件地使用ID为LOG_ID_RADIO的日志缓冲区作为写入缓冲区，接着，把传进来的参数prio、tag和msg分别存放在一个向量数组中，调用write_to_log函数来进入下一步操作。write_to_log是一个函数指针，定义在文件开始的位置上：

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

static int __write_to_log_init(log_id_t, struct iovec *vec, size_t nr); static int (*write_to_log)(log_id_t, struct iovec *vec, size_t nr) = __write_to_log_init; 并且初始化为__write_to_log_init函数：

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);
}

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); } 这里我们可以看到，如果是第一次调write_to_log函数，write_to_log == __write_to_log_init判断语句就会true，于是执行log_open函数打开设备文件，并把文件描述符保存在log_fds数组中。如果打开/dev/LOGGER_LOG_SYSTEM文件失败，即log_fds[LOG_ID_SYSTEM] < 0，就把log_fds[LOG_ID_SYSTEM]设置为log_fds[LOG_ID_MAIN]，这就是我们上面描述的如果不存在ID为LOG_ID_SYSTEM的日志缓冲区，就把LOG_ID_SYSTEM设置为和LOG_ID_MAIN对应的日志缓冲区了。LOGGER_LOG_MAIN、LOGGER_LOG_RADIO、LOGGER_LOG_EVENTS和LOGGER_LOG_SYSTEM四个宏定义在system/core/include/cutils/logger.h文件中：

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

#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，把write_to_log函数指针指向__write_to_log_kernel函数：

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;
}

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; } 函数调用log_writev来实现Log的写入，注意，这里通过一个循环来写入Log，直到写入成功为止。这里log_writev是一个宏，在文件开始的地方定义为：

#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)
#define log_writev(filedes, vector, count) writev(filedes, vector, count)
#define log_close(filedes) close(filedes)
#endif

#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) #define log_writev(filedes, vector, count) writev(filedes, vector, count) #define log_close(filedes) close(filedes) #endif 这里，我们看到，一般情况下，log_writev就是writev了，这是个常见的批量文件写入函数，就不多说了。
至些，整个调用过程就结束了。总结一下，首先是从应用程序层调用应用程序框架层的Java接口，应用程序框架层的Java接口通过调用本层的JNI方法进入到系统运行库层的C接口，系统运行库层的C接口通过设备文件来访问内核空间层的Logger驱动程序。这是一个典型的调用过程，很好地诠释Android的系统架构，希望读者好好领会。