C Printf Format

| 分类 C-Family 

1 printf family format:

ordinary characters + conversion sepcifications

  • Ordinary Characters
    ordinary characters (not %), are copied unchanged to the output stream.

  • Conversion Specifications
    Each specification results in fetching zero or more subsequent arguments.

    Conversion specification is introduced by:

    % + flags + field width + precision + length modifier + conversion specifier

    • %
      Mark of the start of an convertion specifier

    • flags % 之后可以有 0 或多个 flags。
      • # The value should be converted to an "alternate form".
        • For o conversions
          the first character of the output string is made zero (by prefixing a 0 if it was not zero already).
        • For x and X conversions
          a nonzero result has the string "0x" (or "0X" for X conversions) prepended to

        it.

        • For a, A, e, E, f, F, g, and G conversions
          The result will always contain a decimal point, even if no digits follow it

        (normally, a decimal point appears in the results of those conversions only if a digit follows). For g and G conversions, trailing zeros are not removed from the result as they would otherwise be. For other conversions, the result is undefined.

      • 0
        The value should be zero padded. For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, the converted value is padded on the left with zeros rather than blanks. If the 0 and - flags both appear, the 0 flag is ignored. If a precision is given with a numeric conversion (d, i, o, u, x, and X), the 0 flag is ignored. For other conversions, the behavior is undefined.
      • -
        The converted value is to be left adjusted on the field boundary. (The default is right justification.) The converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A - overrides a 0 if both are given.
      • SPACE
        (a space) 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. By default a sign is used only for negative numbers. A + overrides a space if both are used.

  • field width
    An optional decimal digit string (with nonzero first digit) specifying a minimum field width. If the converted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if the left-adjustment flag has been given). Instead of a decimal digit string one may write "*" or "*m$" (for some decimal integer m) to specify that the field width is given in the next argument, or in the m-th argument, respectively, which must be of type int. A negative field width is taken as a '-' flag followed by a positive field width. In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.

  • precision
    An optional precision, in the form of a period ('.') followed by an optional decimal digit string. Instead of a decimal digit string one may write "*" or "*m$" (for some decimal integer m) to specify that the precision is given in the next argument, or in the m-th argument, respectively, which must be of type int. If the precision is given as just '.', the precision is taken to be zero. A negative precision is taken as if the precision were omitted. This gives the minimum number of digits to appear for d, i, o, u, x, and X conversions, 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, or the maximum number of characters to be printed from a string for s and S conversions.

  • length modifier
    Here, "integer conversion" stands for d, i, o, u, =x, or X conversion.
    • hh
      A following integer conversion corresponds to a signed char or unsigned char argument, or a following n conversion corresponds to a pointer to a signed char argument.
    • h
      A following integer conversion corresponds to a short int or unsigned short int argument, or a following n conversion corresponds to a pointer to a short int argument.
    • l
      A following integer conversion corresponds to a long int or unsigned long int argument, or a following n conversion corresponds to a pointer to a long int argument, or a following c conversion corresponds to a wint_t argument, or a following s conversion corresponds to a pointer to wchar_t argument.
    • ll
      A following integer conversion corresponds to a long long int or unsigned long long int argument, or a following n conversion corresponds to a pointer to a long long int argument.
    • L
      A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argument. (C99 allows %LF, but SUSv2 does not.) This is a synonym for ll.
    • j
      A following integer conversion corresponds to an intmax_t or uintmax_t argument, or a following n conversion corresponds to a pointer to an intmax_t argument.
    • z
      A following integer conversion corresponds to a size_t or ssize_t argument, or a following n conversion corresponds to a pointer to a size_t argument.
    • t
      A following integer conversion corresponds to a ptrdiff_t argument, or a following n conversion corresponds to a pointer to a ptrdiff_t argument.

  • conversion specifier
    A character that specifies the type of conversion to be applied. The conversion specifiers and their meanings are:
    • d, i
      The int argument is converted to signed decimal notation. The precision, if any, gives the minimum number of digits that must appear; if the converted value requires fewer digits, it is padded on the left with zeros. The default precision is 1. When 0 is printed with an explicit precision 0, the output is empty.
    • o, u, x, X
      The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (x and X) notation. The letters abcdef are used for x conversions; the letters ABCDEF are used for X conversions. The precision, if any, gives the minimum number of digits that must appear; if the converted value requires fewer digits, it is padded on the left with zeros. The default precision is 1. When 0 is printed with an explicit precision 0, the output is empty.
    • e, E
      The double argument is rounded and converted in the style [-]d.ddde+/-dd where there is one digit before the decimal-point character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero, no decimal-point character appears. An E conversion uses the letter E (rather than e) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00.
    • f, F
      The double argument is rounded and converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly zero, no decimal-point character appears. If a decimal point appears, at least one digit appears before it.

      (SUSv2 does not know about F and says that character string representations for infinity and NaN may be made available. SUSv3 adds a specification for F. The C99 standard specifies "[-]inf" or "[-]infinity" for infinity, and a string starting with "nan" for NaN, in the case of f conversion, and "[-]INF" or "[-]INFINITY" or "NAN*" in the case of F conversion.)

    • g, G
      The double argument is converted in style f or e (or F or E for G conversions). The precision specifies the number of significant digits. If the precision is missing, 6 digits are given; if the precision is zero, it is treated as 1. Style e is used if the exponent from its conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional part of the result; a decimal point appears only if it is followed by at least one digit.
    • a, A
      (C99; not in SUSv2, but added in SUSv3) For a conversion, the double argument is converted to hexadecimal notation (using the letters abcdef) in the style [-]0xh.hhhhp+/-; for A conversion the prefix 0X, the letters ABCDEF, and the exponent separator P is used. There is one hexadecimal digit before the decimal point, and the number of digits after it is equal to the precision. The default precision suffices for an exact representation of the value if an exact representation in base 2 exists and otherwise is sufficiently large to distinguish values of type double. The digit before the decimal point is unspecified for nonnormalized numbers, and nonzero but otherwise unspecified for normalized numbers.
    • c
      If no l modifier is present, the int argument is converted to an unsigned char, and the resulting character is written. If an l modifier is present, the wint_t (wide character) argument is converted to a multibyte sequence by a call to the wcrtomb(3) function, with a conversion state starting in the initial state, and the resulting multibyte string is written.
    • s
      If no l modifier is present: The const char * argument is expected to be a pointer to an array of character type (pointer to a string). Characters from the array are written up to (but not including) a terminating null byte ('\0'); if a precision is specified, no more than the number specified are written. If a precision is given, no null byte need be present; if the precision is not specified, or is greater than the size of the array, the array must contain a terminating null byte.

      If an l modifier is present: The const wchar_t * argument is expected to be a pointer to an array of wide characters. Wide characters from the array are converted to multibyte characters (each by a call to the wcrtomb(3) function, with a conversion state starting in the initial state before the first wide character), up to and including a terminating null wide character. The resulting multibyte characters are written up to (but not including) the terminating null byte. If a precision is specified, no more bytes than the number specified are written, but no partial multibyte characters are written. Note that the precision determines the number of bytes written, not the number of wide characters or screen positions. The array must contain a terminating null wide character, unless a precision is given and it is so small that the number of bytes written exceeds it before the end of the array is reached.

    • n
      The number of characters written so far is stored into the integer pointed to by the corresponding argument. That argument shall be an int *, or variant whose size matches the (optionally) supplied integer length modifier. No argument is converted. The behavior is undefined if the conversion specification includes any flags, a field width, or a precision.
    • %
      A '%' is written. No argument is converted. The complete conversion specification is '%%'.

2 C99 Enhancement

为了让代码可移植性更好,C99 提供了两个极为有用的头文件。

2.1 stdint.h

该文件提供了诸如 int32, uint32, uint16 等等类型的定义

2.2 inttypes.h

该文件提供了若干宏用于格式化输出,可以用于 printf 以及 scanf

可以参考这里: http://en.wikipedia.org/wiki/C_data_types#inttypes.h


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