The printf(1) utility takes its first argument as a
format, and uses that format for printing all remaining
arguments. The format is a character string that contains
three types of objects: plain characters, which are simply copied
to standard output; character escape sequences, which are converted
and copied to the standard output; and format specifications, each
of which causes printing of the next successive
argument.
The arguments after the first are treated as strings if
the corresponding format is either c or s; otherwise
it is evaluated as a C constant, with the following extensions:
A leading plus or minus sign is allowed.
If the leading character is a single or double quote, or not a
digit, plus, or minus sign, the value is the ASCII code of the next
character.
The format string is reused as often as necessary to satisfy the
arguments. Any extra format specifications are evaluated
with zero or the null string.
Character escape sequences are in backslash (\) notation
as defined in the ISO or ANSI C Standard. The characters and their
meanings are as follows:
\a
Write a <bell> character.
\b
Write a <back-space> character.
\f
Write a <form-feed> character.
\n
Write a <newline> character.
\r
Write a <carriage-return> character.
\t
Write a <tab> character.
\v
Write a <vertical-tab> character.
\'
Write a <single-quote> character.
\\
Write a backslash character.
\ddd
Write an eight-bit character whose ASCII value is the one-,
two-, or three-digit octal number ddd.
Each format specification is introduced by the percent character
(%).
The remainder of the format specification includes, in the
following order, zero or more of the following flags:
#
Specifies that the value should be printed in an "alternate
form." For c, d, and s, formats, this option
has no effect. For the o formats the precision of the number
is increased to force the first character of the output string to a
zero. For the x (X) format, a non-zero result has the string
0x (0X) prepended to it. For e, E, f,
g, and G, formats, the result will always contain a
decimal point, even if no digits follow the point (normally, a
decimal point only appears in the results of those formats if a
digit follows the decimal point). For g and G
formats, trailing zeros are not removed from the result as they
would otherwise be.
-
Specifies left adjustment of the output in the indicated
field.
+
Specifies that there should always be a sign placed before the
number when using signed formats.
' '
A space; specifies that a blank should be left before a
positive number for a signed format. A '+' overrides a space if
both are used.
0
A zero; indicates that zero-padding should be used rather than
blank-padding. A '-' overrides a '0' if both are used.
Field width
An optional digit string specifying a field width. If
the output string has fewer characters than the field width, it
will be blank-padded on the left (or right, if the left-adjustment
indicator has been given) to make up the field width (note that a
leading zero is a flag, but an embedded zero is part of a field
width).
Precision
An optional period, '.', followed by an optional digit string
giving a precision that specifies the number of digits to
appear after the decimal point, for e and f formats,
or the maximum number of characters to be printed from a string. If
the digit string is missing, the precision is treated as zero.
Format
A character that indicates the type of format to use (one of
diouxXfwEgGcs).
A field width or precision can be '*' instead of a digit string.
In this case, an argument supplies the field width or
precision.
The format characters and their meanings are:
diouXx
The argument is printed as a signed decimal (d or
i), unsigned decimal, unsigned octal, or unsigned
hexadecimal (X or x), respectively. The
argument is printed in the style '[-]ddd.ddd' where the
number of occurrences of d after the decimal point is equal
to the precision specification for the argument. If the precision
is missing, six digits are given; if the precision is explicitly 0,
no digits and no decimal point are printed.
eE
The argument is printed in the style e where
there is one digit before the decimal point, and the number after
is equal to the precision specification for the argument; when the
precision is missing, six digits are produced. An uppercase E is
used for an E format.
gG
The argument is printed in style f or in style
e (E), whichever gives full precision in minimum
space.
c
The first character of argument is printed.
s
Characters from the string argument are printed until
the end is reached, or until the number of characters indicated by
the precision specification is reached. If the precision is 0 or
missing, however, all characters in the string are printed.
%
Print a '%'; no argument is used.
In no case does a nonexistent or small field width cause
truncation of a field. Padding takes place only if the specified
field width exceeds the actual width.