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../kernel/Vsprintf.c

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/* vsprintf.c -- Lars Wirzenius & Linus Torvalds. */

/*
 * Wirzenius wrote this portably, Torvalds fucked it up :-)
 */


#include <stdarg.h>
#include <string.h>


/* we use this so that we can do without the ctype library */
#define is_digit(c) ((c) >= '0' && (c) <= '9')


static int skip_atoi(const char **s)
{
int i=0;


while (is_digit(**s))
i = i*10 + *((*s)++) - '0';
return i;
}


#define ZEROPAD 1 /* pad with zero */
#define SIGN 2 /* unsigned/signed long */
#define PLUS 4 /* show plus */
#define SPACE 8 /* space if plus */
#define LEFT 16 /* left justified */
#define SPECIAL 32 /* 0x */
#define SMALL 64 /* use 'abcdef' instead of 'ABCDEF' */


#define do_div(n,base) ({ \
int __res; \
//下面的内嵌汇编的意思是：
// movl n %eax
// movl 0 %edx
// movl base %ebx
// divl %ebx //将eax中的数除操作数，商放入EAX，余数放入EDX
// movl %eax n
// movl %edx _res
__asm__("divl %4":"=a" (n),"=d" (__res):"0" (n),"1" (0),"r" (base)); \
__res; })


static char * number(char * str, int num, int base, int size, int precision
,int type) //将数字转变为字符串
{
char c,sign,tmp[36];
const char *digits="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
int i;


if (type&SMALL) digits="0123456789abcdefghijklmnopqrstuvwxyz";
if (type&LEFT) type &= ~ZEROPAD;
if (base<2 || base>36)
return 0;
c = (type & ZEROPAD) ? '0' : ' ' ; //如果type是ZEROPAD，那么就填0，否则填“”
if (type&SIGN && num<0) {
sign='-';
num = -num;
} else
sign=(type&PLUS) ? '+' : ((type&SPACE) ? ' ' : 0);
if (sign) size--;
if (type&SPECIAL)
if (base==16) size -= 2; //16进制数的处理
else if (base==8) size--; //8进制数的处理
i=0;
if (num==0)
tmp[i++]='0';
else while (num!=0)
tmp[i++]=digits[do_div(num,base)];
if (i>precision) precision=i;
size -= precision;
if (!(type&(ZEROPAD+LEFT)))
while(size-->0)
*str++ = ' ';
if (sign)
*str++ = sign;
if (type&SPECIAL)
if (base==8)
*str++ = '0';
else if (base==16) {
*str++ = '0';
*str++ = digits[33];
}
if (!(type&LEFT))
while(size-->0)
*str++ = c;
while(i<precision--)
*str++ = '0';
while(i-->0)
*str++ = tmp[i];
while(size-->0)
*str++ = ' ';
return str;
}


int vsprintf(char *buf, const char *fmt, va_list args)
{
int len;
int i;
char * str;
char *s;
int *ip;


int flags; /* flags to number() */


int field_width; /* width of output field */
int precision; /* min. # of digits for integers; max
  number of chars for from string */
int qualifier; /* 'h', 'l', or 'L' for integer fields */


for (str=buf ; *fmt ; ++fmt) {
if (*fmt != '%') { //将指针定位在第一个%处理
*str++ = *fmt;
continue;
}

/* process flags */
flags = 0;
repeat:
++fmt; /* this also skips first '%' */
switch (*fmt) { //看看第一个%之后是什么flags
case '-': flags |= LEFT; goto repeat;
case '+': flags |= PLUS; goto repeat;
case ' ': flags |= SPACE; goto repeat;
case '#': flags |= SPECIAL; goto repeat;
case '0': flags |= ZEROPAD; goto repeat;
}

/* get field width */
field_width = -1;
if (is_digit(*fmt))
field_width = skip_atoi(&fmt); //提取出全部的数字
else if (*fmt == '*') {
/* it's the next argument */
field_width = va_arg(args, int);
if (field_width < 0) {
field_width = -field_width;
flags |= LEFT;
}
}


/* get the precision */
precision = -1;
if (*fmt == '.') { //现在要小树点以后的数字
++fmt;
if (is_digit(*fmt))
precision = skip_atoi(&fmt);
else if (*fmt == '*') {
/* it's the next argument */
precision = va_arg(args, int);
}
if (precision < 0)
precision = 0;
}


/* get the conversion qualifier */
qualifier = -1;
if (*fmt == 'h' || *fmt == 'l' || *fmt == 'L') {
qualifier = *fmt;
++fmt;
}


switch (*fmt) {
case 'c': //字符处理
if (!(flags & LEFT))
while (--field_width > 0)
*str++ = ' ';
*str++ = (unsigned char) va_arg(args, int);
while (--field_width > 0)
*str++ = ' ';
break;


case 's': //字符串处理
s = va_arg(args, char *);
len = strlen(s);
if (precision < 0)
precision = len;
else if (len > precision)
len = precision;


if (!(flags & LEFT))
while (len < field_width--)
*str++ = ' ';
for (i = 0; i < len; ++i)
*str++ = *s++;
while (len < field_width--)
*str++ = ' ';
break;


case 'o':
str = number(str, va_arg(args, unsigned long), 8,
field_width, precision, flags);
break;


case 'p':
if (field_width == -1) {
field_width = 8;
flags |= ZEROPAD;
}
str = number(str,
(unsigned long) va_arg(args, void *), 16,
field_width, precision, flags);
break;


case 'x':
flags |= SMALL;
case 'X':
str = number(str, va_arg(args, unsigned long), 16,
field_width, precision, flags);
break;


case 'd':
case 'i':
flags |= SIGN;
case 'u':
str = number(str, va_arg(args, unsigned long), 10,
field_width, precision, flags);
break;


case 'n':
ip = va_arg(args, int *);
*ip = (str - buf);
break;


default:
if (*fmt != '%')
*str++ = '%';
if (*fmt)
*str++ = *fmt;
else
--fmt;
break;
}
}
*str = '\0';
return str-buf;

}