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DESCRIPTION

The printf() and vprintf() functions write output to the standard output stream, stdout; fprintf() and vfprintf() write output to the supplied stream pointer stream; dprintf() and vdprintf() write output to the given file descriptor; sprintf(), snprintf(), vsprintf(), and vsnprintf() write to the character string str; asprintf() and vasprintf() write to a dynamically allocated string that is stored in ret.

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 va_start(3)) are converted for output.

snprintf() and vsnprintf() write at most size-1 characters to str, followed by a terminating ‘\0’. If size is zero, no characters are written and str may be a NULL pointer.

sprintf() and vsprintf() effectively assume an infinite size; their use is not recommended.

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. The arguments must correspond properly (after type promotion) with the conversion specifiers.

The overall syntax of a conversion specification is:

Not all combinations of these parts are meaningful; see the description of the individual conversion specifiers for details.

The parts of a conversion specification are as follows:

%

A literal percent character begins a conversion specification.

argno$

An unsigned decimal digit string followed by a dollar character specifies the index of the next argument to access. By default, the argument following the last argument accessed is used. Arguments are numbered starting at 1.

flags

Zero or more of the following flag characters can be given:

# (hash)

Use an alternate form for the output. The effect differs depending on the conversion specifier.

‘ ’ (space)

For signed conversions, print a space character before a positive number.

+ (plus)

For signed conversions, always print a sign before the number, even if it is positive. This overrides the space flag if both are specified.

0 (zero)

Pad numbers with leading zeros instead of space characters to fill the field width. This flag is ignored if the precision modifier is also given, which in this case specifies mindigits.

- (minus)

Left adjust: pad to the field width with space characters on the right rather than on the left. This overrides the ‘0’ flag if both are specified.

width

An unsigned decimal digit string specifies a minimum field width in bytes. Unless the ‘0’ or ‘-’ flag is given, the value is right adjusted in the field and padded with space characters on the left. By default, no padding is added. 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.

.precision

The meaning of an unsigned decimal digit string prefixed with a period character depends on the conversion specifier: it provides the minimum number of digits for integer conversions, of decimals for some floating point conversions and of significant digits for others, or the maximum number of bytes to print for string conversions.

A field width or precision, or both, may alternatively be indicated as *[argno$], i.e. as an asterisk optionally followed by an unsigned decimal digit string and a dollar sign. In this case, an additional int argument supplies the field width or precision. If a single conversion specification tries to use arguments both with and without argno$ modifiers, the result is undefined.

size

An argument size modifier. The syntax, the precise meaning, and the default size of the argument depend on the following conversion character.

conversion

Each conversion specification ends with a conversion specifier, which is a single letter determining which argument type is expected and how it is formatted.

The conversion specifiers are:

%a

%[argno$][#][ |+][-|0][width][.hexadecimals][L|l]a

The double argument is converted to the hexadecimal notation [-]0xh.hhhp±d with one digit before the hexadecimal point. If specified, the number is rounded to hexadecimals after the hexadecimal point; otherwise, enough digits are printed to represent it exactly. The hexadecimal point is only printed if at least one digit follows it or if the ‘#’ flag is given.

The exponent is expressed in base 2, not in base 16. Consequently, there are multiple ways to represent a number in this format. For example, 0x3.24p+0, 0x6.48p-1, and 0xc.9p-2 are all equivalent. The format chosen depends on the internal representation of the number, but the implementation guarantees that the length of the mantissa is minimized. Zeroes are always represented with a mantissa of ‘0’ (preceded by a sign if appropriate) and an exponent of ‘+0’.

If the argument is infinity, it is converted to ‘[-]inf’. If the argument is not-a-number (NaN), it is converted to ‘[-]nan’.

%La is similar to %a except that it takes an argument of long double. %la (ell a) is an alias for %a.

%A

Identical to %a except that upper case is used, i.e. ‘0X’ for the prefix, ‘0123456789ABCDEF’ for the digits, ‘P’ to introduce the exponent, and ‘[-]INF’ and ‘[-]NAN’ for infinity and not-a-number, respectively.

%c

%[argno$][-][width]c

The int argument is converted to an unsigned char, and the resulting single-byte character is written, with optional padding.

%lc

%[argno$][-][width]lc

The wint_t argument is converted to a multibyte character according to the current LC_CTYPE locale(1), and that character is written. For example, under a UTF-8 locale on OpenBSD, ‘printf("%lc", 0x03c0)’ writes the greek letter pi, whereas the same call fails under the default POSIX locale. Padding assures at least width bytes are printed; the number of characters printed may be smaller, and the number of display columns occupied may be smaller or larger.

%d

%[argno$][ |+][-|0][width][.mindigits][size]d

The int argument is converted to signed decimal notation. If specified, at least mindigits are printed, padding with leading zeros if needed. The following are similar to %d except that they take an argument of a different size:

%hhd	signed char
%hd	signed short
%d	signed int
%ld	signed long (percent ell dee)
%lld	signed long long (percent ell ell dee)
%jd	intmax_t
%td	ptrdiff_t
%zd	ssize_t
%qd	quad_t (deprecated)

%D

A deprecated alias for %ld.

%e

%[argno$][#][ |+][-|0][width][.decimals][L|l]e

The double argument is rounded and converted to the scientific notation [-]d.dddddde±dd with one digit before the decimal point and decimals, or six digits by default, after it. If decimals is zero and the ‘#’ flag is not given, the decimal point is omitted. The exponent always contains at least two digits; if the value is zero, the exponent is ‘+00’. If the argument is infinity, it is converted to ‘[-]inf’. If the argument is not-a-number (NaN), it is converted to ‘[-]nan’.

%Le is similar to %e except that it takes an argument of long double. %le (ell e) is an alias for %e.

%E

Identical to %e except that upper case is used, i.e. ‘E’ instead of ‘e’ to introduce the exponent and ‘[-]INF’ and ‘[-]NAN’ for infinity and not-a-number, respectively.

%f

%[argno$][#][ |+][-|0][width][.decimals][L|l]f

The double argument is rounded and converted to the decimal notation [-]ddd.dddddd with decimals, or six digits by default, after the decimal point. If decimals is zero and the ‘#’ flag is not given, the decimal point is omitted. If a decimal point appears, at least one digit appears before it. If the argument is infinity, it is converted to ‘[-]inf’. If the argument is not-a-number (NaN), it is converted to ‘[-]nan’.

%Lf is similar to %f except that it takes an argument of long double. %lf (ell eff) is an alias for %f.

%F

Identical to %f except that upper case is used, i.e. ‘[-]INF’ and ‘[-]NAN’ for infinity and not-a-number, respectively.

%g

%[argno$][#][ |+][-|0][width][.significant][L|l]g

The double argument is converted in style %f or %e (general floating point notation) with significant digits, or six significant digits by default. If significant is zero, one is used instead. Style %e is used if the exponent from its conversion is less than -4 or greater than or equal to significant. Unless the ‘#’ flag is given, trailing zeros are removed from the fractional part of the result, and the decimal point only appears if it is followed by at least one digit.

%Lg is similar to %g except that it takes an argument of long double. %lg (ell gee) is an alias for %g.

%G

Identical to %g except that upper case is used, i.e. ‘E’ instead of ‘e’ to introduce the exponent and ‘[-]INF’ and ‘[-]NAN’ for infinity and not-a-number, respectively.

%i

An alias for %d, supporting the same modifiers.

%n

%[argno$][size]n

The %n conversion specifier has serious security implications, so it was changed to no longer store the number of bytes written so far into the variable indicated by the pointer argument. Instead a syslog(3) message will be generated, after which the program is aborted with SIGABRT.

%o

%[argno$][#][-|0][width][.mindigits][size]o

Similar to %u except that the unsigned int argument is converted to unsigned octal notation. If the ‘#’ flag is given, mindigits is increased such that the first digit printed is a zero, except if a zero value is printed with an explicit mindigits of zero.

%O

A deprecated alias for %lo.

%p

The void * pointer argument is printed in hexadecimal, similar to %#x or %#lx depending on the size of pointers.

%s

%[argno$][-][width][.maxbytes]s

Characters from the char * (string) argument are written up to (but not including) a terminating NUL character. If maxbytes is specified, at most maxbytes bytes are written; in that case, no NUL character needs to be present.

%ls

%[argno$][-][width][.maxbytes]ls

The wchar_t * (wide character string) argument is converted to a multibyte character string according to the current LC_CTYPE locale(1) up to (but not including) a terminating NUL character, and that multibyte character string is written. If maxbytes is specified, at most maxbytes bytes are written; in that case, no NUL character needs to be present. If a multibyte character does not fit into the rest of maxbytes, it is omitted together with the rest of the argument string; partial characters are not written. Locale dependency and padding work in the same way as for %lc.

%u

%[argno$][-|0][width][.mindigits][size]u

The unsigned int argument is converted to unsigned decimal notation. If specified, at least mindigits are printed, padding with leading zeros if needed. The following are similar to %u except that they take an argument of a different size:

%hhu	unsigned char
%hu	unsigned short
%u	unsigned int
%lu	unsigned long (percent ell u)
%llu	unsigned long long (percent ell ell u)
%ju	uintmax_t
%tu	unsigned type of same size as ptrdiff_t
%zu	size_t
%qu	u_quad_t (deprecated)

%U

A deprecated alias for %lu.

%x

%[argno$][#][-|0][width][.mindigits][size]x

Similar to %u except that the unsigned int argument is converted to unsigned hexadecimal notation using the digits ‘0123456789abcdef’. If the ‘#’ flag is given, the string ‘0x’ is prepended unless the value is zero.

%X

Identical to %x except that upper case is used, i.e. ‘0X’ for the optional prefix and ‘0123456789ABCDEF’ for the digits.

%%

A single percent sign (‘%’) is written. No argument is converted. The complete conversion specification is ‘%%’; no modifiers can be inserted between the two percent signs.

#include <stdio.h>

fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
    weekday, month, day, hour, min);

#include <math.h>
#include <stdio.h>

fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

#include <stdarg.h>
#include <stdio.h>
#include <stdlib.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);
}

int ret = snprintf(buffer, sizeof(buffer), "%s", string);
if (ret < 0 || (size_t)ret >= sizeof(buffer))
	goto toolong;