5. File Format Notation

The STDIN, STDOUT, STDERR, INPUT FILES, and OUTPUT FILES sections of the utility descriptions use a syntax to describe the data organization within the files, when that organization is not otherwise obvious. The syntax is similar to that used by the System Interfaces volume of POSIX.1-2017 printf() function, as described in this chapter. When used in STDIN or INPUT FILES sections of the utility descriptions, this syntax describes the format that could have been used to write the text to be read, not a format that could be used by the System Interfaces volume of POSIX.1-2017 scanf() function to read the input file.

The description of an individual record is as follows:

"<format>", [<arg1>, <arg2>,..., <argn>]

The format is a character string that contains three types of objects defined below:

1. Characters that are not "escape sequences" or "conversion specifications", as described below, shall be copied to the output.

2. Escape Sequences represent non-graphic characters and the escape character ( <backslash>).

3. Conversion Specifications specify the output format of each argument; see below.

The following characters have the following special meaning in the format string:

''

(An empty character position.) Represents one or more <blank> characters.

Represents exactly one <space> character.

Escape Sequences and Associated Actions lists escape sequences and associated actions on display devices capable of the action.

Each conversion specification is introduced by the <percent-sign> character ( '%' ). After the character '%', the following shall appear in sequence:

flags

Zero or more flags, in any order, that modify the meaning of the conversion specification.

field width

An optional string of decimal digits to specify a minimum field width. For an output field, if the converted value has fewer bytes than the field width, it shall be padded on the left (or right, if the left-adjustment flag ( '-' ), described below, has been given) to the field width.

precision

Gives the minimum number of digits to appear for the d, o, i, u, x, or X conversion specifiers (the field is padded with leading zeros), the number of digits to appear after the radix character for the e and f conversion specifiers, the maximum number of significant digits for the g conversion specifier; or the maximum number of bytes to be written from a string in the s conversion specifier. The precision shall take the form of a <period> ( '.' ) followed by a decimal digit string; a null digit string is treated as zero.

conversion specifier characters

A conversion specifier character (see below) that indicates the type of conversion to be applied.

The flag characters and their meanings are:

-

The result of the conversion shall be left-justified within the field.

+

The result of a signed conversion shall always begin with a sign ( '+' or '-' ).

<space>

If the first character of a signed conversion is not a sign, a <space> shall be prefixed to the result. This means that if the <space> and '+' flags both appear, the <space> flag shall be ignored.

#

The value shall be converted to an alternative form. For c, d, i, u, and s conversion specifiers, the behavior is undefined. For the o conversion specifier, it shall increase the precision to force the first digit of the result to be a zero. For x or X conversion specifiers, a non-zero result has 0x or 0X prefixed to it, respectively. For a, A, e, E, f, F, g, and G conversion specifiers, the result shall always contain a radix character, even if no digits follow the radix character. For g and G conversion specifiers, trailing zeros shall not be removed from the result as they usually are.

0

For a, A, d, e, E, f, F, g, G, i, o, u, x, and X conversion specifiers, leading zeros (following any indication of sign or base) shall be used to pad to the field width rather than performing space padding, except when converting an infinity or NaN. If the '0' and '-' flags both appear, the '0' flag shall be ignored. For d, i , o, u, x, and X conversion specifiers, if a precision is specified, the '0' flag shall be ignored. For other conversion specifiers, the behavior is undefined.

Each conversion specifier character shall result 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 shall be ignored.

The conversion specifiers and their meanings are:

a,A

The floating-point number argument representing a floating-point number shall be converted in the style "[-]0xh.hhhhp±d", where there is one hexadecimal digit (which shall be non-zero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point character and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision shall be sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision shall be sufficient to distinguish different floating-point values in the internal representation used by the utility, except that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no decimal-point character shall appear. The letters "abcdef" shall be used for a conversion and the letters "ABCDEF" for A conversion. The A conversion specifier produces a number with X and P instead of x and p. The exponent shall always contain at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent shall be zero. A floating-point number argument representing an infinity or NaN shall be converted in the style of an f or F conversion specifier.

d,i,o,u,x,X

The integer argument shall be written as signed decimal ( d or i ), unsigned octal ( o ), unsigned decimal ( u ), or unsigned hexadecimal notation ( x and X ). The d and i specifiers shall convert to signed decimal in the style "[-]dddd". The x conversion specifier shall use the numbers and letters "0123456789abcdef" and the X conversion specifier shall use the numbers and letters "0123456789ABCDEF". The precision component of the argument shall specify the minimum number of digits to appear. If the value being converted can be represented in fewer digits than the specified minimum, it shall be expanded with leading zeros. The default precision shall be 1. The result of converting a zero value with a precision of 0 shall be no characters. If both the field width and precision are omitted, the implementation may precede, follow, or precede and follow numeric arguments of types d, i, and u with <blank> characters; arguments of type o (octal) may be preceded with leading zeros.

f,F

The floating-point number argument shall be written in decimal notation in the style [-]ddd.ddd, where the number of digits after the radix character (shown here as a decimal point) shall be equal to the precision specification. The LC_NUMERIC locale category shall determine the radix character to use in this format. If the precision is omitted from the argument, six digits shall be written after the radix character; if the precision is explicitly 0, no radix character shall appear.

A floating-point number argument representing an infinity shall be converted in one of the styles "[-]inf" or "[-]infinity"; which style is implementation-defined. A floating-point number argument representing a NaN shall be converted in one of the styles "[-]nan(n-char-sequence)" or "[-]nan"; which style, and the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces "INF", "INFINITY", or "NAN" instead of "inf", "infinity", or "nan", respectively.

e,E

The floating-point number argument shall be written in the style [-]d.ddde±dd (the symbol '±' indicates either a <plus-sign> or <hyphen-minus>), where there is one digit before the radix character (shown here as a decimal point) and the number of digits after it is equal to the precision. The LC_NUMERIC locale category shall determine the radix character to use in this format. When the precision is missing, six digits shall be written after the radix character; if the precision is 0, no radix character shall appear. The E conversion specifier shall produce a number with E instead of e introducing the exponent. The exponent shall always contain at least two digits. However, if the value to be written requires an exponent greater than two digits, additional exponent digits shall be written as necessary.

A floating-point number argument representing an infinity or NaN shall be converted in the style of an f or F conversion specifier.

g,G

The floating-point number argument shall be written in style f or e (or in style F or E in the case of a G conversion specifier), with the precision specifying the number of significant digits. The style used depends on the value converted: style e (or E ) shall 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 result. A radix character shall appear only if it is followed by a digit.

A floating-point number argument representing an infinity or NaN shall be converted in the style of an f or F conversion specifier.

c

The single-byte character argument shall be written.

s

The argument shall be taken to be a string and bytes from the string shall be written until the end of the string or the number of bytes indicated by the precision specification of the argument is reached. If the precision is omitted from the argument, it shall be taken to be infinite, so all bytes up to the end of the string shall be written.

%

Write a '%' character; no argument is converted.

In no case does a nonexistent or insufficient 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. The term "field width" should not be confused with the term "precision" used in the description of %s.

Examples

To represent the output of a program that prints a date and time in the form Sunday, July 3, 10:02, where weekday and month are strings:

"%s,%s%d,%d:%.2d\n" <weekday>, <month>, <day>, <hour>, <min>

To show '' written to 5 decimal places:

"pi=%.5f\n",<value of >

To show an input file format consisting of five <colon>-separated fields:

"%s:%s:%s:%s:%s\n", <arg1>, <arg2>, <arg3>, <arg4>, <arg5>