Linux内核打印printk

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#linux #c语言

本文介绍了Linux内核中的printk函数，其提供了灵活的格式化输出，支持8个打印级别。还详细讲解了console_printk数组和如何查看当前控制台的日志级别设置。

Linux内核打印-----printk

本文将展开介绍内核打印函数printk
printk函数是Linux内核中常用的记录消息的函数之一。能在任何上下文中使用，并提供灵活的格式化输出功能。通过使用 printk函数，开发者可以在内核代码中记录关键消息，进行调试以及了解系统的运行状况。

函数原型

/**
 * printk - print a kernel message
 * @fmt: format string
 *
 * This is printk(). It can be called from any context. We want it to work.
 *
 * We try to grab the console_lock. If we succeed, it's easy - we log the
 * output and call the console drivers.  If we fail to get the semaphore, we
 * place the output into the log buffer and return. The current holder of
 * the console_sem will notice the new output in console_unlock(); and will
 * send it to the consoles before releasing the lock.
 *
 * One effect of this deferred printing is that code which calls printk() and
 * then changes console_loglevel may break. This is because console_loglevel
 * is inspected when the actual printing occurs.
 *
 * See also:
 * printf(3)
 *
 * See the vsnprintf() documentation for format string extensions over C99.
 */
asmlinkage __visible int printk(const char *fmt, ...)
{
	printk_func_t vprintk_func;
	va_list args;
	int r;

	va_start(args, fmt);

	/*
	 * If a caller overrides the per_cpu printk_func, then it needs
	 * to disable preemption when calling printk(). Otherwise
	 * the printk_func should be set to the default. No need to
	 * disable preemption here.
	 */
	vprintk_func = this_cpu_read(printk_func);
	r = vprintk_func(fmt, args);

	va_end(args);

	return r;
}
EXPORT_SYMBOL(printk);

printk函数使用asmlinkage和__visible修饰符，表示这是一个可见的内联函数，并可以在任意上下文中调用。
EXPORT_SYMBOL(printk)表示将printk函数导出为符号，使得其他模块能够使用该函数。

打印级别

共有如下8个级别，这些宏通常与printk函数一起使用，用于在内核中输出不同级别的消息到系统日志。

#define KERN_EMERG	KERN_SOH "0"	/* system is unusable */
#define KERN_ALERT	KERN_SOH "1"	/* action must be taken immediately */
#define KERN_CRIT	KERN_SOH "2"	/* critical conditions */
#define KERN_ERR	KERN_SOH "3"	/* error conditions */
#define KERN_WARNING	KERN_SOH "4"	/* warning conditions */
#define KERN_NOTICE	KERN_SOH "5"	/* normal but significant condition */
#define KERN_INFO	KERN_SOH "6"	/* informational */
#define KERN_DEBUG	KERN_SOH "7"	/* debug-level messages */

#define KERN_DEFAULT	KERN_SOH "d"	/* the default kernel loglevel */

默认打印级别

console_printk数组

Linux内核源代码中，console_printk是一个名为console_printk的整型数组。该数组的定义如下所示：

int console_printk[4] = {
    CONSOLE_LOGLEVEL_DEFAULT,     /* console_loglevel */
    MESSAGE_LOGLEVEL_DEFAULT,     /* default_message_loglevel */
    CONSOLE_LOGLEVEL_MIN,         /* minimum_console_loglevel */
    CONSOLE_LOGLEVEL_DEFAULT,     /* default_console_loglevel */
};