Debugging by Printing

                The most common debugging technique is monitoring, which in applications programming is done by calling printf at suitable points. When you are debugging kernel code, you can accomplish the same goal with printk.

printk(KERN_DEBUG "Here I am: %s:%i\n", __FILE__, __LINE__);
printk(KERN_CRIT "I'm trashed; giving up on %p\n", ptr);

KERN_EMERG
Used for emergency messages, usually those that precede a crash.
KERN_ALERT
A situation requiring immediate action.
KERN_CRIT
Critical conditions, often related to serious hardware or software failures.
KERN_ERR
Used to report error conditions; device drivers often use KERN_ERR to report hard-
ware difficulties.
KERN_WARNING
Warnings about problematic situations that do not, in themselves, create seri-
ous problems with the system.
KERN_NOTICE
Situations that are normal, but still worthy of note. A number of security-related
conditions are reported at this level.
KERN_INFO
Informational messages. Many drivers print information about the hardware
they find at startup time at this level.
KERN_DEBUG
Used for debugging messages.

                 Each string (in the macro expansion) represents an integer in angle brackets. Integers range from 0 to 7,with smaller values representing higher priorities.

/*
 * 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);
}

The kernel has provided a function that can be helpful in such cases:

int printk_ratelimit(void);

                 This function should be called before you consider printing a message that could be repeated often. If the function returns a nonzero value, go ahead and print your message, otherwise skip it. Thus, typical calls look like this:

if (printk_ratelimit( ))
printk(KERN_NOTICE "The printer is still on fire\n");

printk_ratelimit works by tracking how many messages are sent to the console. When the level of output exceeds a threshold, printk_ratelimit starts returning 0 and causing messages to be dropped.

int print_dev_t(char *buffer, dev_t dev);
char *format_dev_t(char *buffer, dev_t dev);

                  Both macros encode the device number into the given buffer ; the only difference is that print_dev_t returns the number of characters printed, while format_dev_t returns buffer ; therefore, it can be used as a parameter to a printk call directly, although one must remember that printk doesn’t flush until a trailing newline is provided. The buffer should be large enough to hold a device number; given that 64-bit device numbers are a distinct possibility in future kernel releases, the buffer should probably be at least 20 bytes long.

#define print_dev_t(buffer, dev)						sprintf((buffer), "%u:%u\n", MAJOR(dev), MINOR(dev))


#define format_dev_t(buffer, dev)						({										sprintf(buffer, "%u:%u", MAJOR(dev), MINOR(dev));			buffer;								})

#include <stdio.h>

int main()
{
	printf("Hello world!\n");

	return 0;
}

可以见得，一开始先调用了execve(),去执行./hello这个可执行程序

然后去open了libc.so.6这个库，最后write()把“hello world”字符串写入到标准输出