printf.3s

unix v7是最后一个广泛发布的研究型UNIX版本

.TH PRINTF 3S 
.SH NAME
printf, fprintf, sprintf \- formatted output conversion
.SH SYNOPSIS
.B #include <stdio.h>
.PP
.B printf(format
.RB [ ,
arg ] ...
.B )
.br
.B char *format;
.PP
.B fprintf(stream, format
.RB [ ,
arg ] ...
.B )
.br
.SM
.B FILE
.B *stream;
.br
.B char *format;
.PP
.B sprintf(s, format
.RB [ ,
arg ] ...
.B )
.br
.B char *s, format;
.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' in the string
.I s,
followed by the character `\\0'.
.PP
Each of these functions 
converts, formats, and prints its arguments after the first
under control of the first argument.
The first argument is a character string
which contains
two types of objects:
plain characters, which are simply copied to the
output stream,
and conversion specifications,
each of which causes conversion and printing
of the next successive
.I arg
.IR printf .
.PP
Each conversion specification is introduced by
the character
.BR % .
Following the
.BR % ,
there may be
.TP
\-
an optional minus sign `\-' which specifies
.I "left adjustment"
of the converted value
in the
indicated field;
.TP
\-
an optional digit string specifying a
.I "field width;"
if the converted value has fewer characters
than the field width
it will be blank-padded on the left (or right,
if the left-adjustment indicator has been
given) to make up the field width;
if the field width begins with a zero,
zero-padding will be done instead of blank-padding;
.TP
\-
an optional period
.RB ` . '
which serves to
separate the field width from the
next digit string;
.TP
\-
an optional digit string
specifying a
.I precision
which specifies
the number of digits to appear after the
decimal point, for e- and f-conversion,
or the maximum number of characters
to be printed from a string;
.TP
\-
the character
.B l
specifying that a following
.BR d ,
.BR o ,
.BR x ,
or
.B u
corresponds to a long integer
.I arg.
(A capitalized conversion code accomplishes
the same thing.)
.TP
\-
a character which indicates the type of
conversion to be applied.
.PP
A field width or precision may be `*' instead of a digit string.
In this case an integer
.I arg
supplies
the field width or precision.
.PP
The conversion characters
and their meanings are
.TP
.B dox
The integer
.I arg
is converted to decimal, octal, or
hexadecimal notation respectively.
.TP
.B f
The float or double
.I arg
is converted to decimal notation
in the style `[\fB\-\fR]ddd.ddd'
where the number of d's after the decimal point
is equal to the precision specification
for the argument.
If the precision
is missing,
6 digits are given;
if the precision is explicitly 0, no digits and
no decimal point are printed.
.TP
.B e
The float or double
.I arg
is converted in the style
`[\fB\-\fR]d\fB.\fRddd\fBe\fR\(+-dd'
where there is one digit before the decimal point and
the number after is equal to the
precision specification for the argument;
when the precision is missing,
6 digits are produced.
.TP
.B g
The float or double
.I arg
is printed in style
.BR d ,
in style
.BR f ,
or in
style
.BR e ,
whichever gives full precision in minimum space.
.TP
.B c
The character
.I arg
is printed.
Null characters are ignored.
.TP
.B s
.I Arg
is taken to be a string (character pointer)
and characters from the string are printed until
a null character or until
the number of characters indicated by the precision
specification is reached;
however if the precision is 0 or missing
all characters up to a null are printed.
.TP
.B u
The unsigned integer
.I arg
is converted to decimal
and printed (the result will be in the
range 0 to 65535).
.TP
.B %
Print a `%'; no argument is converted.
.PP
In no case does a non-existent or small field width
cause truncation of a field;
padding takes place only if the specified field
width exceeds the actual width.
Characters generated by
.I printf
are printed by 
.IR putc (3).
.PP
.B Examples
.br
To 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:
.RS
.HP
.nh
printf("%s, %s %d, %02d:%02d", weekday, month, day, hour, min);
.RE
.hy
.PP
To print
.if n pi
.if t \(*p
to 5 decimals:
.IP
printf("pi = %.5f", 4*atan(1.0));
.SH "SEE ALSO"
putc(3),
scanf(3),
ecvt(3)
.SH BUGS
Very wide fields (>128 characters) fail.