FPRINTF(2) FPRINTF(2)
NAME
fprintf, printf, sprintf, vfprintf, vprintf, vsprintf -
print formatted output
SYNOPSIS
#include <stdio.h>
int fprintf(FILE *f, char *format, ...);
int printf(char *format, ...);
int sprintf(char *s, char *format, ...);
int vfprintf(FILE *f, char *format, char *args);
int vprintf(char *format, char *args);
int vsprintf(char *s, char *format, char *args);
DESCRIPTION
Fprintf places output on the named output stream f (see
fopen(2)). Printf places output on the standard output
stream stdout. 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. Vfprintf, vprintf, and vsprintf are the same,
except the args argument is a pointer to an argument in an
argument list of the calling function, and the effect is as
if the calling function's argument list from that point on
is passed to the printf routines.
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.
These functions convert, format, and print their trailing
arguments under control of a format string. The format con-
tains two types of objects: plain characters, which are sim-
ply copied to the output stream, and conversion specifica-
tions, each of which results in fetching of zero or more
arguments. The results are undefined if there are arguments
of the wrong type or too few arguments for the format. If
the format is exhausted while arguments remain, the excess
are 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.
Page 1 Plan 9 (printed 11/17/24)
FPRINTF(2) FPRINTF(2)
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 with
spaces on the left (or right, if the left adjustment,
described later, 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, and X conver-
sions, the number of digits to appear after the decimal
point for the e, E, and f conversions, the maximum num-
ber of significant digits for the g and G conversions,
or the maximum number of characters to be written from
a string in s conversion. The precision takes the form
of a period (.) followed by an optional decimal inte-
ger; if the integer is omitted, it is treated as zero.
An optional h specifying that a following d, i, o, u, x
or X conversion specifier applies to a short int or
unsigned short argument (the argument will have been
promoted according to the integral promotions, and its
value shall be converted to short or unsigned short
before printing); an optional h specifying that a fol-
lowing n conversion specifier applies to a pointer to a
short argument; an optional l (ell) specifying that a
following d, i, o, u, x, or X conversion character
applies to a long or unsigned long argument; an
optional l specifying that a following n conversion
specifier applies to a pointer to a long int argument;
or an optional L specifying that a following e, E, f,
g, or G conversion specifier applies to a long double
argument. If an h, l, or L appears with any other con-
version specifier, the behavior is undefined.
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
int arg supplies the field width or precision. The argu-
ments specifying field width or precision, or both, shall
appear (in that order) before the argument (if any) to be
converted. A negative field width argument is taken as a -
flag followed by a positive field width. A negative preci-
sion is taken as if it were missing.
The flag characters and their meanings are:
- The result of the conversion is left-justified
within the field.
+ The result of a signed conversion always begins
with a sign (+ or -).
blank If the first character of a signed conversion is
not a sign, or a signed conversion results in no
Page 2 Plan 9 (printed 11/17/24)
FPRINTF(2) FPRINTF(2)
characters, a blank is prefixed to the result.
This implies that if the blank and + flags both
appear, the blank flag is ignored.
# The result is to be converted to an ``alternate
form.'' For o conversion, it increases the preci-
sion to force the first digit of the result to be
a zero. For x or X conversion, a non-zero result
has 0x or 0X prefixed to it. For e, E, f, g, and
G conversions, the result always contains a deci-
mal 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 zeros are not be
removed from the result as they normally are. For
other conversions, the behavior is undefined.
0 For d, i, o, u, x, X, e, E, f, g, and G conver-
sions, leading zeros (following any indication of
sign or base) are used to pad 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.
The conversion characters and their meanings are:
d,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 or X); 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 is expanded with leading zeros.
The default precision is 1. The result of con-
verting a zero value with a precision of zero is
no characters.
f The double argument is converted to decimal nota-
tion 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, it is taken as 6; if the precision is
explicitly `0', no decimal point appears.
e,E The double argument 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, it is taken as 6; if the precision is
zero, no decimal point appears. The E format code
produces a number with E instead of e introducing
the exponent. The exponent always contains at
least two digits.
Page 3 Plan 9 (printed 11/17/24)
FPRINTF(2) FPRINTF(2)
g,G The double argument is printed in style f or e (or
in style E in the case of a G conversion speci-
fier), with the precision specifying the number of
significant digits. If an explicit precision is
zero, it is taken as 1. The style used depends on
the value converted: style e is 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
portion of the result; a decimal point appears
only if it is followed by a digit.
c The int argument is converted to an unsigned char,
and the resulting character is written.
s The argument 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 zero value for the argument yields
undefined results.
P The void * argument is printed in an implementa-
tion defined way (for Plan 9: the address as hex-
adecimal number).
n The argument shall be a pointer to an integer into
which is written the number of characters written
to the output stream so far by this call to
fprintf. No argument is converted.
% Print a %; no argument is converted.
If a conversion specification is invalid, the behavior is
undefined.
If any argument is, or points to, a union or an aggregate
(except for an array of character type using %s conversion,
or a pointer cast to be a pointer to void using %P conver-
sion), the behavior is undefined.
In no case does a nonexistent or small field width cause
truncation of a field; if the result of a conversion is
wider than the field width, the field is expanded to contain
the conversion result.
SOURCE
/sys/src/libstdio
SEE ALSO
fopen(2), fscanf(2), print(2)
BUGS
There is no way to print a wide character (rune); use
Page 4 Plan 9 (printed 11/17/24)
FPRINTF(2) FPRINTF(2)
print(2) or bio(2).
Page 5 Plan 9 (printed 11/17/24)