判断机器大小端

#include <iostream>

using namespace std;

int main() {

    union endian
    {
        int data;
        char ch;
    }test;
    test.data=0x12345678;
    if(test.ch == 0x78)
        cout << "little endian" << endl;
    else {
        cout << "big endian" << endl;
    }
    test.data=1;
    if(test.ch == 1)
        cout << "little endian!" << endl;
    else {
        cout << "big endian" << endl;
    }

    int data = 0x12345678;
    int i;
    for(i=0;i<4;i++)
    {
        printf("%#x---->%p ",*((char *)&data+i),(char *)&data+i);
    }
    cout << "buffer" << sizeof(char) << endl;
    cout << "after reverse! ";
    exit(EXIT_SUCCESS);
}
C语言规定，如果在printf()的格式控制符中使用了%#x，就表示在输出时是以带0x前缀的十六进制形式输出后面的对应参数。
也就是说，它的作用和%x相同，只不过%x不会输出前面的“0x"，而这个会输出前面的“0x"

在32位小端的机器上，如下代码输出是什么：

char array[12] = {0x01 , 0x02 , 0x03 , 0x04 , 0x05 , 0x06 , 0x07 , 0x08};     

 short *pshort = (short *)array;     

 int *pint = (int *)array;     

 int64 *pint64 = (int64 *)array;     

 printf("0x%x , 0x%x , 0x%llx , 0x%llx", *pshort , *(pshort+2) , *pint64 , *(pint+2));

 

需要注意的是：(short *)array后pshort指向的不再是array原始的地址，应该是

15         short *pshort = (short *)array;
(gdb) p array
$1 = "010203040506a000000"
(gdb) p/x array
$2 = {0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8, 0x0, 0x0, 0x0, 0x0}
(gdb) p &array
$3 = (char (*)[12]) 0x7fffffffe120
(gdb) p &pshort
$6 = (short **) 0x7fffffffe138

C string that contains the text to be written to stdout.
It can optionally contain embedded format specifiers that are replaced by the values specified in subsequent additional arguments and formatted as requested.

A format specifier follows this prototype: [see compatibility note below] 
%[flags][width][.precision][length]specifier 

Where the specifier character at the end is the most significant component, since it defines the type and the interpretation of its corresponding argument:

specifier	Output	Example
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.	%

The format specifier can also contain sub-specifiers: flags, width, .precision and modifiers (in that order), which are optional and follow these specifications:

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	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	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.

The length sub-specifier modifies the length of the data type. This is a chart showing the types used to interpret the corresponding arguments with and without length specifier (if a different type is used, the proper type promotion or conversion is performed, if allowed):

	specifiers
length	d i	u o x X	f F e E g G a A	c	s	p	n
(none)	int	unsigned int	double	int	char*	void*	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

Note regarding the c specifier: it takes an int (or wint_t) as argument, but performs the proper conversion to a char value (or awchar_t) before formatting it for output.

Note: Yellow rows indicate specifiers and sub-specifiers introduced by C99. See <cinttypes> for the specifiers for extended types.