xprintf format string

说明：xPRintf表示printf系列的函数。

The  format string is a character string, beginning and ending in its initial shift state, if any. 

The format string is composed of zero or more directives:

格式说明串主要包含两部分，普通字符和转换说明符

1. ordinary characters (not %), which are copied unchanged to the output stream;

    普通字符直接输出

2. conversion specifications, each of which results in fetching  zero  or  more subsequent arguments.  

    Each conversion specification is introduced by the character %, and ends with a conversion specifier. 

     转换说明符形如%d，可能需要0个或多个后序参数

  "%  [flags ...]  [minimum field width] [precision]  [length modifier] conversion-specifications"

[flags ...]

#  加前缀，o->0, x->0x

0  converted value is padded on the left with zeros rather than blanks.

-   The converted value is to be left adjusted on the field  boundary.

' '  A blank should be left before a positive  number  (or empty string) produced by a signed conversion.

+  A  sign  (+  or -) should always be placed before a number produced by a signed conversion.

[minimum field width]

            如果结果位宽不足，则用空格填充；如果位宽超过限制也不会造成结果被截断，而是忽略位宽限制

[precision]    

格式为.[num]，如果只有小数点则取精度为0；负数忽略该限制；

This gives the minimum number of digits to appear for d, i, o, u, x, and X conversions

This gives the  number of digits to appear after the radix character for a, A, e, E, f, and F conversions

the maximum number of significant digits for g and G conversions

the maximum  number of characters to be printed from a string for s and S conversions.

[length modifier]

hh --> char

h   --> short int

l    --> long int

ll   --> long long int

L  --> long double

z  --> sizt_t

t   --> ptrdiff_t

conversion specifications (A  character that specifies the type of conversion to be applied. )

d,i : int --> signed decimal

o,u,x,X : unsigned int --> unsigned octal,hexadecimal

e,E: The double argument is  rounded  and  converted  in  the  style [-]d.ddde±dd