printf(3S) UNIX System V printf(3S)
NAME
printf, fprintf, sprintf - print formatted output
SYNOPSIS
#include <stdio.h>
int printf(const char *format, .../* args */);
int fprintf(FILE *strm, const char *format, .../* args */);
int sprintf(char *s, const char *format, .../* args */);
DESCRIPTION
printf places output on the standard output stream stdout.
fprintf places output on strm.
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
three types of objects defined below:
1. plain characters that are simply copied to the output stream;
2. escape sequences that represent non-graphic characters;
3. conversion specifications.
The following escape sequences produce the associated action on display
devices capable of the action:
\a Alert. Ring the bell.
\b Backspace. Move the printing position to one character before the
current position, unless the current position is the start of a
line.
\f Form feed. Move the printing position to the initial printing
position of the next logical page.
\n Newline. Move the printing position to the start of the next line.
\r Carriage return. Move the printing position to the start of the
current line.
\t Horizontal tab. Move the printing position to the next
implementation-defined horizontal tab position on the current line.
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printf(3S) UNIX System V printf(3S)
\v Vertical tab. Move the printing position to the start of the next
implementation-defined vertical tab position.
All forms of the printf functions allow for the insertion of a language-
dependent decimal-point character. The decimal-point character is
defined by the program's locale (category LCNUMERIC). In the C locale,
or in a locale where the decimal-point character is not defined, the
decimal-point character defaults to a period (.).
Each conversion specification is introduced by the character %. After
the character %, the following appear in sequence:
An optional field, consisting of a decimal digit string followed by
a $, specifying the next args to be converted. If this field is
not provided, the args following the last args converted will be
used.
Zero or more flags, which modify the meaning of the conversion
specification.
An optional string of decimal digits to specify 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.
An optional precision that gives the minimum number of digits to
appear for the d, i, o, u, x, or X conversions (the field is padded
with leading zeros), the number of digits to appear after the
decimal-point character for the e, E, and f conversions, the
maximum number of significant digits for the g and G conversions,
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 h specifies that a following d, i, o, u, x, or X
conversion specifier applies to a short int or unsigned short int
argument (the argument will be promoted according to the integral
promotions and its value converted to short int or unsigned short
int before printing); an optional h specifies that a following n
conversion specifier applies to a pointer to a short int argument.
An optional l (ell) specifies that a following d, i, o, u, x, or X
conversion specifier applies to a long int or unsigned long int
argument; an optional l (ell) specifies that a following n
conversion specifier applies to a pointer to long int argument. An
optional L specifies that a following e, E, f, g, or G conversion
specifier applies to a long double argument. If an h, l, or L
appears before any other conversion specifier, the behavior is
undefined.
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A conversion character (see below) 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 args supplies the field width
or precision. The args 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 args (if any) to be converted. If the
precision argument is negative, it will be changed to zero. A negative
field width argument is taken as a - flag, followed by a positive field
width.
In format strings containing the *digits$ form of a conversion
specification, a field width or precision may also be indicated by the
sequence *digits$, giving the position in the argument list of an integer
args containing the field width or precision.
When numbered argument specifications are used, specifying the Nth
argument requires that all the leading arguments, from the first to the
(N-1)th, be specified in the format string.
The flag characters and their meanings are:
- The result of the conversion will be left-justified within the
field. (It will be right-justified if this flag is not specified.)
+ The result of a signed conversion will always begin with a sign (+
or -). (It will begin with a sign only when a negative value is
converted if this flag is not specified.)
space If the first character of a signed conversion is not a sign, a
space will be placed before the result. This means that if the
space and + flags both appear, the space flag will be ignored.
# The value is to be converted to an alternate form. For c, d, i, s,
and u conversions, the flag has no effect. For an 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) prepended to it. For e, E, f, g, and G conversions, the
result will always contain a decimal-point character, 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 zeros will not be removed from the result
as they normally 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 will be ignored. For d, i, o, u, x, and X
conversions, if a precision is specified, the 0 flag will be
ignored. For other conversions, the behavior is undefined.
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Each conversion character results in fetching 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 ignored.
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), unsigned decimal (u), or unsigned
hexadecimal notation (x and X). The x conversion uses the
letters abcdef and the X conversion uses the letters
ABCDEF. The precision specifies the minimum number of
digits to appear. If the value being converted can be
represented in fewer digits than the specified minimum, it
will be expanded with leading zeros. The default
precision is 1. The result of converting a zero value
with a precision of zero is no characters.
f The double args is converted to decimal notation in the
style [-]ddd.ddd, where the number of digits after the
decimal-point character [see setlocale(3C)] is equal to
the precision specification. If the precision is omitted
from arg, six digits are output; if the precision is
explicitly zero and the # flag is not specified, no
decimal-point character appears. If a decimal-point
character appears, at least 1 digit appears before it.
The value is rounded to the appropriate number of digits.
e,E The double args is converted to the style [-]d.ddde+dd,
where there is one digit before the decimal-point
character (which is non-zero if the argument is non-zero)
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 and the # flag is not
specified, no decimal-point character appears. The E
conversion character will produce a number with E instead
of e introducing the exponent. The exponent always
contains at least two digits. The value is rounded to the
appropriate number of digits.
g,G The double args is printed 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. If
the precision is zero, it is taken as one. The style used
depends on the value converted: style e (or E) will be
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 fractional part of the
result. A decimal-point character appears only if it is
followed by a digit.
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printf(3S) UNIX System V printf(3S)
c The int args is converted to an unsigned char, and the
resulting character is printed.
s The args is taken to be a string (character pointer) and
characters from the string are written up to (but not
including) a terminating null character; if the precision
is specified, no more than that many characters are
written. If the precision is not specified, it is taken
to be infinite, so all characters up to the first null
character are printed. A NULL value for args will yield
undefined results.
p The args should be a pointer to void. The value of the
pointer is converted to an implementation-defined set of
sequences of printable characters, which should be the
same as the set of sequences that are matched by the %p
conversion of the scanf function.
n The argument should be a pointer to an integer into which
is written the number of characters written to the output
standard I/O stream so far by this call to printf,
fprintf, or sprintf. No argument is converted.
% Print a %; no argument is converted.
If the character after the % or %digits$ sequence is not a valid
conversion character, the results of the conversion are undefined.
If a floating-point value is the internal representation for infinity,
the output is [+]inf, where inf is either inf or INF, depending on the
conversion character. Printing of the sign follows the rules described
above.
If a floating-point value is the internal representation for ``not-a-
number,'' the output is [+]nan0xm. Depending on the conversion
character, nan is either nan or NAN. Additionally, 0xm represents the
most significant part of the mantissa. Again depending on the conversion
character, x will be x or X, and m will use the letters abcdef or ABCDEF.
Printing of the sign follows the rules described above.
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 the putc routine had
been called.
EXAMPLE
To print a date and time in the form Sunday, July 3, 10:02, where weekday
and month are pointers to null-terminated strings:
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printf("%s, %s %i, %d:%.2d",
weekday, month, day, hour, min);
To print π to 5 decimal places:
printf("pi = %.5f", 4 * atan(1.0));
SEE ALSO
exit(2), lseek(2), write(2), abort(3C), ecvt(3C), putc(3S), scanf(3S),
setlocale(3C), stdio(3S).
DIAGNOSTICS
printf, fprintf, and sprintf return the number of characters transmitted,
or return a negative value if an error was encountered.
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