term% cat index.txt 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 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 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.
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 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 integer 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 letter 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 ig‐
nored.
# This flag specifies that the value is to be converted to an
``alternate form.'' For c, d, 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 pre‐
fixed 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 re‐
sult (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 oc‐
tal, decimal, or hexadecimal notation (x and X), respec‐
tively; 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 con‐
verted can be represented in fewer digits, it will be ex‐
panded with leading zeroes. (For compatibility 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
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 pre‐
cision; when the precision is missing, six digits are pro‐
duced; 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 char‐
acters 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. (For compatibility with other
versions 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 wider than the field width,
the field is simply expanded to contain the conversion result. Charac‐
ters 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).
PRINTF(3S)