illumos: manual page: formats.7

FORMATS(7) Standards, Environments, and Macros FORMATS(7)

NAME

formats - file format notation

DESCRIPTION

Utility descriptions use a syntax to describe the data organization
within files--stdin, stdout, stderr, input files, and output files--when
that organization is not otherwise obvious. The syntax is similar to that
used by the printf(3C) function. When used for stdin or input file
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
scanf(3C) function to read the input file.

Format

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:

characters
Characters that are not escape sequences or
conversion specifications, as described
below, are copied to the output.

escape sequences
Represent non-graphic characters.

conversion specifications
Specifies the output format of each
argument. (See below.)

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

`` ''
(An empty character position.) One or more blank characters.

/\
Exactly one space character.

The notation for spaces allows some flexibility for application output.
Note that an empty character position in format represents one or more
blank characters on the output (not white space, which can include
newline characters). Therefore, another utility that reads that output as
its input must be prepared to parse the data using scanf(3C), awk(1), and
so forth. The character is used when exactly one space character is
output.

Escape Sequences

The following table lists escape sequences and associated actions on
display devices capable of the action.

Sequence Character Terminal Action
--------------------------------------------------
\\ backslash None.
\a alert Attempts to alert
the user through
audible or visible
notification.
\b backspace Moves the printing
position to one
column before the
current position,
unless the current
position is the
start of a line.
\f form-feed Moves the printing
position to the
initial printing
position of the next
logical page.
\n newline Moves the printing
position to the
start of the next
line.
\r carriage-return Moves the printing
position to the
start of the current
line.
\t tab Moves the printing
position to the next
tab position on the
current line. If
there are no more
tab positions left
on the line, the
behavior is
undefined.
\v vertical-tab Moves the printing
position to the
start of the next
vertical tab
position. If there
are no more vertical
tab positions left
on the page, the
behavior is
undefined.

Conversion Specifications

Each conversion specification is introduced by the percent-sign character
(%). After the character %, the following 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 is 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 conversions (the
field is padded with leading zeros), the number
of digits to appear after the radix character
for the e and f conversions, the maximum number
of significant digits for the g conversion; or
the maximum number of bytes to be written from
a string in s conversion. The precision takes
the form of a period (.) followed by a decimal
digit string; a null digit string is treated as
zero.

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

flags
The flags and their meanings are:

-
The result of the conversion is left-justified within the
field.

+
The result of a signed conversion always begins with a sign
(+ or -).

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

#
The value is to be converted to an alternative form. For c,
d, i, u, and s conversions, the behaviour is undefined. For o
conversion, it increases the precision to force the first
digit of the result to be a zero. For x or X conversion, a
non-zero result has 0x or 0X prefixed to it, respectively.
For e, E, f, g, and G conversions, the result always contains
a radix character, even if no digits follow the radix
character. For g and G conversions, trailing zeros are not
removed from the result as they usually 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 is ignored. For d, i,
o, u, x and X conversions, if a precision is specified, the 0
flag is ignored. For other conversions, the behaviour is
undefined.

Conversion Characters

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,o,u,x,X
The integer argument is 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
convert to signed decimal in the style [-]dddd. The x
conversion uses the numbers and letters 0123456789abcdef
and the X conversion uses the numbers and letters
0123456789ABCDEF. The precision component of the argument
specifies the minimum number of digits to appear. If the
value being converted can be represented in fewer digits
than the specified minimum, it is expanded with leading
zeros. The default precision is 1. The result of
converting a zero value with a precision of 0 is 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.

The treatment of integers and spaces is different from
the printf(3C) function in that they can be surrounded
with blank characters. This was done so that, given a
format such as:

"%d\n",<foo>

the implementation could use a printf() call such as:

printf("%6d\n", foo);

and still conform. This notation is thus somewhat like
scanf() in addition to printf().

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

e,E
The floating point number argument is written in the
style [-]d.ddde+-dd (the symbol +- indicates either a
plus or minus sign), 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 determines the radix character
to use in this format. When the precision is missing, six
digits are written after the radix character; if the
precision is 0, no radix character appears. The E
conversion character produces a number with E instead of
e introducing the exponent. The exponent always contains
at least two digits. However, if the value to be written
requires an exponent greater than two digits, additional
exponent digits are written as necessary.

g,G
The floating point number argument is written 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. The style used depends on the value
converted: style g is 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 appears only if it is
followed by a digit.

c
The integer argument is converted to an unsigned char and
the resulting byte is written.

s
The argument is taken to be a string and bytes from the
string are 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 is taken to be infinite, so all
bytes up to the end of the string are written.

%
Write a % character; no argument is converted.

In no case does a non-existent 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.

One difference from the C function printf() is that the l and h
conversion characters are not used. There is no differentiation between
decimal values for type int, type long, or type short. The
specifications %d or %i should be interpreted as an arbitrary length
sequence of digits. Also, no distinction is made between single precision
and double precision numbers (float or double in C). These are simply
referred to as floating point numbers.

Many of the output descriptions use the term line, such as:

"%s", <input line>

Since the definition of line includes the trailing newline character
already, there is no need to include a \n in the format; a double newline
character would otherwise result.

EXAMPLES

Example 1: 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>

Example 2: To show pi written to 5 decimal places:

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

Example 3: To show an input file format consisting of five colon-separated

fields:

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