PRINTF(3S) 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 responsi-
bility 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 insuffi-
cient 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
conversion specification.
An optional decimal digit string specifying a minimum
field width. If the converted value has fewer charac-
ters 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.
A precision that gives the minimum number of digits to
appear for the d, o, u, x, or X conversions, the number
of digits to appear after the decimal point for the e
PRINTF(3S) PRINTF(3S)
and f conversions, the maximum number of significant
digits for the 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.
An optional l (ell) specifying that a following d, o,
u, x, or X conversion character applies to a long inte-
ger arg. A l before any other conversion character is
ignored.
A character that indicates the type of conversion to be
applied.
A field width or precision 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 let-
ter is seen, so the args specifying field width or precision
must appear before the arg (if any) to be converted.
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 con-
verted to an ``alternate form.'' For c, d, s, and
u conversions, the flag has no effect. For o con-
version, 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 conver-
sions, the result will always contain a decimal
point, even if no digits follow the point (nor-
mally, 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,o,u,x,X The integer arg is converted to signed decimal,
unsigned octal, decimal, or hexadecimal notation
(x and X), respectively; the letters abcdef are
used for x conversion and the letters ABCDEF for X
PRINTF(3S) PRINTF(3S)
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. (For com-
patibility with other versions of printf, a field
width with a leading zero will result in padding
with leading zeroes. This does not imply an octal
value for the field width.) 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
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 preci-
sion 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 preci-
sion 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 sig-
nificant 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 pre-
cision 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 unde-
fined results. (For compatibility with other ver-
sions of printf, a field width with a leading zero
will result in zero-padding the string instead of
blank-padding it. This does not imply an octal
value for the field width.)
% Print a %; no argument is converted.
In no case does a non-existent or small field width cause
truncation of a field; if the result of a conversion is
PRINTF(3S) PRINTF(3S)
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 %d, %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).