PRINTF(3) Library Routines PRINTF(3)
NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vs����
nprintf - 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, ...);
int snprintf (char *str, size_t size, 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);
int vsnprintf (char *str, size_t size, const 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 stan‐
dard output stream; fprintf and vfprintf write output to the given out‐
put stream. sprintf, snprintf, vsprintf, and vsnprintf write to the
character string str. These functions write the output under the con‐
trol of a format string that specifies how subsequent arguments (or ar‐
guments 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). Snprintf and vsnprintf will write at most size of the
characters printed into the output string (the size character then gets
the terminating \0); if the return value is greater than or equal to
the size argument, the string was too short and some of the printed
characters were discarded. Sprintf and vsprintf effectively assume an
infinite size.
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 intro‐
duced by the character %. The arguments must correspond properly (af‐
ter type promotion) with the conversion specifier. After the %, the
following appear in sequence:
Zero or more of the following flags:
A # character specifying that the value should be con‐
verted 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 pre‐
cision of zero). For x and X conversions, a non-zero re‐
sult 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 fol‐
lows). For g and G conversions, trailing zeros are not
removed from the result as they would otherwise be.
A zero (0) character 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. Ex‐
cept 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).
A + character specifying that a sign always be placed be‐
fore a number produced by a signed conversion. A + over‐
rides 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 num‐
ber 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 optional character q, specifying that a following d, i, o,
u, x, or X conversion corresponds to a quad int or unsigned quad
int argument, or that a following n conversion corresponds to a
pointer to a quad int argument.
The character L specifying that a following e, E, f, g, or G
conversion corresponds to a long double argument (but note that
long double values are not currently supported by the ORCA/C,
VAX, and Tahoe compilers).
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 preci‐
sion 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 let‐
ters ABCDEF are used for X conversions. The precision,
if any, gives the minimum number of digits that must ap‐
pear; 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, un‐
signed octal, or unsigned decimal, as if the format had
been ld, lo, or lu respectively. These conversion char‐
acters are deprecated, and will eventually disappear.
eE The double argument is rounded and converted in the style
[-]d.ddde(+|-)dd
where there is one digit before the decimal-point charac‐
ter 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 ap‐
pears. 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 char‐
acter 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 conver‐
sion is less than -4 or greater than or equal to the pre‐
cision. 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 ar‐
ray of character type (pointer to a string). Characters
from the array are written up to (but not including) a
terminating NULL character; if a precision is specified,
no more than the number specified are written. If a pre‐
cision 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 Pi 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/C.
HISTORY
The functions snprintf and vsnprintf are new to this release.
BUGS
The ORCA/C stdio implementation also provides a %b format specifier,
for Pascal-type strings. This is not currently implemented.
The conversion formats %D, %O, and %U are not standard and are provided
only for backward compatibility. 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 impos‐
sible to assure. For safety, programmers should use the snprintf in‐
terface instead. Unfortunately, this interface is not portable.
GNO 15 September 1997 PRINTF(3)
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