PRINTF(3) 386BSD Programmer's Manual PRINTF(3)
NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,
vsnprintf - 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
standard output stream; fprintf() and vfprintf() write output to the
given output stream; sprintf(), snprintf(), vsprintf(), and vsnprintf()
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). Snprintf() and vsnprintf() will write at most size-1 of the
characters printed into the output string (the size'th 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 infinte 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 introduced by the
character %. The arguments must correspond properly (after type
promotion) with the conversion specifier. After the %, the following
appear in sequence:
o Zero or more of the following flags:
- a # character 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.
- 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 (Mc 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).
- a `+' character specifying that a sign always be placed
before a number produced by a signed conversion. A `+'
overrides a space if both are used.
o 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.
o 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.
o 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.
o 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.
o 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 VAX and Tahoe
compilers).
o 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
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+-dd 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 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 C3.159-1989 (``ANSI C'').
HISTORY
The functions snprintf() and vsnprintf() are new to this release.
BUGS
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
impossible to assure. For safety, programmers should use the snprintf()
interface instead. Unfortunately, this interface is not portable.
BSD Experimental July 30, 1991 4