V10/man/man3/printf.3

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.TH PRINTF 3S
.CT 2 file_io
.SH NAME
printf, fprintf, sprintf, snprintf \(mi print formatted output
.SH SYNOPSIS
.nf
.B "#include <stdio.h>"
.PP
.B "int printf(char *format, ... );
.PP
.B "int fprintf (FILE *stream, char *format, ... );
.PP
.B "int sprintf (char *s, char *format, ... );
.PP
.B "int snprintf (char *s, int len, char *format, ... );
.fi
.SH DESCRIPTION
.I Printf
places output on the standard output stream
.IR stdout .
.I Fprintf
places output on the named output
.IR stream .
.I Sprintf
places output
followed by the null character
.RB ( \e0 ),
in consecutive bytes starting at
.IR s ;
it is the user's responsibility to ensure that
enough storage is available.
.I Snprintf
corresponds to
.IR sprintf
except that no more than
.IR len
bytes are placed into
.IR s .
Each function returns the number of characters
transmitted (not including the
.B \e0
in the case of
.IR sprintf ),
or
a negative value if an output error was encountered.
.PP
Each of these functions
converts, formats, and prints its
trailing arguments
under control of a
.IR format 
string.
The
.I format
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
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.
.PP
Each conversion specification is introduced by
the character
.BR % .
After the
.BR % ,
the following
appear in sequence:
.PP
.RS
Zero or more
.IR flags ,
which modify the meaning of
the conversion specification.
.PP
An optional decimal digit string specifying a minimum
.IR "field width" .
If the converted value has fewer characters
than the field width,
it is padded to the field width.
When the width specification begins with zero, padding is
with leading zeros.
Otherwise padding is with leading spaces (trailing spaces,
with the left-adjustment flag 
.LR - ,
described below) to the field width.
.PP
A
.I precision\^
that gives
the minimum number of digits to appear for the
.BR d ,
.BR o ,
.BR u ,
.BR x ,
or
.B X
conversions,
the number of digits to appear after the
decimal point for the
.B e
and
.B f
conversions,
the maximum number of significant digits
for the
.B g
conversion,
or the maximum number of characters
to be printed from a string in
.B s
conversion.
The precision takes the form of a period
.RB ( \&. )
followed by a decimal digit string;
a null digit string is treated as zero.
.PP
An optional
.B l
(ell) specifying that a following
.BR d ,
.BR o ,
.BR u ,
.BR x ,
or
.B X
conversion character applies to a long
integer
.IR arg .
An
.B l
before any other conversion character is ignored.
.PP
A character that indicates the type of
conversion to be applied.
.RE
.PP
A field width or precision may be
indicated by an asterisk
.RB ( * )
or an exclamation point
.RB ( ! )
instead of a digit string.
In this case, an integer
.I arg\^
supplies
the field width or precision.
.PP
The flag characters and their meanings are:
.PD 0
.TP 10
.B \-
The result of the conversion is left-justified within the field.
.TP
.B +
The result of a signed
conversion always begins with a sign
.RB ( +
or
.BR - ).
.TP
blank
If the first character of a signed conversion is not a sign, a blank
is prefixed to the result.
This implies that if the blank and
.B +
flags both appear, the blank flag is ignored.
.TP
.B #
This flag specifies that the value is to be converted
to an ``alternate form.''
For
.BR c ,
.BR d ,
.BR s ,
and
.B u
conversions, the flag has no effect.
For
.B o
conversion, it increases the precision to force
the first digit of the result to be a zero.
For
.B x or X
conversion, a non-zero result has
.B 0x or 0X
prefixed to it.
For
.BR e ,
.BR E ,
.BR f ,
.BR g ,
and
.B G
conversions, the result always contains 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
.B g
and
.B G
conversions, trailing zeros are
.I not\^
be removed from the result
as they normally are.
.PD
.PP
The conversion characters
and their meanings are:
.PP
.PD 0
.TP 10
\fLd\fP,\fLo\fP,\fLu\fP,\fLx\fP,\fLX\fP
The integer
.I arg\^
is converted to signed decimal,
unsigned octal, decimal, or
hexadecimal notation
.RB ( x
and
.BR X ),
respectively;
the letters
.B abcdef
are used for
.B x
conversion and the letters
.B ABCDEF
for
.B 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.
(For compatibility with other versions of
.IR printf ,
a field width with a leading zero
results in padding with leading zeros.
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.
.TP
.BR f
The float or double
.I arg\^
is converted to decimal notation
in the style
[\fB-\fR]\fId\fB.\fIddd\fR,
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
.LR 0 ,
no decimal point appears.
.TP
.BR e ", " E
The float or double
.I arg\^
is converted in the style
[\fB-\fR]\fId\fB.\fIddd\fBe\(+-\fIdd\fR,
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
.B E
format code produces a number with
.B E
instead of
.B e
introducing the exponent.
The exponent always contains at least two digits.
.TP
.BR g ", " G
The float or double
.I arg\^
is printed in style
.BR f
or
.BR e
(or in style
.B E
in the case of a
.B G
format code),
with the precision specifying the number of significant digits.
The style used depends on the value converted:
style
.B e
is used only if the exponent resulting from
the conversion is less than -4
or greater than the precision.
Trailing zeros are removed from the result; a decimal point
appears only if it is followed by a digit.
Precision 0 yields a result with just enough significance
to round to exactly the original value when converted
back to binary as by
.IR scanf (3).
.TP
.B c
The character
argument is printed.
.TP
.B s
The
argument is taken to be a string (character pointer)
and characters from the string are printed until
a null character
.RB ( \e0 )
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
zero
value for
the argument yields undefined results.
(For compatibility with other versions of
.IR printf ,
a field width with
a leading zero results in zero-padding the string instead
of blank-padding it.
This does not imply an octal value for the field width.)
.TP
.B %
Print a
.BR % ;
no argument is converted.
.PD
.PP
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
.I printf\^
and
.I fprintf\^
are printed as if
.IR putc
had been called; see
.IR getc (3).
.SH EXAMPLES
.TP
.L
printf("%s, %s %d, %d:%.2d", weekday, month, day, hour, min);
Print a date and time in the form `Sunday, July 3, 10:02',
where
.I weekday\^
and
.I month\^
are pointers to null-terminated strings.
.TP
.L
printf("pi = %.5f", 4*atan(1.0));
Print
.if n .I pi\^
.if t \(*p
to 5 decimal places.
.SH SEE ALSO
.IR ecvt (3),
.IR scanf (3),
.IR stdio (3),
.IR print (3)
.SH BUGS
The
.L !
indicator for field width is nonstandard.