PRINTF(3S) (C Programming Language Utilities) PRINTF(3S)
NAME
printf, fprintf, sprintf - print formatted output
SYNOPSIS
#include <stdio.h>
int printf (format , arg ... )
char *format;
int fprintf (stream, format , arg ... )
FILE *stream;
char *format;
int sprintf (s, format [ , arg ] ... )
char *s, *format;
DESCRIPTION
printf places output on the standard output stream stdout.
Fprintf places output on the named output stream. Sprintf
places ``output,'' followed by the null character (\0), in
consecutive bytes starting at *s; it is the user's
responsibility to ensure that enough storage is available.
Each function returns the number of characters transmitted
(not including the \0 in the case of sprintf), or a negative
value if an output error was encountered.
Each of these functions converts, formats, and prints its
args under control of the format. The format is a character
string that contains two types of objects: plain
characters, which are simply copied to the output stream,
and conversion specifications, each of which results in
fetching of zero or more args. The results are undefined if
there are insufficient args for the format. If the format
is exhausted while args remain, the excess args are simply
ignored.
Each conversion specification is introduced by the character
%. After the %, the following appear in sequence:
Zero or more flags, which modify the meaning of the
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PRINTF(3S) (C Programming Language Utilities) PRINTF(3S)
conversion specification.
An optional decimal digit string specifying a minimum
field width. If the converted value has fewer
characters than the field width, it will be padded on
the left (or right, if the left-adjustment flag `-',
described below, has been given) to the field width.
The padding is with blanks unless the field width digit
string starts with a zero, in which case the padding is
with zeros.
A precision that gives the minimum number of digits to
appear for the d, i, o, u, x, or X conversions, the
number of digits to appear after the decimal point for
the e, E, and f conversions, the maximum number of
significant digits for the g and G conversion, or the
maximum number of characters to be printed 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. Padding
specified by the precision overrides the padding
specified by the field width.
An optional l (ell) specifying that a following d, i,
o, u, x, or X conversion character applies to a long
integer arg. An l before any other conversion
character is ignored.
A character that indicates the type of conversion to be
applied.
A field width or precision or both may be indicated by an
asterisk (*) instead of a digit string. In this case, an
integer arg supplies the field width or precision. The arg
that is actually converted is not fetched until the
conversion letter is seen, so the args specifying field
width or precision must appear before the arg (if any) to be
converted. A negative field width argument is taken as a
`-' flag followed by a positive field width. If the
precision argument is negative, it will be changed to zero.
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The flag characters and their meanings are:
- The result of the conversion will be left-
justified within the field.
+ The result of a signed conversion will always
begin with a sign (+ or -).
blank If the first character of a signed conversion is
not a sign, a blank will be prefixed to the
result. This implies that if the blank and +
flags both appear, the blank flag will be ignored.
# This flag specifies that the value is to be
converted to an ``alternate form.'' For c, d, i,
s, and u conversions, the flag has no effect. 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 will have 0x or
0X prefixed to it. For e, E, f, g, and G
conversions, the result will always contain a
decimal point, even if no digits follow the point
(normally, a decimal point appears in the result
of these conversions only if a digit follows it).
For g and G conversions, trailing zeroes will not
be removed from the result (which they normally
are).
The conversion characters and their meanings are:
d,i,o,u,x,X
The integer arg is converted to signed decimal (d
or i), unsigned octal, (o), decimal (u), or
hexadecimal notation (x or X), respectively; the
letters abcdef are used for x conversion and the
letters ABCDEF for X conversion. 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
zeroes. The default precision is 1. The result
of converting a zero value with a precision of
zero is a null string.
f The float or double arg is converted to decimal
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PRINTF(3S) (C Programming Language Utilities) PRINTF(3S)
notation in the style ``[-]ddd.ddd,'' where the
number of digits after the decimal point is equal
to the precision specification. If the precision
is missing, six digits are output; if the
precision is explicitly 0, no decimal point
appears.
e,E The float or double arg is converted in the style
``[-]d.ddde+dd,'' where there is one digit before
the decimal point and the number of digits after
it is equal to the precision; when the precision
is missing, six digits are produced; if the
precision is zero, no decimal point appears. The
E format code will produce a number with E instead
of e introducing the exponent. The exponent
always contains at least two digits.
g,G The float or double arg is printed in style f or e
(or in style E in the case of a G format code),
with the precision specifying the number of
significant digits. The style used depends on the
value converted: style e will be used only if the
exponent resulting from the conversion is less
than -4 or greater than the precision. Trailing
zeroes are removed from the result; a decimal
point appears only if it is followed by a digit.
c The character arg is printed.
s The arg is taken to be a string (character
pointer) and characters from the string are
printed until a null character (\0) is encountered
or the number of characters indicated by the
precision specification is reached. If the
precision is missing, it is taken to be infinite,
so all characters up to the first null character
are printed. A NULL value for arg will yield
undefined results.
% Print a %; no argument is converted.
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In printing floating point types (float and double), if the
exponent is 0x7FF and the mantissa is not equal to zero,
then the output is
[-]NaN0xdddddddd
where 0xdddddddd is the hexadecimal representation of the
leftmost 32 bits of the mantissa. If the mantissa is zero,
the output is
[+]inf.
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 putc(3S) had been
called.
EXAMPLES
To print a date and time in the form ``Sunday, July 3,
10:02,'' where weekday and month are pointers to null-
terminated strings:
printf("%s, %s %i, %d:%.2d", weekday, month, day, hour, min);
To print pi to 5 decimal places:
printf("pi = %.5f", 4 * atan(1.0));
SEE ALSO
ecvt(3C), putc(3S), scanf(3S), stdio(3S).
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