adb logcat打印内核调试信息

在调试内核的时候，如果用串口总要费县到串口，如果能直接用adb logcat 命令就好了，插上USB就可以了，也省去了飞线的步骤：

如何才能实现呢，经过搜索找到了如下的方法：

更改logcat.cpp文件：

第一步：添加头文件 #include <sys/klog.h> //add 

第二步：添加宏 #define KERNEL_TAG "Kernel"

第三部：修改readLogLines函数

在函数中添加红色部分函数就可以了，其他不变

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 }; // 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(); 
                     
                    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(); 
                             
                            entry->entry.msg[0] = Android_LOG_INFO; 
                             
                            strcpy(entry->entry.msg+1, KERNEL_TAG); 
                             
                            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'; 
                             
                        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: 
        ; 
    } 
} 

第四步：连接USB 进入调试模式，打开终端 输入命令 adb logcat

此时在内核中用函数PRINTK输出的TRACE就能打印出来了，

如何治显示内核部分的LOG呢,可以用命令

adb logcat -s Kernel

-s参数说明只显示包含LOG_TAG 为Kernel的，这样就只显示内核LOG了