让adb logcat打印内核调试信息

在默认情况下，adb logcat只能显示应用程序的调试信息，我把logcat.cpp修改了一下，让它同时可以打印内核调试信息：

修改的文件：system/core/logcat/logcat.cpp

1、首先先加入头文件
#include //add by 

2、定义所使用的TAG
#define KERNEL_TAG "Kernel"

3、替换readLogLines函数

static void readLogLines(log_device_t* devices)
{
    log_device_t* dev;
    int max = 0;
    int ret;
    int queued_lines = 0;
    bool sleep = true;
    char buffer[256] = {0}; //add by zhaofei


    int result;
    fd_set readset;


    for (dev=devices; dev; dev = dev->next) {
        if (dev->fd > max) {
            max = dev->fd;
        }
    }


    while (1) {
        do {
            timeval timeout = { 0, 5000 /* 5ms */ }; // If we oversleep it's ok, i.e. ignore EINTR.
            FD_ZERO(&readset);
            for (dev=devices; dev; dev = dev->next) {
                FD_SET(dev->fd, &readset);
            }
            result = select(max + 1, &readset, NULL, NULL, sleep ? NULL : &timeout);
        } while (result == -1 && errno == EINTR);


        if (result >= 0) {
            for (dev=devices; dev; dev = dev->next) {
                if (FD_ISSET(dev->fd, &readset)) {
                    queued_entry_t* entry = new queued_entry_t();
                    /* NOTE: driver guarantees we read exactly one full entry */
                    ret = read(dev->fd, entry->buf, LOGGER_ENTRY_MAX_LEN);
                    if (ret < 0) {
                        if (errno == EINTR) {
                            delete entry;
                            goto next;
                        }
                        if (errno == EAGAIN) {
                            delete entry;
                            break;
                        }
                        perror("logcat read");
                        exit(EXIT_FAILURE);
                    }
                    else if (!ret) {
                        fprintf(stderr, "read: Unexpected EOF!\n");
                        exit(EXIT_FAILURE);
                    }


                    entry->entry.msg[entry->entry.len] = '\0';


                    dev->enqueue(entry);
                    ++queued_lines;


#if 1 //read kernel log
                    if((ret = klogctl(9, buffer, sizeof(buffer))) > 0) {
                        if((ret = klogctl(2, buffer, sizeof(buffer))) > 0) {
                            entry->entry.tid = 0;
                            entry->entry.pid = getpid();
                            /*priority*/
                            entry->entry.msg[0] = ANDROID_LOG_INFO;
                            /*tag*/
                            strcpy(entry->entry.msg+1, KERNEL_TAG);
                            /*message*/
                            strncpy(entry->entry.msg+1+sizeof(KERNEL_TAG), buffer, ret);
                            entry->entry.len = 1 + sizeof(KERNEL_TAG) + ret + 1;
                            entry->entry.msg[entry->entry.len] = '/0';
                            /*
                            if (g_printBinary) {
                                printBinary(dev, entry->entry);
                            } else {
                                (void) processBuffer(dev, entry->entry);
                            }
                            */
                        printNextEntry(dev);
                        }
                    }
#endif
                }
            }


            if (result == 0) {
                // we did our short timeout trick and there's nothing new
                // print everything we have and wait for more data
                sleep = true;
                while (true) {
                    chooseFirst(devices, &dev);
                    if (dev == NULL) {
                        break;
                    }
                    if (g_tail_lines == 0 || queued_lines <= g_tail_lines) {
                        printNextEntry(dev);
                    } else {
                        skipNextEntry(dev);
                    }
                    --queued_lines;
                }


                // the caller requested to just dump the log and exit
                if (g_nonblock) {
                    exit(0);
                }
            } else {
                // print all that aren't the last in their list
                sleep = false;
                while (g_tail_lines == 0 || queued_lines > g_tail_lines) {
                    chooseFirst(devices, &dev);
                    if (dev == NULL || dev->queue->next == NULL) {
                        break;
                    }
                    if (g_tail_lines == 0) {
                        printNextEntry(dev);
                    } else {
                        skipNextEntry(dev);
                    }
                    --queued_lines;
                }
            }
        }
next:
        ;
    }
}

这里没有把内核调试信息的级别转换成Androind的LOG级别，entry->entry.msg[0] = Android_LOG_INFO;使用了ANDROID_LOG_INFO级别，进程ID用了当前的进程ID。

然后就可以使用logcat来抓取kernel的log了！