Each conversion specification is introduced by the % character or by the character sequence %n$, after which the following appear in sequence:

• An optional field, consisting of a decimal digit string followed by a $, specifying the next argument to be converted. If this field is not provided, the args following the last argument converted will be used.
• Zero or more flags (in any order), which modify the meaning of the conversion specification.
• An optional minimum field width. If the converted value has fewer bytes than the field width, it will be padded with spaces by default on the left; it will be padded on the right, if the left-adjustment flag (-), described below, is given to the field width. The field width takes the form of an asterisk (*), described below, or a decimal integer.

  If the conversion character is s, a standard-conforming application (see standards(5)) interprets the field width as the minimum number of bytes to be printed; an application that is not standard-conforming interprets the field width as the minimum number of columns of screen display. For an application that is not standard-conforming, %10s means if the converted value has a screen width of 7 columns, 3 spaces would be padded on the right.

  If the format is %ws, then the field width should be interpreted as the minimum number of columns of screen display.

• An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversions (the field is padded with leading zeros); the number of digits to appear after the radix character for the e, E, and f conversions, the maximum number of significant digits for the g and G conversions; or the maximum number of bytes to be printed from a string in s and S conversions. The precision takes the form of a period (.) followed either by an asterisk (*), described below, or an optional decimal digit string, where a null digit string is treated as 0. If a precision appears with any other conversion character, the behavior is undefined.

  If the conversion character is s or S, a standard-conforming application (see standards(5)) interprets the precision as the maximum number of bytes to be written; an application that is not standard-conforming interprets the precision as the maximum number of columns of screen display. For an application that is not standard-conforming, %.5s would print only the portion of the string that would display in 5 screen columns. Only complete characters are written.

  For %ws, the precision should be interpreted as the maximum number of columns of screen display. The precision takes the form of a period (.) followed by a decimal digit string; a null digit string is treated as zero. Padding specified by the precision overrides the padding specified by the field width.

• An optional h specifies that a following d, i, o, u, x, or X conversion character applies to a type short int or type unsigned short int argument (the argument will be promoted according to the integral promotions, and its value converted to type short int or unsigned short int before printing); an optional h specifying that a following n conversion character applies to a pointer to a type short int argument; an optional l (ell) specifying that a following d, i, o, u, x, or X conversion character applies to a type long int or unsigned long int argument; an optional l (ell) specifying that a following n conversion character applies to a pointer to a type long int argument; an optional ll (ell ell) specifying that a following d, i, o, u, x, or X conversion character applies to a type long long or unsigned long long argument; an optional ll (ell ell) specifying that a following n conversion character applies to a pointer to a long long argument; or an optional L specifying that a following e, E, f, g, or G conversion character applies to a type long double argument. If an h, l, ll, or L appears with any other conversion character, the behavior is undefined.
• An optional l (ell) specifying that a following c conversion character applies to a wint_t argument; an optional l (ell) specifying that a following s conversion character applies to a pointer to a wchar_t argument.
• A conversion character (see below) that indicates the type of conversion to be applied.

A field width, or precision, or both may be indicated by an asterisk (*) . In this case, an argument of type int supplies the field width or precision. Arguments specifying field width, or precision, or both must appear in that order before the argument, if any, to be converted. A negative field width is taken as a - flag followed by a positive field width. A negative precision is taken as if the precision were omitted. In format strings containing the %n$ form of a conversion specification, a field width or precision may be indicated by the sequence *m$, where m is a decimal integer in the range [1, NL_ARGMAX] giving the position in the argument list (after the format argument) of an integer argument containing the field width or precision, for example:

printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

The format can contain either numbered argument specifications (that is, %n$ and *m$), or unnumbered argument specifications (that is, % and *), but normally not both. The only exception to this is that %% can be mixed with the %n$ form. The results of mixing numbered and unnumbered argument specifications in a format string are undefined. When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, are specified in the format string.

The flag characters and their meanings are:

'

The integer portion of the result of a decimal conversion (%i, %d, %u, %f, %g, or %G) will be formatted with thousands' grouping characters. For other conversions the behavior is undefined. The non-monetary grouping character is used.

-

The result of the conversion will be left-justified within the field. The conversion will be right-justified if this flag is not specified.

+

The result of a signed conversion will always begin with a sign (+ or -). The conversion will begin with a sign only when a negative value is converted if this flag is not specified.

space

If the first character of a signed conversion is not a sign or if a signed conversion results in no characters, a space will be placed before the result. This means that if the space and + flags both appear, the space flag will be ignored.

#

The value is to be converted to an alternate form. For c, d, i, s, and u conversions, the flag has no effect. For an o conversion, it increases the precision (if necessary) to force the first digit of the result to be a zero. For x or X conversion, a non-zero result will have 0x (or 0X) prepended to it. For e, E, f, g, and G conversions, the result will always contain a radix character, even if no digits follow the radix character. Without this flag, the radix character appears in the result of these conversions only if a digit follows it. For g and G conversions, trailing zeros will not be removed from the result as they normally are.

0

For d, i, o, u, x, X, e, E, f, g, and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the 0 and - flags both appear, the 0 flag will be ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag will be ignored. If the 0 and ' flags both appear, the grouping characters are inserted before zero padding. For other conversions, the behavior is undefined.

Each conversion character results in fetching zero or more arguments. The results are undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments are ignored.

The conversion characters and their meanings are:

d,i

The int argument is converted to a signed decimal in the style [-]dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

o

The unsigned int argument is converted to unsigned octal format in the style dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

u

The unsigned int argument is converted to unsigned decimal format in the style dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

x

The unsigned int argument is converted to unsigned hexadecimal format in the style dddd; the letters abcdef are used. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

X

Behaves the same as the x conversion character except that letters ABCDEF are used instead of abcdef.

f

The double argument is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the radix character (see setlocale(3C)) is equal to the precision specification. If the precision is missing it is taken as 6; if the precision is explicitly 0 and the # flag is not specified, no radix character appears. If a radix character appears, at least 1 digit appears before it. The value is rounded to the appropriate number of digits.

e,E

The double argument is converted to the style [-]d.ddde+-dd, where there is one digit before the radix character (which is non-zero if the argument is non-zero) and the number of digits after it is equal to the precision. When the precision is missing it is taken as 6; if the precision is 0 and the # flag is not specified, no radix character appears. The E conversion character will produce a number with E instead of e introducing the exponent. The exponent always contains at least two digits. The value is rounded to the appropriate number of digits.

g,G

The double argument is printed in style f or e (or in style E in the case of a G conversion character), with the precision specifying the number of significant digits. If an explicit precision is 0, it is taken as 1. The style used depends on the value converted: style e (or E) will be used only if the exponent resulting from the 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 radix character appears only if it is followed by a digit.

c

The int argument is converted to an unsigned char, and the resulting byte is printed.

If an l (ell) qualifier is present, the wint_t argument is converted as if by an ls conversion specification with no precision and an argument that points to a two-element array of type wchar_t, the first element of which contains the wint_t argument to the ls conversion specification and the second element contains a null wide-character.

C

Same as lc.

wc

The int argument is converted to a wide character (wchar_t), and the resulting wide character is printed.

s

The argument must be a pointer to an array of char. Bytes from the array are written up to (but not including) any terminating null byte. If a precision is specified, a standard-conforming application (see standards(5)) will write only the number of bytes specified by precision; an application that is not standard-conforming will write only the portion of the string that will display in the number of columns of screen display specified by precision. If the precision is not specified, it is taken to be infinite, so all bytes up to the first null byte are printed. An argument with a null value will yield undefined results.

If an l (ell) qualifier is present, the argument must be a pointer to an array of type wchar_t. Wide-characters from the array are converted to characters (each as if by a call to the wcrtomb(3C) function, with the conversion state described by an mbstate_t object initialized to zero before the first wide-character is converted) up to and including a terminating null wide-character. The resulting characters are written up to (but not including) the terminating null character (byte). If no precision is specified, the array must contain a null wide-character. If a precision is specified, no more than that many characters (bytes) are written (including shift sequences, if any), and the array must contain a null wide-character if, to equal the character sequence length given by the precision, the function would need to access a wide-character one past the end of the array. In no case is a partial character written.

S

Same as ls.

ws

The argument must be a pointer to an array of wchar_t. Bytes from the array are written up to (but not including) any terminating null character. If the precision is specified, only that portion of the wide-character array that will display in the number of columns of screen display specified by precision will be written. If the precision is not specified, it is taken to be infinite, so all wide characters up to the first null character are printed. An argument with a null value will yield undefined results.

p

The argument must be a pointer to void. The value of the pointer is converted to a set of sequences of printable characters, which should be the same as the set of sequences that are matched by the %p conversion of the scanf(3C) function.

n

The argument must be a pointer to an integer into which is written the number of bytes written to the output standard I/O stream so far by this call to one of the printf() functions. No argument is converted.

%

Print a %; no argument is converted. The entire conversion specification must be %%.

If a conversion specification does not match one of the above forms, the behavior is undefined.

If a floating-point value is the internal representation for infinity, the output is [+-]Infinity, where Infinity is either Infinity or Inf, depending on the desired output string length. Printing of the sign follows the rules described above.

If a floating-point value is the internal representation for "not-a-number," the output is [+-]NaN. Printing of the sign follows the rules described above.

In no case does a non-existent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is simply expanded to contain the conversion result. Characters generated by printf() and fprintf() are printed as if the putc(3C) function had been called.

The st_ctime and st_mtime fields of the file will be marked for update between the call to a successful execution of printf() or fprintf() and the next successful completion of a call to fflush(3C) or fclose(3C) on the same stream or a call to exit(3C) or abort(3C).

It is common to use the following escape sequences built into the C language when entering format strings for the printf() functions, but these sequences are processed by the C compiler, not by the printf() function.

\a

Alert. Ring the bell.

\b

Backspace. Move the printing position to one character before the current position, unless the current position is the start of a line.

\f

Form feed. Move the printing position to the initial printing position of the next logical page.

\n

Newline. Move the printing position to the start of the next line.

\r

Carriage return. Move the printing position to the start of the current line.

\t

Horizontal tab. Move the printing position to the next implementation-defined horizontal tab position on the current line.

\v

Vertical tab. Move the printing position to the start of the next implementation-defined vertical tab position.

In addition, the C language supports character sequences of the form

\octal-number

\hex-number

printf (format, weekday, month, day, hour, min);

For American usage, format could be a pointer to the string:

"%s, %s %d, %d:%.2d\n"

producing the message:

Sunday, July 3, 10:02

whereas for German usage, format could be a pointer to the string:

"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"

producing the message:

Sonntag, 3. Juli, 10:02

printf("%s, %s %i, %d:%.2d", weekday, month, day, hour, min);

printf("pi = %.5f", 4 * atan(1.0));