PRINTF
Section: Linux Programmer's Manual (3)
Updated: 29 November 1993
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NAME
printf, fprintf, sprintf, vprintf, vfprintf, vsprintf - formatted output conversion
SYNOPSIS
#include <stdio.h>
int printf( const char *format, ...);
int fprintf( FILE *stream, const char *format, ...);
int sprintf( char *str, const char *format, ...);
#include <stdarg.h>
int vprintf( const char *format, va_list ap);
int vfprintf( FILE *stream, const char *format, va_list ap);
int vsprintf( char *str, char *format, va_list ap);
DESCRIPTION
The
printf
family of functions produces output according to a
format
as described below.
Printf
and
vprintf
write output to
stdout,
the standard output stream;
fprintf
and
vfprintf
write output to the given output
stream;
sprintf,
and
vsprintf
write to the character string
str.
These functions write the output under the control of a
format
string that specifies how subsequent arguments (or arguments accessed via
the variable-length argument facilities of
stdarg(3)
are converted for output. These functions return the number of characters
printed (not including the trailing `\0' used to end output to strings).
The format string is composed of zero or more directives: ordinary
characters (not
%),
which are copied unchanged to the output stream;
and conversion specifications, each of which results in fetching zero or
more subsequent arguments. Each conversion specification is introduced by
the character
%.
The arguments must correspond properly (after type promotion) with the
conversion specifier. After the
%,
the following appear in sequence:
- *
-
Zero or more of the following flags:
-
- #
-
specifying that the value should be converted to an ``alternate form''.
For
c,
d,
i,
n,
p,
s,
and
u
conversions, this option has no effect. For
o
conversions, the precision of the number is increased to force the first
character of the output string to a zero (except if a zero value is printed
with an explicit precision of zero).
For
x
and
X
conversions, a non-zero result has the string `0x' (or `0X' for
X
conversions) prepended to it. For
e,
E,
f,
g,
and
G
conversions, the result will always contain a decimal point, even if no
digits follow it (normally, a decimal point appears in the results of those
conversions only if a digit follows). For
g
and
G
conversions, trailing zeros are not removed from the result as they would
otherwise be.
- 0
-
specifying zero padding. For all conversions except
n,
the converted value is padded on the left with zeros rather than blanks.
If a precision is given with a numeric conversion
(d,
i,
o,
u,
i,
x,
and
X),
the
0
flag is ignored.
- -
-
(a negative field width flag) indicates the converted value is to be left
adjusted on the field boundary. Except for
n
conversions, the converted value is padded on the right with blanks, rather
than on the left with blanks or zeros. A
-
overrides a
0
if both are given.
-
-
(a space) specifying that a blank should be left before a positive number
produced by a signed conversion
(d,
e,
E,
f,
g,
G,
or
i).
- +
-
specifying that a sign always be placed before a number produced by a
signed conversion. A
+
overrides a space if both are used.
- *
-
An optional decimal digit string specifying a minimum field width. If the
converted value has fewer characters than the field width, it will be
padded with spaces on the left (or right, if the left-adjustment flag has
been given) to fill out the field width.
- *
-
An optional precision, in the form of a period (`.') followed by an
optional digit string. If the digit string is omitted, the precision is
taken as zero. This gives the minimum number of digits to appear for
d,
i,
o,
u,
x,
and
X
conversions, the number of digits to appear after the decimal-point for
e,
E,
and
f
conversions, the maximum number of significant digits for
g
and
G
conversions, or the maximum number of characters to be printed from a
string for
s
conversions.
- *
-
The optional character
h,
specifying that a following
d,
i,
o,
u,
x,
or
X
conversion corresponds to a
short int
or
unsigned short int
argument, or that a following
n
conversion corresponds to a pointer to a
short int
argument.
- *
-
The optional character
l
(ell) specifying that a following
d,
i,
o,
u,
x,
or
X
conversion applies to a pointer to a
long int
or
unsigned long int
argument, or that a following
n
conversion corresponds to a pointer to a
long int
argument.
- *
-
The character
L
specifying that a following
e,
E,
f,
g,
or
G
conversion corresponds to a
long double
argument.
- *
-
A character that specifies 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
argument supplies the field width or precision. A negative field width is
treated as a left adjustment flag followed by a positive field width; a
negative precision is treated as though it were missing.
The conversion specifiers and their meanings are:
- diouxX
-
The
int
(or appropriate variant) argument is converted to signed decimal
(d
and
i),
unsigned octal
(o,
unsigned decimal
(u,
or unsigned hexadecimal
(x
and
X)
notation. The letters
abcdef
are used for
x
conversions; the letters
ABCDEF
are used for
X
conversions. The precision, if any, gives the minimum number of digits
that must appear; if the converted value requires fewer digits, it is
padded on the left with zeros.
- DOU
-
The
long int
argument is converted to signed decimal, unsigned octal, or unsigned
decimal, as if the format had been
ld,
lo,
or
lu
respectively. These conversion characters are deprecated, and will
eventually disappear.
- eE
-
The
double
argument is rounded and converted in the style
[-]d.ddde\*(Pmdd
where there is one digit before the decimal-point character and the number
of digits after it is equal to the precision; if the precision is missing,
it is taken as 6; if the precision is zero, no decimal-point character
appears. An
E
conversion uses the letter
E
(rather than
e)
to introduce the exponent. The exponent always contains at least two
digits; if the value is zero, the exponent is 00.
- f
-
The
double
argument is rounded and converted to decimal notation in the style
[-]ddd.ddd,
where the number of digits after the decimal-point character is equal to
the precision specification. If the precision is missing, it is taken as
6; if the precision is explicitly zero, no decimal-point character appears.
If a decimal point appears, at least one digit appears before it.
- g
-
The
double
argument is converted in style
f
or
e
(or
E
for
G
conversions). The precision specifies the number of significant digits.
If the precision is missing, 6 digits are given; if the precision is zero,
it is treated as 1. Style
e
is used if the exponent from its 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 appears only if it is
followed by at least one digit.
- c
-
The
int
argument is converted to an
unsigned char,
and the resulting character is written.
- s
-
The
``char *''
argument is expected to be a pointer to an array of character type (pointer
to a string). Characters from the array are written up to (but not
including) a terminating
NUL
character; if a precision is specified, no more than the number specified
are written. If a precision is given, no null character need be present;
if the precision is not specified, or is greater than the size of the
array, the array must contain a terminating
NUL
character.
- p
-
The
``void *''
pointer argument is printed in hexadecimal (as if by
%#x
or
%#lx).
- n
-
The number of characters written so far is stored into the integer
indicated by the
``int *''
(or variant) pointer argument. No argument is converted.
- %
-
A `%' is written. No argument is converted. The complete conversion
specification is `%%'.
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 expanded to contain the conversion result.
EXAMPLES
To print a date and time in the form `Sunday, July 3, 10:02',
where
weekday
and
month
are pointers to strings:
-
#include <stdio.h>
fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
weekday, month, day, hour, min);
To print
to five decimal places:
-
#include <math.h>
#include <stdio.h>
fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
To allocate a 128 byte string and print into it:
-
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
char *newfmt(const char *fmt, ...)
{
char *p;
va_list ap;
if ((p = malloc(128)) == NULL)
return (NULL);
va_start(ap, fmt);
(void) vsnprintf(p, 128, fmt, ap);
va_end(ap);
return (p);
}
SEE ALSO
printf(1), scanf(3)
STANDARDS
The
fprintf,
printf,
sprintf,
vprintf,
vfprintf,
and
vsprintf
functions conform to ANSI C3.159-1989 (``ANSI C'').
BUGS
Some floating point conversions under Linux cause memory leaks.
The conversion formats
%D,
%O,
and
%U
are not standard and are provided only for backward compatibility. These
may not be provided under Linux.
The effect of padding the
%p
format with zeros (either by the
0
flag or by specifying a precision), and the benign effect (i.e., none) of
the
#
flag on
%n
and
%p
conversions, as well as other nonsensical combinations such as
%Ld,
are not standard; such combinations should be avoided.
Because
sprintf
and
vsprintf
assume an infinitely long string, callers must be careful not to overflow
the actual space; this is often impossible to assure.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- EXAMPLES
-
- SEE ALSO
-
- STANDARDS
-
- BUGS
-
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