printf.3

操作系统设计与实现源码

.\"	@(#)printf.3s	6.3 (Berkeley) 6/5/86
.\"
.TH PRINTF 3 "June 5, 1986"
.AT 3
.SH NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf \- formatted output conversion
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <stdio.h>
#include <stdarg.h>

int printf(const char *\fIformat\fP \fR[\fP, \fIarg\fP\fR] ...\fP);
int fprintf(FILE *\fIstream\fP, const char *\fIformat\fP \fR[\fP, \fIarg\fP\fR] ...\fP);
int sprintf(char *\fIs\fP, const char *\fIformat\fP \fR[\fP, \fIarg\fP\fR] ...\fP);
int snprintf(char *\fIs\fP, size_t \fIn\fP, const char *\fIformat\fP \fR[\fP, \fIarg\fP\fR] ...\fP);
int vprintf(const char *\fIformat\fP, va_list \fIargs\fP);
int vfprintf(FILE *\fIstream\fP, const char *\fIformat\fP, va_list \fIargs\fP);
int vsprintf(char *\fIs\fP, const char *\fIformat\fP, va_list \fIargs\fP);
int vsnprintf(char *\fIs\fP, size_t \fIn\fP, const char *\fIformat\fP, va_list \fIargs\fP);
.ft R
.fi
.SH DESCRIPTION
.B Printf
places output on the standard output stream
.BR stdout .
.B Fprintf
places output on the named output
.IR stream .
.B Sprintf
places `output' in the string
.IR s ,
followed by the character `\e0'.
.B Snprintf
(Minix-vmd only)
is like
.B sprintf
except that no more than
.IR n \-1
characters are written to
.I s
followed by a `\e0'.
.PP
The
.B v*printf
functions can be used to make functions like the first four by using the
.BR stdarg (3)
method to process the argument.
.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
.IR arg .
.PP
Each conversion specification is introduced by the character
.BR % .
The remainder of the conversion specification includes
in the following order
.TP
\(bu
Zero or more of following flags:
.RS
.TP
\(bu
a `#' character
specifying that the value should be converted to an ``alternate form''.
For 
.BR c ,
.BR d ,
.BR s ,
and
.BR u
conversions, this option has no effect.  For 
.B o
conversions, the precision of the number is increased to force the first
character of the output string to a zero.  For 
.BR x ( X )
conversion, a non-zero result has the string 
.BR 0x ( 0X )
prepended to it.  For 
.BR e ,
.BR E ,
.BR f ,
.BR g ,
and
.BR G
conversions, the result will always contain a decimal point, even if no
digits follow the point (normally, a decimal point only appears in the
results of those conversions if a digit follows the decimal point).  For
.B g
and
.B G
conversions, trailing zeros are not removed from the result as they
would otherwise be.
.TP
\(bu
a minus sign `\-' which specifies
.I "left adjustment"
of the converted value in the indicated field;
.TP
\(bu
a `+' character specifying that there should always be
a sign placed before the number when using signed conversions.
.TP
\(bu
a space specifying that a blank should be left before a positive number
during a signed conversion.  A `+' overrides a space if both are used.
.RE
.TP
\(bu
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
\(bu
an optional period
.RB ` . '
which serves to separate the field width from the next digit string;
.TP
\(bu
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
\(bu
the character
.B l
specifying that a following
.BR d ,
.BR o ,
.BR x ,
or
.B u
corresponds to a long integer
.IR arg .
.TP
\(bu
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 X
Like
.BR x ,
but use upper case instead of lower case.
.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.
.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.
.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
.B printf
are printed by 
.BR 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:
.PP
.RS
printf("%s, %s %d, %02d:%02d", weekday, month, day, hour, min);
.RE
.PP
To print
.if n pi
.if t \(*p
to 5 decimals:
.IP
printf("pi = %.5f", 4*atan(1.0));
.SH "SEE ALSO"
.BR putc (3),
.BR scanf (3),
.BR ecvt (3),
.BR stdarg (3).