使用printk在Linux終端上打印數據

Question

我目前開始學習在Linux下編程的Linux設備驅動程序。在那裏我發現了這個使用printk()函數打印hello world的一小段代碼。

#include <linux/module.h> 
#include <linux/kernel.h> 
#include <linux/init.h> 

MODULE_LICENSE("Dual BSD/GPL"); 

static int hello_init(void) 
{ 
     printk(KERN_ALERT "Hello World!!!\n"); 
     return 0; 
} 

static void hello_exit(void) 
{ 
     printk(KERN_ALERT "Goodbye Hello World!!!\n"); 
} 

module_init(hello_init); 
module_exit(hello_exit); 

使用make命令和使用命令insmod負載驅動器編譯代碼後。我沒有得到「你好世界」打印在屏幕上，而不是隻打印在日誌文件/var/log/kern.log。但我想在我的ubuntu終端上打印printk。我正在使用Ubuntu（14.04）。可能嗎？

Answer 1

printk打印到內核日誌。就核心而言，沒有「屏幕」。如果您想要實時查看printk的輸出，可以打開一個終端並輸入以下內容dmesg -w。請注意，-w標誌僅支持dmesg（由util-linux包提供）的最新版本。

Answer 2

將內核日誌和按摩重定向到gnome-terminal是不可能的，並且您必須使用dmesg。 但是在一個虛擬終端（打開一個與ctrl+F1-F6），你可以將它們重定向到標準輸出。
首先通過在虛擬終端中輸入tty命令來確定tty編號。輸出可能是/dev/tty（1-6）。
使用您指定的參數編譯並運行此代碼。

/* 
* setconsole.c -- choose a console to receive kernel messages 
* 
* Copyright (C) 1998,2000,2001 Alessandro Rubini 
* 
* This program is free software; you can redistribute it and/or modify 
* it under the terms of the GNU General Public License as published by 
* the Free Software Foundation; either version 2 of the License, or 
* (at your option) any later version. 
* 
* This program is distributed in the hope that it will be useful, 
* but WITHOUT ANY WARRANTY; without even the implied warranty of 
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
* GNU General Public License for more details. 
* 
* You should have received a copy of the GNU General Public License 
* along with this program; if not, write to the Free Software 
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA. 
*/ 

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <errno.h> 
#include <unistd.h> 
#include <sys/ioctl.h> 

int main(int argc, char **argv) 
{ 
    char bytes[2] = {11,0}; /* 11 is the TIOCLINUX cmd number */ 
    if (argc==2) bytes[1] = atoi(argv[1]); /* the chosen console */ 
    else { 
     fprintf(stderr, "%s: need a single arg\n",argv[0]); exit(1); 
    } 
    if (ioctl(STDIN_FILENO, TIOCLINUX, bytes)<0) { /* use stdin */ 
     fprintf(stderr,"%s: ioctl(stdin, TIOCLINUX): %s\n", 
      argv[0], strerror(errno)); 
     exit(1); 
    } 
    exit(0); 
} 

例如，如果你的tty命令的輸出爲的/ dev/tty1上然後鍵入這兩個命令：

gcc setconsole.c -o setconsole 
sudo ./setconsole 1 

這將設置您的TTY接收內核消息。
然後編譯並運行這段代碼。

/* 
* setlevel.c -- choose a console_loglevel for the kernel 
* 
* Copyright (C) 1998,2000,2001 Alessandro Rubini 
* 
* This program is free software; you can redistribute it and/or modify 
* it under the terms of the GNU General Public License as published by 
* the Free Software Foundation; either version 2 of the License, or 
* (at your option) any later version. 
* 
* This program is distributed in the hope that it will be useful, 
* but WITHOUT ANY WARRANTY; without even the implied warranty of 
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
* GNU General Public License for more details. 
* 
* You should have received a copy of the GNU General Public License 
* along with this program; if not, write to the Free Software 
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA. 
*/ 

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <errno.h> 
#include <sys/klog.h> 

int main(int argc, char **argv) 
{ 
    int level; 

    if (argc==2) { 
    level = atoi(argv[1]); /* the chosen console */ 
    } else { 
     fprintf(stderr, "%s: need a single arg\n",argv[0]); exit(1); 
    } 
    if (klogctl(8,NULL,level) < 0) { 
     fprintf(stderr,"%s: syslog(setlevel): %s\n", 
       argv[0],strerror(errno)); 
     exit(1); 
    } 
    exit(0); 
} 

有8級內核信息爲你設置的代碼KERN_ALERT是them.To之一使控制檯接收所有的人，你應該用8上面運行代碼arguement。
gcc setlevel.c -o setlevel
sudo ./setlevel 8

現在，您可以將您的模塊到內核，看看在控制檯內核日誌。
順便說一下這些代碼來自ldd3的例子。