Format Specification Fields: printf and wprintf Functions
A format specification, which consists of optional and required fields, has the following form:
%[flags] [width] [.precision] [{h | l | I64 | L}]type
Each field of the format specification is a single character or a number signifying a particular format option. The simplest format specification contains only the percent sign and a type character (for example, %s). If a percent sign is followed by a character that has no meaning as a format field, the character is copied to stdout. For example, to print a percent-sign character, use %%.
The optional fields, which appear before the type character, control other aspects of the formatting, as follows:
type
Required character that determines whether the associated argument is interpreted as a character, a string, or a number (see Table R.3).
flags
Optional character or characters that control justification of output and printing of signs, blanks, decimal points, and octal and hexadecimal prefixes (see Table R.4). More than one flag can appear in a format specification.
width
Optional number that specifies the minimum number of characters output. (See printf Width Specification.)
precision
Optional number that specifies the maximum number of characters printed for all or part of the output field, or the minimum number of digits printed for integer values (see Table R.5).
h | l | I64 | L
Optional prefixes to type-that specify the size of argument (see Table R.6).
Flag Directives
The first optional field of the format specification is flags. A flag directive is a character that justifies output and prints signs, blanks, decimal points, and octal and hexadecimal prefixes. More than one flag directive may appear in a format specification.
Table R.4 Flag Characters
| Flag | Meaning | Default |
| – | Left align the result within the given field width. | Right align. |
| + | Prefix the output value with a sign (+ or –) if the output value is of a signed type. | Sign appears only for negative signed values (–). |
| 0 | If width is prefixed with 0, zeros are added until the minimum width is reached. If 0 and – appear, the 0 is ignored. If 0 is specified with an integer format (i, u, x, X, o, d) the 0 is ignored. | No padding. |
| blank (' ') | Prefix the output value with a blank if the output value is signed and positive; the blank is ignored if both the blank and + flags appear. | No blank appears. |
| # | When used with the o, x, or X format, the # flag prefixes any nonzero output value with 0, 0x, or 0X, respectively. | No blank appears. |
| When used with the e, E, or f format, the # flag forces the output value to contain a decimal point in all cases. | Decimal point appears only if digits follow it. | |
| When used with the g or G format, the # flag forces the output value to contain a decimal point in all cases and prevents the truncation of trailing zeros. Ignored when used with c, d, i, u, or s. | Decimal point appears only if digits follow it. Trailing zeros are truncated. |
printf Width Specification
The second optional field of the format specification is the width specification. The width argument is a nonnegative decimal integer controlling the minimum number of characters printed. If the number of characters in the output value is less than the specified width, blanks are added to the left or the right of the values — depending on whether the – flag (for left alignment) is specified — until the minimum width is reached. If width is prefixed with 0, zeros are added until the minimum width is reached (not useful for left-aligned numbers).
The width specification never causes a value to be truncated. If the number of characters in the output value is greater than the specified width, or if width is not given, all characters of the value are printed (subject to the precision specification).
If the width specification is an asterisk (*), an int argument from the argument list supplies the value. The width argument must precede the value being formatted in the argument list. A nonexistent or small field width does not cause the truncation of a field; if the result of a conversion is wider than the field width, the field expands to contain the conversion result.
Precision Specification
The third optional field of the format specification is the precision specification. It specifies a nonnegative decimal integer, preceded by a period (.), which specifies the number of characters to be printed, the number of decimal places, or the number of significant digits (see Table R.5). Unlike the width specification, the precision specification can cause either truncation of the output value or rounding of a floating-point value. If precision is specified as 0 and the value to be converted is 0, the result is no characters output, as shown below:
printf( "%.0d", 0 ); /* No characters output */If the precision specification is an asterisk (*), an int argument from the argument list supplies the value. The precision argument must precede the value being formatted in the argument list.
The type determines the interpretation of precision and the default when precision is omitted, as shown in Table R.5.
Table R.5 How Precision Values Affect Type
| Type | Meaning | Default |
| c, C | The precision has no effect. | Character is printed. |
| d, i, u, o, x, X | The precision specifies the minimum number of digits to be printed. If the number of digits in the argument is less than precision, the output value is padded on the left with zeros. The value is not truncated when the number of digits exceeds precision. | Default precision is 1. |
| e, E | The precision specifies the number of digits to be printed after the decimal point. The last printed digit is rounded. | Default precision is 6; if precision is 0 or the period (.) appears without a number following it, no decimal point is printed. |
| f | The precision value specifies the number of digits after the decimal point. If a decimal point appears, at least one digit appears before it. The value is rounded to the appropriate number of digits. | Default precision is 6; if precision is 0, or if the period (.) appears without a number following it, no decimal point is printed. |
| g, G | The precision specifies the maximum number of significant digits printed. | Six significant digits are printed, with any trailing zeros truncated. |
| s, S | The precision specifies the maximum number of characters to be printed. Characters in excess of precision are not printed. | Characters are printed until a null character is encountered. |
If the argument corresponding to a floating-point specifier is infinite, indefinite, or NaN, printf gives the following output.
| Value | Output |
| + infinity | 1.#INFrandom-digits |
| – infinity | –1.#INFrandom-digits |
| Indefinite (same as quiet NaN) | digit.#INDrandom-digits |
| NAN | digit.#NANrandom-digits |
printf Type Field Characters
The type character is the only required format field ; it appears after any optional format fields. The type character determines whether the associated argument is interpreted as a character, string, or number. The types C and S, and the behavior of c and s with printf functions, are Microsoft extensions and are not ANSI-compatible.
Table R.3 printf Type Field Characters
| Character | Type | Output Format |
| c | int or wint_t | When used with printf functions, specifies a single-byte character; when used with wprintf functions, specifies a wide character. |
| C | int or wint_t | When used with printf functions, specifies a wide character; when used with wprintf functions, specifies a single-byte character. |
| d | int | Signed decimal integer. |
| i | int | Signed decimal integer. |
| o | int | Unsigned octal integer. |
| u | int | Unsigned decimal integer. |
| x | int | Unsigned hexadecimal integer, using “abcdef.” |
| X | int | Unsigned hexadecimal integer, using “ABCDEF.” |
| e | double | Signed value having the form [ – ]d.dddd e [sign]ddd where d is a single decimal digit, dddd is one or more decimal digits, ddd is exactly three decimal digits, and sign is + or –. |
| E | double | Identical to the e format except that E rather than e introduces the exponent. |
| f | double | Signed value having the form [ – ]dddd.dddd, where dddd is one or more decimal digits. The number of digits before the decimal point depends on the magnitude of the number, and the number of digits after the decimal point depends on the requested precision. |
| g | double | Signed value printed in f or e format, whichever is more compact for the given value and precision. The e format is used only when the exponent of the value is less than –4 or greater than or equal to the precision argument. Trailing zeros are truncated, and the decimal point appears only if one or more digits follow it. |
| G | double | Identical to the g format, except that E, rather than e, introduces the exponent (where appropriate). |
| n | Pointer to integer | Number of characters successfully written so far to the stream or buffer; this value is stored in the integer whose address is given as the argument. |
| p | Pointer to void | Prints the address pointed to by the argument in the form xxxx:yyyy where xxxx is the segment and yyyy is the offset, and the digits x and y are uppercase hexadecimal digits. |
| s | String | When used with printf functions, specifies a single-byte–character string; when used with wprintf functions, specifies a wide-character string. Characters are printed up to the first null character or until the precision value is reached. |
| S | String | When used with printf functions, specifies a wide-character string; when used with wprintf functions, specifies a single-byte–character string. Characters are printed up to the first null character or until the precision value is reached. |
Size and Distance Specification
The optional prefixes to type, h, l, and L, specify the “size” of argument (long or short, single-byte character or wide character, depending upon the type specifier that they modify). These type-specifier prefixes are used with type characters in printf functions or wprintf functions to specify interpretation of arguments, as shown in the following table. These prefixes are Microsoft extensions and are not ANSI-compatible.
Table R.6 Size Prefixes for printf and wprintf Format-Type Specifiers
| To Specify | Use Prefix | With Type Specifier |
| long int | l | d, i, o, x, or X |
| long unsigned int | l | u |
| short int | h | d, i, o, x, or X |
| short unsigned int | h | u |
| __int64 | I64 | d, i, o, u, x, or X |
| Single-byte character with printf functions | h | c or C |
| Single-byte character with wprintf functions | h | c or C |
| Wide character with printf functions | l | c or C |
| Wide character with wprintf functions | l | c or C |
| Single-byte – character string with printf functions | h | s or S |
| Single-byte – character string with wprintf functions | h | s or S |
| Wide-character string with printf functions | l | s or S |
| Wide-character string with wprintf functions | l | s or S |
Thus to print single-byte or wide-characters with printf functions and wprintf functions, use format specifiers as follows.
| To Print Character As | Use Function | With Format Specifier |
| single byte | printf | c, hc, or hC |
| single byte | wprintf | C, hc, or hC |
| wide | wprintf | c, lc, or lC |
| wide | printf | C, lc, or lC |
To print strings with printf functions and wprintf functions, use the prefixes h and l analogously with format type-specifiers s and S.
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