Androi系统提供了一套完整的API供其他程序调用输出log,这套API分为Java 层和 native 层,不过两个API最终都是通过file system将log写入kernel 层的logger device.
ALOGX 系列
以native层为例,如果我们要开发’.cpp’或’.c’程序,那么可以call下列API之以写出不同level的log
#define LOG_TAG "HeloWorld"ALOGV("hello world,level verbose");ALOGD("hello world,level debug");ALOGI("hello world,level info");ALOGE("hello world,level error");ALOGW("hello world,level warning");这里通常都需要定义一个LOG_TAG, 作为一个完整log的一部分,可以唯一的定位一个module. ALOGX()系列API的实现通过宏定位到共同的一组函数.
#ifndef ALOGE#define ALOGE(...) ((void)ALOG(LOG_ERROR, LOG_TAG, __VA_ARGS__))#endif#ifndef ALOG#define ALOG(priority, tag, ...) \
LOG_PRI(ANDROID_##priority, tag, __VA_ARGS__)#endif#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)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);}__android_log_print()通过va_list变量把format形式字符串生成最终的字符串,然后调用__android_log_write(),这里的参数tag就是之前定义的 LOG_TAG. 而prio是一个整数值,中logcat讲到过,最后通过logcat抓出来后,会将整形log level转换为字符型.
int __android_log_write(int prio, const char *tag, const char *msg){
struct iovec vec[3];
log_id_t log_id = LOG_ID_MAIN;
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;
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);}Android log 系统目前有四种类型的log:main,system,radio,events. 后三种一般都是系统的一些特殊的log,除此之外,自己开发的程序,log都默认写到main中. 所以程序最开始把 log_id 设为 LOG_ID_MAIN. 不过程序接下来会判断tag参数,如果tag符合radio log的规则的话,则将log_id改为 LOG_ID_RADIO. 接着把传入的三个参数放到一个iovec变量中. 并调用write_to_log()
struct iovec {
const void* iov_base;
size_t iov_len;};static int (*write_to_log)(log_id_t, struct iovec *vec, size_t nr) = __write_to_log_init;write_to_log()是一个指针函数,这里的实现用了一点小伎俩. 最开始这个指针就被赋值为__write_to_log_init, 所以,在第一次调用该函数的时候,调用的就是 __write_to_log_init()
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);}之所以要这样做,是因为在系统开启后第一次写通过ALOGX函数写log的时候,kernel 层的logger device还未被打开,所以要将这些device都打开,然后,将write_to_log改成__write_to_log_kernel. 在函数的最后,接着再调用一次write_to_log(),这次调用的就是__write_log_log_kernel 了.
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;}函数将log_id转为log_fd后,就直接调用 log_writev()函数
#define log_writev(filedes, vector, count) writev(filedes, vector, count)
log_writev()就被映射到具体的driver层的writev()函数.这样,一条log就被写入到了kernel层的device中.
SLOGX
SLOGX()API族用于生成system log,log被写到system这个logger device中,SLOGX的实现跟main log基本相同,只是默认的log id是system而不是main
#define SLOGV(...) ((void)__android_log_buf_print(LOG_ID_SYSTEM, ANDROID_LOG_VERBOSE, LOG_TAG, __VA_ARGS__))int __android_log_buf_print(int bufID, 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_buf_write(bufID, prio, tag, buf);}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 */
!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;
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);}Events Log
Events log是一种特殊的log,经常被用来记录系统的一些参数:例如电池的当前状态,剩余电量……等等
Java层写event log的api为 writeEvent(),不过系统有四种该函数的实现.
public static native int writeEvent(int tag, int value);public static native int writeEvent(int tag, long value);public static native int writeEvent(int tag, String str);public static native int writeEvent(int tag, Object... list);
系统会根据模块中call api时传入的参数对应调用不同的writeEvent()函数.这些函数分别对应到不同的JNI实现.首先看下参数为int/long时的JNI实现.
static jint android_util_EventLog_writeEvent_Integer(JNIEnv* env, jobject clazz,
jint tag, jint value){
return android_btWriteLog(tag, EVENT_TYPE_INT, &value, sizeof(value));}static jint android_util_EventLog_writeEvent_Long(JNIEnv* env, jobject clazz,
jint tag, jlong value){
return android_btWriteLog(tag, EVENT_TYPE_LONG, &value, sizeof(value));}这两个函数调用了一个共同的函数 android_btWriteLog()
#define android_btWriteLog(tag, type, payload, len) __android_log_btwrite(tag, type, payload, len)int __android_log_btwrite(int32_t tag, char type, const void *payload,
size_t len){
struct iovec vec[3];
vec[0].iov_base = &tag;
vec[0].iov_len = sizeof(tag);
vec[1].iov_base = &type;
vec[1].iov_len = sizeof(type);
vec[2].iov_base = (void*)payload;
vec[2].iov_len = len;
return write_to_log(LOG_ID_EVENTS, vec, 3);}是不是很熟悉?没错,最后跟main log流程一样,都调用write_to_log(0函数. 下面是参数为string时的JNI实现:
static jint android_util_EventLog_writeEvent_String(JNIEnv* env, jobject clazz,
jint tag, jstring value) {
uint8_t buf[MAX_EVENT_PAYLOAD];
const char *str = value != NULL ? env->GetStringUTFChars(value, NULL) : "NULL";
jint len = strlen(str);
const int max = sizeof(buf) - sizeof(len) - 2; // Type byte, final newline
if (len > max) len = max;
buf[0] = EVENT_TYPE_STRING;
memcpy(&buf[1], &len, sizeof(len));
memcpy(&buf[1 + sizeof(len)], str, len);
buf[1 + sizeof(len) + len] = '\n';
if (value != NULL) env->ReleaseStringUTFChars(value, str);
return android_bWriteLog(tag, buf, 2 + sizeof(len) + len);}该函数把type,string长度,string都放到了同一个buffer中,然后call android_bWriteLog()
int __android_log_bwrite(int32_t tag, const void *payload, size_t len){
struct iovec vec[2];
vec[0].iov_base = &tag;
vec[0].iov_len = sizeof(tag);
vec[1].iov_base = (void*)payload;
vec[1].iov_len = len;
return write_to_log(LOG_ID_EVENTS, vec, 2);}该函数与__android_log_bwrite的不同是后者是把type(int/long)跟payload分开的,而该函数放到了一起.
writeEvent的第四种形式:写入的是int/long/string的组合体,则会循环遍历该组合,转换成格式化字符串放到同一个buffer中.
static jint android_util_EventLog_writeEvent_Array(JNIEnv* env, jobject clazz,
jint tag, jobjectArray value) {
uint8_t buf[MAX_EVENT_PAYLOAD];
const size_t max = sizeof(buf) - 1; // leave room for final newline
size_t pos = 2; // Save room for type tag & array count
jsize copied = 0, num = env->GetArrayLength(value);
for (; copied < num && copied < 255; ++copied) {
jobject item = env->GetObjectArrayElement(value, copied);
if (item == NULL || env->IsInstanceOf(item, gStringClass)) {
if (pos + 1 + sizeof(jint) > max) break;
const char *str = item != NULL ? env->GetStringUTFChars((jstring) item, NULL) : "NULL";
jint len = strlen(str);
if (pos + 1 + sizeof(len) + len > max) len = max - pos - 1 - sizeof(len);
buf[pos++] = EVENT_TYPE_STRING;
memcpy(&buf[pos], &len, sizeof(len));
memcpy(&buf[pos + sizeof(len)], str, len);
pos += sizeof(len) + len;
if (item != NULL) env->ReleaseStringUTFChars((jstring) item, str);
} else if (env->IsInstanceOf(item, gIntegerClass)) {
jint intVal = env->GetIntField(item, gIntegerValueID);
if (pos + 1 + sizeof(intVal) > max) break;
buf[pos++] = EVENT_TYPE_INT;
memcpy(&buf[pos], &intVal, sizeof(intVal));
pos += sizeof(intVal);
} else if (env->IsInstanceOf(item, gLongClass)) {
jlong longVal = env->GetLongField(item, gLongValueID);
if (pos + 1 + sizeof(longVal) > max) break;
buf[pos++] = EVENT_TYPE_LONG;
memcpy(&buf[pos], &longVal, sizeof(longVal));
pos += sizeof(longVal);
} else {
jniThrowException(env,
"java/lang/IllegalArgumentException",
"Invalid payload item type");
return -1;
}
env->DeleteLocalRef(item);
}
buf[0] = EVENT_TYPE_LIST;
buf[1] = copied;
buf[pos++] = '\n';
return android_bWriteLog(tag, buf, pos);}最后同样是调用函数 android_bWriteLog()
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