linux的内核源码

				/*
				 * When in kernel-mode, we cannot use printf, as fs is liable to
				 * point to 'interesting' things. Make a printf with fs-saving, and
				 * all is well.
				 */
				#include 
				#include 
				
				#include 
				
				static char buf[1024];
				
				int printk(const char *fmt, ...)
				{
					va_list args;
					int i;
				
					va_start(args, fmt);			//定位要打印的串在args中的开始位置
					i=vsprintf(buf,fmt,args);		//要打印的字符个数
					va_end(args);
					__asm__("push %%fs\n\t"
						"push %%ds\n\t"
						"pop %%fs\n\t"
						"pushl %0\n\t"		//要打印的字符串个数i，tty_write函数的第3个参数
						"pushl $_buf\n\t"		//要打印的字符串指针，tty_write函数的第2个参数
						"pushl $0\n\t"		//tty_write函数的第一个参数，为0
						"call _tty_write\n\t"
						"addl $8,%%esp\n\t"
						"popl %0\n\t"
						"pop %%fs"
						::"r" (i):"ax","cx","dx");
					return i;
				}