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FORMATS(7) Standards, Environments, and Macros FORMATS(7)
NNAAMMEE
formats - file format notation
DDEESSCCRRIIPPTTIIOONN
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 pprriinnttff(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 ssccaannff(3C) function to read the input file.
FFoorrmmaatt
The description of an individual record is as follows:
"<ffoorrmmaatt>", [<_a_r_g_1>, <_a_r_g_2>, ..., <_a_r_g_n>]
The ffoorrmmaatt is a character string that contains three types of objects
defined below:
_c_h_a_r_a_c_t_e_r_s
Characters that are not _e_s_c_a_p_e _s_e_q_u_e_n_c_e_s
or _c_o_n_v_e_r_s_i_o_n _s_p_e_c_i_f_i_c_a_t_i_o_n_s, as
described below, are copied to the
output.
_e_s_c_a_p_e _s_e_q_u_e_n_c_e_s
Represent non-graphic characters.
_c_o_n_v_e_r_s_i_o_n _s_p_e_c_i_f_i_c_a_t_i_o_n_s
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 ffoorrmmaatt represents one or more
blank characters on the output (not _w_h_i_t_e _s_p_a_c_e, which can include
newline characters). Therefore, another utility that reads that output
as its input must be prepared to parse the data using ssccaannff(3C),
aawwkk(1), and so forth. The character is used when exactly one space
character is output.
EEssccaappee SSeeqquueenncceess
The following table lists escape sequences and associated actions on
display devices capable of the action.
SSeeqquueennccee CChhaarraacctteerr TTeerrmmiinnaall AAccttiioonn
────────────────────────────────────────────────
\\\\ backslash None.
\\aa alert Attempts to alert
the user through
audible or visible
notification.
\\bb backspace Moves the printing
position to one
column before the
current position,
unless the current
position is the
start of a line.
\\ff form-feed Moves the printing
position to the
initial printing
position of the
next logical page.
\\nn newline Moves the printing
position to the
start of the next
line.
\\rr carriage-return Moves the printing
position to the
start of the
current line.
\\tt 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.
\\vv 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.
CCoonnvveerrssiioonn SSppeecciiffiiccaattiioonnss
Each conversion specification is introduced by the percent-sign
character (%). After the character %, the following appear in
sequence:
_f_l_a_g_s
Zero or more _f_l_a_g_s, in any order, that modify
the meaning of the conversion specification.
_f_i_e_l_d _w_i_d_t_h
An optional string of decimal digits to
specify a minimum _f_i_e_l_d _w_i_d_t_h. 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).
_p_r_e_c_i_s_i_o_n
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.
_c_o_n_v_e_r_s_i_o_n _c_h_a_r_a_c_t_e_r_s
A conversion character (see below) that
indicates the type of conversion to be
applied.
_f_l_a_g_s
The _f_l_a_g_s 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 −).
_<_s_p_a_c_e_>
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.
CCoonnvveerrssiioonn CChhaarraacctteerrss
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 _c_o_n_v_e_r_s_i_o_n _c_h_a_r_a_c_t_e_r_s 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
[[−]]_d_d_d_d. The x conversion uses the numbers and letters
0123456789abcdef and the X conversion uses the numbers
and letters 0123456789ABCDEF. The _p_r_e_c_i_s_i_o_n 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 pprriinnttff(3C) function in that they can be surrounded
with blank characters. This was done so that, given a
format such as:
"%d\n",<_f_o_o>
the implementation could use a pprriinnttff(()) call such as:
printf("%6d\n", _f_o_o);
and still conform. This notation is thus somewhat like
ssccaannff(()) in addition to pprriinnttff(())..
_f
The floating point number argument is written in
decimal notation in the style [[−]]_d_d_d._d_d_d, where the
number of digits after the radix character (shown here
as a decimal point) is equal to the _p_r_e_c_i_s_i_o_n
specification. The LLCC__NNUUMMEERRIICC locale category
determines the radix character to use in this format.
If the _p_r_e_c_i_s_i_o_n is omitted from the argument, six
digits are written after the radix character; if the
_p_r_e_c_i_s_i_o_n is explicitly 0, no radix character appears.
_e_,_E
The floating point number argument is written in the
style [[−]]_d._d_d_de±dddd (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 LLCC__NNUUMMEERRIICC 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 uunnssiiggnneedd cchhaarr
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 _p_r_e_c_i_s_i_o_n
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 _f_i_e_l_d _w_i_d_t_h 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 _f_i_e_l_d _w_i_d_t_h should not be confused with the term
_p_r_e_c_i_s_i_o_n used in the description of %s.
One difference from the C function pprriinnttff(()) is that the l and h
conversion characters are not used. There is no differentiation between
decimal values for type iinntt, type lloonngg, or type sshhoorrtt. 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 (ffllooaatt or ddoouubbllee in C). These
are simply referred to as floating point numbers.
Many of the output descriptions use the term lliinnee, such as:
"%s", <_i_n_p_u_t _l_i_n_e>
Since the definition of lliinnee includes the trailing newline character
already, there is no need to include a \\nn in the format; a double
newline character would otherwise result.
EEXXAAMMPPLLEESS
EExxaammppllee 11 To represent the output of a program that prints a date and
time in the form Sunday, July 3, 10:02, where _<_w_e_e_k_d_a_y_> and _<_m_o_n_t_h_> are
strings:
"%s,/\%s/\%d,/\%d:%.2d\n",<_w_e_e_k_d_a_y>,<_m_o_n_t_h>,<_d_a_y>,<_h_o_u_r>,<_m_i_n>
EExxaammppllee 22 To show pi written to 5 decimal places:
"pi/\=/\%.5f\n",<_v_a_l_u_e _o_f _p_i>
EExxaammppllee 33 To show an input file format consisting of five colon-
separated fields:
"%s:%s:%s:%s:%s\n",<_a_r_g_1>,<_a_r_g_2>,<_a_r_g_3>,<_a_r_g_4>,<_a_r_g_5>
SSEEEE AALLSSOO
aawwkk(1), pprriinnttff(1), pprriinnttff(3C), ssccaannff(3C)
March 28, 1995 FORMATS(7)
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