查看android log系统的源代码 android log系统 0

= 使用选项简介 =
<pre>
Controlling Log Output Format

Log messages contain a number of metadata fields, in addition to the tag and priority. You can modify the output format for messages so that they display a specific metadata field. To do so, you use the -v option and specify one of the supported output formats listed below.

    brief — Display priority/tag and PID of the process issuing the message (the default format).
    process — Display PID only.
    tag — Display the priority/tag only.
    raw — Display the raw log message, with no other metadata fields.
    time — Display the date, invocation time, priority/tag, and PID of the process issuing the message.
    threadtime — Display the date, invocation time, priority, tag, and the PID and TID of the thread issuing the message.
    long — Display all metadata fields and separate messages with blank lines.
</pre>
如:adb logcat -v threadtime，可同时打印时间、PID和TID，详见 http://developer.android.com/tools/debugging/debugging-log.html

= 实现方式剖析 =

== 一、java接口 ==

'''android.util.Log'''
<pre class="prettyprint">
public final class Log {
...
    public static final int VERBOSE = 2;
...
    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));
    }
...
}
</pre>

'''android.util.Slog'''
<pre class="prettyprint">
public final class Slog {
...
    public static int v(String tag, String msg) {
        return Log.println_native(Log.LOG_ID_SYSTEM, Log.VERBOSE, tag, msg);
    }

    public static int v(String tag, String msg, Throwable tr) {
        return Log.println_native(Log.LOG_ID_SYSTEM, Log.VERBOSE, tag,
                msg + '\n' + Log.getStackTraceString(tr));
    }
...
    public static int println(int priority, String tag, String msg) {
        return Log.println_native(Log.LOG_ID_SYSTEM, priority, tag, msg);
    }
}
</pre>

'''com.mediatek.xlog.Xlog'''
<pre class="prettyprint">
public final class Xlog {
    public static int v(String tag, String msg) {
        return Log.println_native(Log.LOG_ID_MAIN, Log.VERBOSE, tag, msg);
    }

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

    ...
}
</pre>

'''com.mediatek.xlog.SXlog'''
<pre class="prettyprint">
public final class SXlog {
    public static int v(String tag, String msg) {
        return Log.println_native(Log.LOG_ID_SYSTEM, Log.VERBOSE, tag, msg);
    }

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

    ...
}
</pre>

== 二、jni接口 ==

'''/frameworks/base/core/jni/android_util_Log.cpp'''
<pre class="prettyprint">
namespace android {

...

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 },
};

...

}; // namespace android
</pre>
println_native 等同于 android_util_Log_println_native.
== 三、hal ==

'''/system/core/liblog/logd_write.c'''

首先来看 jni中 android_util_Log_println_native所调用的函数 __android_log_buf_write：

<pre class="prettyprint">
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);
}
</pre>
此函数只是进行了传入参数的检验，并将 msg和 tag相关信息转存到了 vec[3]这个数组中，接着调用了函数 write_to_log，很容易发现 write_to_log其实是一个函数指针：
<pre class="prettyprint">
static int (*write_to_log)(log_id_t, struct iovec *vec, size_t nr) = __write_to_log_init;
</pre>
但在运行过程中，其指向函数会发生变化：
<pre class="prettyprint">
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);
}
</pre>
即除了第一次调用Log系列函数时会通过 __write_to_log_init进行初始化外，后续的调用都会指向到 __write_to_log_kernel当中：
<pre class = "prettyprint">
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;
}
</pre>
log_writev则是一个宏：
<pre class = "prettyprint">
#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
</pre>
在android中指向 writev(filedes, vector, count)：

'''bionic/libc/arch-sh/syscalls/writev.S'''
<pre class="prettyprint">
/* autogenerated by gensyscalls.py */
#include <sys/linux-syscalls.h>

    .text
    .type writev, @function
    .globl writev
    .align 4

writev:

    /* invoke trap */
    mov.l   0f, r3  /* trap num */
    trapa   #(3 + 0x10)

    /* check return value */
    cmp/pz  r0
    bt      __NR_writev_end

    /* keep error number */
    sts.l   pr, @-r15
    mov.l   1f, r1
    jsr     @r1
    mov     r0, r4
    lds.l   @r15+, pr

__NR_writev_end:
    rts
    nop

    .align  2
0:  .long   __NR_writev
1:  .long   __set_syscall_errno
</pre>
也就是说，log最终是写入到以下设备文件中的：

'''/system/core/include/cutils/logger.h'''
<pre class="prettyprint">
#define LOGGER_LOG_MAIN		"log/main"
#define LOGGER_LOG_RADIO	"log/radio"
#define LOGGER_LOG_EVENTS	"log/events"
#define LOGGER_LOG_SYSTEM	"log/system"
</pre>

== 四、Log驱动 ==

'''kernel/drivers/staging/android/logger.h'''
<pre class="prettyprint">
...
#define LOGGER_LOG_RADIO	"log_radio"	/* radio-related messages */
#define LOGGER_LOG_EVENTS	"log_events"	/* system/hardware events */
#define LOGGER_LOG_SYSTEM	"log_system"	/* system/framework messages */
#define LOGGER_LOG_MAIN		"log_main"	/* everything else */
...
</pre>

'''kernel/drivers/staging/android/logger.c'''
<pre class="prettyprint">
...
static int __init logger_init(void)
{
	int ret;

	ret = init_log(&log_main);
	if (unlikely(ret))
		goto out;

	ret = init_log(&log_events);
	if (unlikely(ret))
		goto out;

	ret = init_log(&log_radio);
	if (unlikely(ret))
		goto out;

	ret = init_log(&log_system);
	if (unlikely(ret))
		goto out;

	init_log_proc();

out:
	return ret;
}
...
</pre>

== 五、疑问 ==

文件 kernel/drivers/staging/android/logger.h 中
<pre class="prettyprint">
...
#define LOGGER_LOG_RADIO	"log_radio"	/* radio-related messages */
...
</pre>
名字为何与实际 dev/ 文件夹下的 log/radio 不一致???