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 stdarg(3)) are converted for output.

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. If size is zero, str may be a null pointer and no characters will be written; the number of bytes that would have been written excluding the terminating ‘\0’ byte, or -1 on error, will be returned.

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 introduced by the character %. The arguments must correspond properly (after type promotion) with the conversion specifier. After the %, the following appear in sequence:

• An optional field, consisting of a decimal digit string followed by a $ specifying the next argument to access. If this field is not provided, the argument following the last argument accessed will be used. Arguments are numbered starting at 1.
• Zero or more of the following flags:
  • A hash ‘#’ character specifying that the value should be converted to an “alternate form”. 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 a, A, e, E, f, 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. For all other formats, behaviour is undefined.
  • 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, 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, a, A, e, E, f, 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.
• 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 a, A, e, E, f, 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.
• An optional length modifier, that specifies the size of the argument. The following length modifiers are valid for the d, i, n, o, u, x, or X conversions:

  Modifier	d, i	o, u, x, X	n
  hh	signed char	unsigned char	signed char *
  h	short	unsigned short	short *
  l (ell)	long	unsigned long	long *
  ll (ell ell)	long long	unsigned long long	long long *
  j	intmax_t	uintmax_t	intmax_t *
  t	ptrdiff_t	(see note)	ptrdiff_t *
  z	(see note)	size_t	(see note)
  q (deprecated)	quad_t	u_quad_t	quad_t *

  Note: the t modifier, when applied to an o, u, x, or X conversion, indicates that the argument is of an unsigned type equivalent in size to a ptrdiff_t. The z modifier, when applied to a d or i conversion, indicates that the argument is of a signed type equivalent in size to a size_t. Similarly, when applied to an n conversion, it indicates that the argument is a pointer to a signed type equivalent in size to a size_t.

  The following length modifiers are valid for the a, A, e, E, f, F, g, or G conversions:

  Modifier	e, E, f, F, g, G
  l (ell)	double (ignored: same behavior as without it)
  L	long double

  The following length modifier is valid for the c or s conversions:

  Modifier	c	s
  l (ell)	wint_t	wchar_t *

• A character that specifies the type of conversion to be applied.

A field width or precision, or both, may be indicated by an asterisk ‘*’ or an asterisk followed by one or more decimal digits and a ‘$’ instead of a digit string. In this case, an int argument supplies the field width or precision. A negative field width is treated as a left adjustment flag followed by a positive field width; a negative precision is treated as though it were missing. If a single format directive mixes positional (nn$) and non-positional arguments, the results are undefined.

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±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.

If the argument is infinity, it will be converted to [-]inf (e) or [-]INF (E), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (e) or [-]NAN (E), respectively.

fF

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.

If the argument is infinity, it will be converted to [-]inf (f) or [-]INF (F), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (f) or [-]NAN (F), respectively.

gG

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.

If the argument is infinity, it will be converted to [-]inf (g) or [-]INF (G), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (g) or [-]NAN (G), respectively.

aA

The double argument is rounded and converted to hexadecimal notation in the style [-]0xh.hhhp[±]d where the number of digits after the hexadecimal-point character is equal to the precision specification. If the precision is missing, it is taken as enough to represent the floating-point number exactly, and no rounding occurs. If the precision is zero, no hexadecimal-point character appears. The p is a literal character ‘p’, and the exponent consists of a positive or negative sign followed by a decimal number representing an exponent of 2. The A conversion uses the prefix “0X” (rather than “0x”), the letters “ABCDEF” (rather than “abcdef”) to represent the hex digits, and the letter ‘P’ (rather than ‘p’) to separate the mantissa and exponent.

Note that there may be multiple valid ways to represent floating-point numbers in this hexadecimal 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 will be minimized. Zeroes are always represented with a mantissa of 0 (preceded by a ‘-’ if appropriate) and an exponent of +0.

If the argument is infinity, it will be converted to [-]inf (a) or [-]INF (A), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (a) or [-]NAN (A), respectively.

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 NUL 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.

#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 == -1 || ret >= sizeof(buffer))
	goto toolong;