printf函数解析

 摘自：

http://www.cplusplus.com/reference/clibrary/cstdio/printf/

function

<cstdio>

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

Print formatted data to stdout

Writes to the standard output (stdout) a sequence of data formatted as the format argument specifies. After the format parameter, the function expects at least as many additional arguments as specified in format.

Parameters

format

String that contains the text to be written to stdout.
It can optionally contain embedded format tags that are substituted by the values specified in subsequent argument(s) and formatted as requested.
The number of arguments following the format parameters should at least be as much as the number of format tags.
The format tags follow this prototype:

%[flags][width][.precision][length]specifier
Where specifier is the most significant one and defines the type and the interpretation of the value of the coresponding argument:

specifier	Output	Example
c	Character	a
d or i	Signed decimal integer	392
e	Scientific notation (mantise/exponent) using e character	3.9265e+2
E	Scientific notation (mantise/exponent) using E character	3.9265E+2
f	Decimal floating point	392.65
g	Use the shorter of %e or %f	392.65
G	Use the shorter of %E or %f	392.65
o	Unsigned octal	610
s	String of characters	sample
u	Unsigned decimal integer	7235
x	Unsigned hexadecimal integer	7fa
X	Unsigned hexadecimal integer (capital letters)	7FA
p	Pointer address	B800:0000
n	Nothing printed. The argument must be a pointer to a signed int, where the number of characters written so far is stored.
%	A % followed by another % character will write % to stdout.	%

The tag can also contain flags, width, .precision and modifiers sub-specifiers, 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 precede 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 e, E and f, it forces the written output to contain a decimal point even if no digits would follow. By default, if no digits follow, no decimal point is written. Used with g or G the result is the same as with e or E but trailing zeros are not removed.
0	Left-pads the number with zeroes (0) instead of spaces, where 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 e, E and f specifiers: this is the number of digits to be printed after the decimal point. 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. For c type: it has no effect. When no precision is specified, the default is 1. 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	description
h	The argument is interpreted as a short int or unsigned short int (only applies to integer specifiers: i, d, o, u, x and X).
l	The argument is interpreted as a long int or unsigned long int for integer specifiers (i, d, o, u, x and X), and as a wide character or wide character string for specifiers c and s.
L	The argument is interpreted as a long double (only applies to floating point specifiers: e, E, f, g and G).

additional arguments

Depending on the format string, the function may expect a sequence of additional arguments, each containing one value to be inserted instead of each %-tag specified in the format parameter, if any. There should be the same number of these arguments as the number of %-tags that expect a value.

Return Value

On success, the total number of characters written is returned.
On failure, a negative number is returned.

Example

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

/* 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 radixes: %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; }

And here is the output:

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