For the signed %d and %i conversions, print a plus sign if the value is positive.
For the signed %d and %i conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead. Since the + flag ensures that the result includes a sign, this flag is ignored if you supply both of them.
#
For the %o conversion, this forces the leading digit to be 0, as if by increasing the precision. For %x or %X, this prefixes a leading 0x or 0X (respectively) to the result. This doesn't do anything useful for the %d, %i, or %u conversions. Using this flag produces output which can be parsed by the strtoul function (see Parsing of Integers) and scanf with the %i conversion (see Numeric Input Conversions).
'
Separate the digits into groups as specified by the locale specified for the LC_NUMERIC category; see General Numeric. This flag is a GNU extension.
0
Pad the field with zeros instead of spaces. The zeros are placed after any indication of sign or base. This flag is ignored if the - flag is also specified, or if a precision is specified.
If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.
Without a type modifier, the corresponding argument is treated as an int (for the signed conversions %i and %d) or unsigned int (for the unsigned conversions %o,
%u, %x, and %X). Recall that since printf and friends are variadic, any char and short arguments are automatically converted to int by the default argument promotions. For arguments of other integer types, you can use these modifiers:
hh
Specifies that the argument is a signed char or unsigned char, as appropriate. A char argument is converted to an int or unsigned int by the default argument promotions anyway, but the h modifier says to convert it back to a char again.
This modifier was introduced in ISO C99.
h
Specifies that the argument is a short int or unsigned short int, as appropriate. A short argument is converted to an int or unsigned int by the default argument promotions anyway, but the h modifier says to convert it back to a short again.
j
Specifies that the argument is a intmax_t or uintmax_t, as appropriate.
This modifier was introduced in ISO C99.
l
Specifies that the argument is a long int or unsigned long int, as appropriate. Two l characters is like the L modifier, below.
If used with %c or %s the corresponding parameter is considered as a wide character or wide character string respectively. This use of l was introduced in Amendment 1 to ISO C90.
L
ll
q
Specifies that the argument is a long long int. (This type is an extension supported by the GNU C compiler. On systems that don't support extra-long integers, this is the same as long int.)
The q modifier is another name for the same thing, which comes from 4.4 BSD; a long long int is sometimes called a "quad" int.
t
Specifies that the argument is a ptrdiff_t.
This modifier was introduced in ISO C99.
z
Z
Specifies that the argument is a size_t.
z was introduced in ISO C99. Z is a GNU extension predating this addition and should not be used in new code.
Here is an example. Using the template string:
"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"
to print numbers using the different options for the %d conversion gives results like:
| 0|0 | +0|+0 | 0|00000| | 00|0|
| 1|1 | +1|+1 | 1|00001| 1| 01|1|
| -1|-1 | -1|-1 | -1|-0001| -1| -01|-1|
|100000|100000|+100000|+100000| 100000|100000|100000|100000|100000|
In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.
Here are some more examples showing how unsigned integers print under various format options, using the template string:
"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"
| 0| 0| 0| 0| 0| 0| 0| 00000000|
| 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001|
|100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|
浮点数
Posted @ 2004-12-28 02:41
Floating-Point Conversions
This section discusses the conversion specifications for floating-point numbers: the %f, %e, %E, %g, and %G conversions.
The %f conversion prints its argument in fixed-point notation, producing output of the form [-]ddd.ddd, where the number of digits following the decimal point is controlled by the precision you specify.
The %e conversion prints its argument in exponential notation, producing output of the form [-]d.ddde[+|-]dd. Again, the number of digits following the decimal point is controlled by the precision. The exponent always contains at least two digits. The %E conversion is similar but the exponent is marked with the letter E instead of e.
The %g and %G conversions print the argument in the style of %e or %E (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the %f style. A precision of 0, is taken as 1. is Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.
The %a and %A conversions are meant for representing floating-point numbers exactly in textual form so that they can be exchanged as texts between different programs and/or machines. The numbers are represented is the form [-]0xh.hhhp[+|-]dd. At the left of the decimal-point character exactly one digit is print. This character is only 0 if the number is denormalized. Otherwise the value is unspecified; it is implementation dependent how many bits are used. The number of hexadecimal digits on the right side of the decimal-point character is equal to the precision. If the precision is zero it is determined to be large enough to provide an exact representation of the number (or it is large enough to distinguish two adjacent values if the FLT_RADIX is not a power of 2, see Floating Point Parameters). For the %a conversion lower-case characters are used to represent the hexadecimal number and the prefix and exponent sign are printed as 0x and p respectively. Otherwise upper-case characters are used and 0X and P are used for the representation of prefix and exponent string. The exponent to the base of two is printed as a decimal number using at least one digit but at most as many digits as necessary to represent the value exactly.
If the value to be printed represents infinity or a NaN, the output is [-]inf or nan respectively if the conversion specifier is %a, %e, %f, or %g and it is [-]INF or NAN respectively if the conversion is %A, %E, or %G.
The following flags can be used to modify the behavior:
-
Left-justify the result in the field. Normally the result is right-justified.
+
Always include a plus or minus sign in the result.
If the result doesn't start with a plus or minus sign, prefix it with a space instead. Since the + flag ensures that the result includes a sign, this flag is ignored if you supply both of them.
#
Specifies that the result should always include a decimal point, even if no digits follow it. For the %g and %G conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed.
'
Separate the digits of the integer part of the result into groups as specified by the locale specified for the LC_NUMERIC category; see General Numeric. This flag is a GNU extension.
0
Pad the field with zeros instead of spaces; the zeros are placed after any sign. This flag is ignored if the - flag is also specified.
The precision specifies how many digits follow the decimal-point character for the %f, %e, and %E conversions. For these conversions, the default precision is 6. If the precision is explicitly 0, this suppresses the decimal point character entirely. For the %g and %G conversions, the precision specifies how many significant digits to print. Significant digits are the first digit before the decimal point, and all the digits after it. If the precision is 0 or not specified for %g or %G, it is treated like a value of 1. If the value being printed cannot be expressed accurately in the specified number of digits, the value is rounded to the nearest number that fits.
Without a type modifier, the floating-point conversions use an argument of type double. (By the default argument promotions, any float arguments are automatically converted to double.) The following type modifier is supported:
L
An uppercase L specifies that the argument is a long double.
Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string:
"|%13.4a|%13.4f|%13.4e|%13.4g|\n"
Here is the output:
| 0x0.0000p+0| 0.0000| 0.0000e+00| 0|
| 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5|
| 0x1.0000p+0| 1.0000| 1.0000e+00| 1|
| -0x1.0000p+0| -1.0000| -1.0000e+00| -1|
| 0x1.9000p+6| 100.0000| 1.0000e+02| 100|
| 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000|
| 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04|
| 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04|
| 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05|
| 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05|
Notice how the %g conversion drops trailing zeros.
其他
Posted @ 2004-12-28 02:41
Other Output Conversions
This section describes miscellaneous conversions for printf.
The %c conversion prints a single character. In case there is no l modifier the int argument is first converted to an unsigned char. Then, if used in a wide stream function, the character is converted into the corresponding wide character. The - flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given. For example:
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
prints hello.
If there is a l modifier present the argument is expected to be of type wint_t. If used in a multibyte function the wide character is converted into a multibyte character before being added to the output. In this case more than one output byte can be produced.
The %s conversion prints a string. If no l modifier is present the corresponding argument must be of type char * (or const char *). If used in a wide stream function the string is first converted in a wide character string. A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream. The - flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion. For example:
printf ("%3s%-6s", "no", "where");
prints nowhere .
If there is a l modifier present the argument is expected to be of type wchar_t (or const wchar_t *).
If you accidentally pass a null pointer as the argument for a %s conversion, the GNU library prints it as (null). We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally.
The %m conversion prints the string corresponding to the error code in errno. See Error Messages. Thus:
fprintf (stderr, "can't open `%s': %m\n", filename);
is equivalent to:
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
The %m conversion is a GNU C library extension.
The %p conversion prints a pointer value. The corresponding argument must be of type void *. In practice, you can use any type of pointer.
In the GNU system, non-null pointers are printed as unsigned integers, as if a %#x conversion were used. Null pointers print as (nil). (Pointers might print differently in other systems.)
For example:
printf ("%p", "testing");
prints 0x followed by a hexadecimal number--the address of the string constant "testing". It does not print the word testing.
You can supply the - flag with the %p conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.
The %n conversion is unlike any of the other output conversions. It uses an argument which must be a pointer to an int, but instead of printing anything it stores the number of characters printed so far by this call at that location. The h and l type modifiers are permitted to specify that the argument is of type short int * or long int * instead of int *, but no flags, field width, or precision are permitted.
For example,
int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);
prints:
3 bears
and sets nchar to 7, because 3 bears is seven characters.
The %% conversion prints a literal % character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted
