C/C++ 中 `printf` 格式化

作为强类型静态语言，类型不仅规定了可以对数据进行的操作，还决定了应该怎样在 printf 中输出。

printf 的签名是：

int printf ( const char * format, ... );

其中 format 为可以参参数格式化的输出内容。具体格式化形式为：

%[flags][width][.precision][length]specifier

以 % 开头，紧跟一些用于格式化的修饰符，其中 [flags][width][.precision][length] 这些为可选部分，称为 sub-specifier，重点是 specifier，它与数据类型便有对应关系了。

一些简单示例：

// 打印整形
int age=20;
printf("My age is %d",age);
// 打印字符串
char[] name="poftut";
printf("Name: %s",name);
// 同时打印多个变量
char[] name="poftut";
int age=2;
char[] city = "ankara";
printf("Name:%s , Age:%d , City:%s",name, age, city);

specifier

可选的 specifier 以及对应的数据类型见如下来自 C++ Reference 的表格：

specifier	输出描述	输出示例
d or i	Signed decimal integer	392
u	Unsigned decimal integer	7235
o	Unsigned octal	610
x	Unsigned hexadecimal integer	7fa
X	Unsigned hexadecimal integer (uppercase)	7FA
f	Decimal floating point, lowercase	392.65
F	Decimal floating point, uppercase	392.65
e	Scientific notation (mantissa/exponent), lowercase	3.9265e+2
E	Scientific notation (mantissa/exponent), uppercase	3.9265E+2
g	Use the shortest representation: %e or %f	392.65
G	Use the shortest representation: %E or %F	392.65
a	Hexadecimal floating point, lowercase	-0xc.90fep-2
A	Hexadecimal floating point, uppercase	-0XC.90FEP-2
c	Character	a
s	String of characters	sample
p	Pointer address	b8000000
n	Nothing printed.
The corresponding argument must be a pointer to a signed int.
The number of characters written so far is stored in the pointed location.
%	A % followed by another % character will write a single % to the stream.	%

flag

flags	description
-	Left-justify within the given field width; Right justification is the default (see width sub-specifier).
+	Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign.
(space)	If no sign is going to be written, a blank space is inserted before the value.
#	Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero.
Used with a, A, e, E, f, F, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written.
0	Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier).

width

width	description
(number)	Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*	The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.

precision

.precision	description
.number	For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0.
For a, A, e, E, f and F specifiers: this is the number of digits to be printed after the decimal point (by default, this is 6).
For g and G specifiers: This is the maximum number of significant digits to be printed.
For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered.
If the period is specified without an explicit value for precision, 0 is assumed.
.*	The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.

length

length 长度 sub-specifier用来补充修饰数据类型的长度。部分数据类型会有长度的变种，便可用此 sub-specifier 来标识。它与 spcifier 的组合所表示的数据类型见下表：

length	int	unsigned int	double	int	char*	int*
hh	signed char	unsigned char				signed char*
h	short int	unsigned short int				short int*
l	long int	unsigned long int		wint_t	wchar_t*	long int*
ll	long long int	unsigned long long int				long long int*
j	intmax_t	uintmax_t				intmax_t*
z	size_t	size_t				size_t*
t	ptrdiff_t	ptrdiff_t				ptrdiff_t*
L			long double

另外一些示例

int encode(const short* buffer_l, int mp3buf_size) {
  printf("addr is %p ,size is %i\n", buffer_l,mp3buf_size);
}

注意这里 %p，对照上面 specifier 表格可知它代表指针，这里用其他类型都不能匹配。

来自的示例：

/* printf example */
#include <stdio.h>
int main()
{
printf ("Characters: %c %c \n", 'a', 65);
printf ("Decimals: %d %ld\n", 1977, 650000L);
printf ("Preceding with blanks: %10d \n", 1977);
printf ("Preceding with zeros: %010d \n", 1977);
printf ("Some different radices: %d %x %o %#x %#o \n", 100, 100, 100, 100, 100);
printf ("floats: %4.2f %+.0e %E \n", 3.1416, 3.1416, 3.1416);
printf ("Width trick: %*d \n", 5, 10);
printf ("%s \n", "A string");
return 0;
}

输出：

Characters: a A
Decimals: 1977 650000
Preceding with blanks:       1977
Preceding with zeros: 0000001977
Some different radices: 100 64 144 0x64 0144
floats: 3.14 +3e+000 3.141600E+000
Width trick:    10
A string