PRINTF(3V)

NAME
printf, fprintf, sprintf - formatted output conversion

SYNOPSIS

#include <stdio.h>

int printf(format [ , arg... ] )
char *format;

int fprintf(stream, format [ , arg... ] )
FILE *stream;
char *format;

char *sprintf(s, format [ , arg... ] )
char *s, *format;

SYSTEM V SYNOPSIS

The routines above are available as shown, except:

int sprintf(s, format [ , arg... ] )
char *s, *format;

The following are provided for XPG2 compatibility:

#define	nl_printf		printf
#define	nl_fprintf	fprintf
#define	nl_sprintf	sprintf

DESCRIPTION

printf() places output on the standard output stream stdout. fprintf() places output on the named output stream. sprintf() places “output”, followed by the null character (\0), in consecutive bytes starting at *s; it is the user's responsibility to ensure that enough storage is available.

Each of these functions converts, formats, and prints its args under control of the format. The format is a character string which contains two types of objects: plain characters, which are simply copied to the output stream, and conversion specifications, each of which causes conversion and printing of zero or more args. The results are undefined if there are insufficient args for the format. If the format is exhausted while args remain, the excess args are simply ignored.

Each conversion specification is introduced by either the % character or by the character sequence %digit $, after which the following appear in sequence:

    •   Zero or more flags, which modify the meaning of the conversion specification.

    •   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 on the left (or right, if the left-adjustment flag `-', described below, has been given) to the field width. The padding is with blanks unless the field width digit string starts with a zero, in which case the padding is with zeros.

    •   A precision that gives the minimum number of digits to appear for the d, i, o, u, x, or X conversions, the number of digits to appear after the decimal point for the e, E, and f conversions, the maximum number of significant digits for the g and G conversion, or the maximum number of characters to be printed from a string in s conversion. The precision takes the form of a period (.) followed by a decimal digit string; a null digit string is treated as zero. Padding specified by the precision overrides the padding specified by the field width.

    •   An optional l (ell) specifying that a following d, i, o, u, x, or X conversion character applies to a long integer arg. An l before any other conversion character is ignored.

    •   A character that indicates 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 integer arg supplies the field width or precision. The arg that is actually converted is not fetched until the conversion letter is seen, so the args specifying field width or precision must appear before the arg (if any) to be converted. A negative field width argument is taken as a `-' flag followed by a positive field width. If the precision argument is negative, it will be changed to zero.

The flag characters and their meanings are:
-   The result of the conversion will be left-justified within the field.
+   The result of a signed conversion will always begin with a sign (+ or -).
blank
    If the first character of a signed conversion is not a sign, a blank will be prefixed to the result. This implies that if the blank and + flags both appear, the blank flag will be ignored.
#   This flag specifies that the value is to be converted to an “alternate form”. For c, d, i, s, and u conversions, the flag has no effect. For o conversion, it increases the precision to force the first digit of the result to be a zero. For x or X conversion, a non-zero result will have 0x or 0X prefixed to it. For e, E, f, g, and G conversions, the result will always contain a decimal point, even if no digits follow the point (normally, a decimal point appears in the result of these conversions only if a digit follows it). For g and G conversions, trailing zeroes will not be removed from the result (which they normally are).

The conversion characters and their meanings are:

d,i,o,p,u,x,X
    The integer arg is converted to signed decimal (d or i), unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal notation (x, p, and X), respectively; the letters abcdef are used for x and p conversion and the letters ABCDEF for X conversion. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeroes. For compatibility with older versions, padding with leading zeroes may alternatively be specified by prepending a zero to the field width. This does not imply an octal value for the field width. The default precision is 1. The result of converting a zero value with a precision of zero is a null string.
f   The float or double arg is converted to decimal notation in the style ``[-]ddd.ddd'' where the number of digits after the decimal point is equal to the precision specification. If the precision is missing, 6 digits are given; if the precision is explicitly 0, no digits and no decimal point are printed.
e,E The float or double arg is converted in the style “[-]d.ddde±ddd,” where there is one digit before the decimal point and the number of digits after it is equal to the precision; when the precision is missing, 6 digits are produced; if the precision is zero, no decimal point appears. The E format code will produce a number with E instead of e introducing the exponent. The exponent always contains at least two digits.
g,G The float or double arg is printed in style f or e (or in style E in the case of a G format code), with the precision specifying the number of significant digits. The style used depends on the value converted: style e or E will be used only if the exponent resulting from the conversion is less than -4 or greater than the precision. Trailing zeroes are removed from the result; a decimal point appears only if it is followed by a digit.

The e, E, f, g, and G formats print IEEE indeterminate values (infinity or not-a-number) as “Infinity” or “NaN” respectively.

c   The character arg is printed.
s   The arg is taken to be a string (character pointer) and characters from the string are printed until a null character (\0) is encountered or until the number of characters indicated by the precision specification is reached. If the precision is missing, it is taken to be infinite, so all characters up to the first null character are printed. A NULL value for arg will yield undefined results.
n   The argument arg is a pointer to an integer into which is written the number of characters written to the output so far by this call to one of the printf() functions. No argument is converted.
%   Print a %; no argument is converted.

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 simply expanded to contain the conversion result. Padding takes place only if the specified field width exceeds the actual width. Characters generated by printf() and fprintf() are printed as if putc(3S) had been called.

All forms of the printf() functions allow for the insertion of a language dependent radix character in the output string. The radix character is defined by the program's locale (category LC_NUMERIC). In the "C" locale, or in a locale where the radix character is not defined, the radix character defaults to `.'.

Conversions can be applied to the nth argument in the argument list, rather than the next unused argument. In this case, the conversion character % is replaced by the sequence %digit $, where digit is a decimal integer n in the range [1,9], giving the position of the argument in the argument list. This feature provides for the definition of format strings that select arguments in an order appropriate to specific languages.

In format strings containing the %digit$ form of a conversion specification, a field width or precision may be indicated by the sequence *digit $, where digit is a decimal integer in the range [1,9] giving the position in the argument list of an integer arg containing the field width or precision.

The format string can contain either numbered argument specifications (that is, %digit$ and *digit $), or unnumbered argument specifications (that is % and *), but not both. The results of mixing numbered and unnumbered specifications is undefined. When numbered argument specifications are used, specifying the nth argument requires that all the leading arguments, from the first to the (n-1)th be specified in the format string.

SYSTEM V DESCRIPTION

XPG2 requires that nl_printf, nl_fprintf and nl_sprintf be defined as printf, fprintf and sprintf, respectively for backward compatibility

RETURN VALUES

On success, printf() and fprintf() return the number of characters transmitted, excluding the null character. On failure, they return EOF.

sprintf() returns s.

SYSTEM V RETURN VALUES

On success, sprintf() returns the number of characters transmitted, excluding the null character. On failure, it returns EOF.

EXAMPLES

    printf(format, weekday, month, day, hour, min);

In American usage, format could be a pointer to the string:

    "%s, %s %d, %d:%.2d\n"

producing the message:

    Sunday, July 3,10:02

Whereas for German usage, format could be a pointer to the string:

    "%1$s, %3$d.%2$s,%4$d:%5$.2d\n"

producing the message:

    Sonntag, 3.Juli,10:02

To print pi to 5 decimal places:

    printf("pi = %.5f", 4 * atan(1. 0));

SEE ALSO
econvert(3), putc(3S), scanf(3V), setlocale(3V), varargs(3), vprintf(3V)

BUGS

Very wide fields (>128 characters) fail.