<日 現在才發現 我們通常用的printf並不是這個printf 找文件找錯了，因爲下面的printf 沒有%f之類的輸出 處理>
在裏面發現宏 #define do_div(n,base) 展開後 奇怪的代碼測試如下：
#include <stdio.h>
2
3 int main()
4 {
5 char tmp[66];
6 int num;
7 static const char digits[16] = "0123456789ABCDEF"; /* "GHIJKLMNOPQR STUVWXYZ"; */
8 int base =10;
9
10 scanf("%d",&num);
11
12 int i=0;
13
14 while(num != 0)
15 {
16 tmp[i++] = (
17 digits[
18 (
19 {
20 int __res;
21 __res = ((unsigned long) num) % (unsigned) base;
22 num = ((unsigned long) n) / (unsigned) base;
23 __res;
24 }
25 )//這樣寫也行 還是我的gcc兼容C99的標準？待會兒回到VC下測試 再寫結果,測試過了,這是c99的標準, 在c89下要將分號改成逗號表達式 即使是c++也不能編譯通過
26 ] | 0)
27 ;
28 } tmp[i]='\0';
29 printf("%s\n",tmp);
30 return 0;
31 }
/**/
#include <stdio.h>
#include <stdarg.h>
//extern int myvsprintf(char *buf, const char *fmt, va_list args);
//#include "myprintf.c"
#include <stdio.h>
#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 SMALL 32 /* Must be 32 == 0x20 */
#define SPECIAL 64 /* 0x */
#define do_div(n,base) ({ \
int __res; \
__res = ((unsigned long) n) % (unsigned) base; \
n = ((unsigned long) n) / (unsigned) base; \
__res; })
/*宏函數 ： */
static char *number(char *str, long num, int base, int size, int precision,
int type)
{/*str 原指向 緩衝數組 調用處 爲 str = number(.....) 自己將自己改變了 */ /*爲%d時 num=2, base=10, size=-1, precision=-1(精度), type=2(???))*/
/* we are called with base 8, 10 or 16, only, thus don't need "G..." */
static const char digits[16] = "0123456789ABCDEF"; /* "GHIJKLMNOPQRSTUVWXYZ"; */
char tmp[66];
char c, sign, locase;
int i;
/* locase = 0 or 0x20. ORing digits or letters with 'locase'
* produces same digits or (maybe lowercased) letters */
locase = (type & SMALL);
if (type & LEFT)
type &= ~ZEROPAD;
if (base < 2 || base > 36)
return NULL;
c = (type & ZEROPAD) ? '0' : ' ';/*d的時候2&2 取後面' '??單獨測試應該取前面 這是怎麼回事？*/
sign = 0;
if (type & SIGN) {/*爲正數 檢測下面 但是不走下面*/
if (num < 0) {
sign = '-';
num = -num;
size--;
} else if (type & PLUS) {
sign = '+';
size--;
} else if (type & SPACE) {
sign = ' ';
size--;
}
}
if (type & SPECIAL) {
if (base == 16)/*多少進制*/
size -= 2;
else if (base == 8)
size--;
}
i = 0;
if (num == 0)
tmp[i++] = '0';
else
while (num != 0)
tmp[i++] = (digits[do_div(num, base)] | locase);/*tmp 是字符串數組 *//*假如 num = 1234 則算後tmp［］ 爲4321*/
if (i > precision)/*注意: i 會隨着上面的i而增加*/
precision = i;
size -= precision;/*精度 減去？？???*/
if (!(type & (ZEROPAD + LEFT)))/*%d 默認會走這裏*/
while (size-- > 0)
*str++ = ' ';/*精度 控制加空格 補齊*/
if (sign)/*默認不走這 默認sign = 0*/
*str++ = sign;
if (type & SPECIAL) {/*%d默認不走這*/
if (base == 8)
*str++ = '0';//原來是空 自己改成0
else if (base == 16) {
*str++ = '0';//原來是空 自己改成0
*str++ = ('X' | locase);
}
}
if (!(type & LEFT))/*默認走這裏*/
while (size-- > 0)
*str++ = c;
while (i < precision--)
*str++ = '0';//原來是空 自己改成0
while (i-- > 0)/*i 表示數字轉換後的大小長度 假如123則長3*/
*str++ = tmp[i];/*正確順序放入*/
while (size-- > 0)/*d默認的size怎麼爲-4??*/
*str++ = ' ';
return str;
}
static inline int isdigit(int ch)/*檢測是否是數字字符*/
{
return (ch >= '0') && (ch <= '9');
}
static int skip_atoi(const char **s)
{
int i = 0;
while (isdigit(**s))
i = i * 10 + *((*s)++) - '0';
return i;
}
int myvsprintf(char *buf, const char *fmt, va_list args)
/* 將要打印的值放入buf中 fmt是格式化， *args是原來第一個參數的地址(第0個參數的位置是原來的 &fmt(不是這裏的&fmt)) */
{
int len;
unsigned long num;
int i, base;
char *str;
const char *s;
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 != '%') {/*比如 \n 之類的 直接存放進去*/
*str++ = *fmt;
continue;
}
/* process flags */
flags = 0;//??????????
repeat://?????????
++fmt; /* this also skips first '%' */
switch (*fmt) {/*不是判斷％d或者％f之類的檢測*//*??????????????與下面的＋－ 0有關*/
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 (isdigit(*fmt))/*檢測是不是數字 以便寬度輸出*/
field_width = skip_atoi(&fmt);/*賦值寬度 */
else if (*fmt == '*') {/*???????*/
++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 == '.') {/*？？？％3.2d這之類的檢測？？？？？*/
++fmt;
if (isdigit(*fmt))
precision = skip_atoi(&fmt);
else if (*fmt == '*') {
++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;
}
/* default base */
base = 10;/*????*/
switch (*fmt) {/*檢測哪種格式輸出*/
case 'c':
if (!(flags & LEFT))
while (--field_width > 0)/*默認的fiel。。爲-1 */
*str++ = ' ';/*把要輸出的內容，全部轉換成字符放入str*/
*str++ = (unsigned char)va_arg(args, int);
while (--field_width > 0)
*str++ = ' ';
continue;
case 's':
s = va_arg(args, char *);
len = strnlen(s, precision);
if (!(flags & LEFT))
while (len < field_width--)
*str++ = ' ';
for (i = 0; i < len; ++i)
*str++ = *s++;
while (len < field_width--)
*str++ = ' ';
continue;
case 'p':
if (field_width == -1) {
field_width = 2 * sizeof(void *);
flags |= ZEROPAD;
}
str = number(str,
(unsigned long)va_arg(args, void *), 16,
field_width, precision, flags);
continue;
case 'n':
if (qualifier == 'l') {
long *ip = va_arg(args, long *);
*ip = (str - buf);
} else {
int *ip = va_arg(args, int *);
*ip = (str - buf);
}
continue;
case '%':/*百分號的輸出*/
*str++ = '%';
continue;
/* integer number formats - set up the flags and "break" */
case 'o':
base = 8;
break;
case 'x':
flags |= SMALL;
case 'X':
base = 16;
break;
case 'd':/*d 要跳走？？*/
case 'i':
flags |= SIGN;/*flags的作用？d後這裏爲2*/
case 'u':/*第一次爲%d 即是這裏*fmt爲'd'爲什麼會從這裏跳*/
break;/*flags |= SIGN已經執行過了再從這裏跳出，基本語法 還是???*/
default:
*str++ = '%';
if (*fmt)
*str++ = *fmt;
else
--fmt;
continue;
}
if (qualifier == 'l')/*???*/
num = va_arg(args, unsigned long);/*按照長整形取參數*/
else if (qualifier == 'h') {
num = (unsigned short)va_arg(args, int);/*按照...取數*/
if (flags & SIGN)
num = (short)num;
} else if (flags & SIGN)/*？？d的時候flags=2 SIGN=2按照int 取數 判斷是不是有符號數*/
num = va_arg(args, int);/* (*(int *) (((args) += (_bnd (int,AUPBND))) - (_bnd (int, _AUPBND)))) 將一個int *的指針指向那裏 並將值賦給num */
else/*無符號數*/
num = va_arg(args, unsigned int);
str = number(str, num, base, field_width, precision, flags);/*將數字轉化爲字符串 放入str*/
}
*str = '\0';
return str - buf;
}
int myprintf(const char *fmt, ...)
{
char printf_buf[1024];
va_list args;
int printed;
va_start(args, fmt);/*取得參數的第一個變量的地址 將其賦給fmt*/
printed = myvsprintf(printf_buf, fmt, args);/* 將要打印的值放入buf中 */
va_end(args);/*將參數值取NULL*/
puts(printf_buf);
return printed;
}
int main()
{
int apple=2,pear=3;
char *p = "this is point p 's content!";
myprintf("%d\n%d\n%s\n",apple,pear,p);
return 0;
}