printf(3S)

NAME

printf, fprintf, sprintf − print formatted output

SYNOPSIS

#include <stdio.h>

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

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

SYNOPSIS (SYSV.2)

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

SYNOPSIS (4.2BSD)

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

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 that contains two types of objects: plain characters, which are simply copied to the output stream, and conversion specifications, each of which results in fetching 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 the character %. After the %, the following appear in sequence:

An optional field, consisting of a decimal digit string followed by a $, specifying the next args to be converted. If this field is not provided, the args following the last args converted will be used.

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. If the field width for an s conversion is preceded by a 0, the string is right adjusted with zero-padding on the left.

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.

An optional h specifying that a following d, i, o, u, x, or X conversion character applies to a short int or unsigned short int arg; an optional h specifying that a following n conversion character applies to a pointer to short int arg; an optional l (ell) specifying that a following d, i, o, u, x, or X conversion character applies to a long int or unsigned long int arg; an optional l specifying that a following n conversion character applies to a pointer to long arg; or an optional L specifying that a following e, E, f, g, or G conversion character applies to a long double arg. A h, l, or L before any other conversion character is ignored.

A character that indicates the type of conversion to be applied.

A field width or precision 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.

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).

0 This flag causes padding with leading zeroes. For d, i, o, u, x, X, e, E, f, g, and G conversions, leading zeroes (following any sign or base indications) are used to pad to the field width (i.e., no space padding is performed). If both the 0 and the - flags appear, the default action is to ignore the the second of the flags. In programs that are linked in one of the ANSI C compilation modes (see hc(1)), and in programs which are linked in 88open OCS-compliant mode, the 0 flag will be ignored. For d, i, o, u, x, and X conversions, the 0 flag will be ignored if a precision is specified. For other conversions the behavior is undefined.

The conversion characters and their meanings are:

d,i,o,u,x,X
The integer arg is converted to signed decimal, unsigned octal, decimal, or hexadecimal notation (x and X), respectively; the letters abcdef are used for x 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, six digits are output; if the precision is explicitly 0, no decimal point appears.

e,E The float or double arg is converted in the style ”[−]d.ddde±dd,” 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, six 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 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.

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 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.

p The args should be a pointer to void. The value of the pointer is converted to an implementation-defined set of sequences of printable characters, which should be the same as the set of sequences that are matched by the %p conversion of the scanf function.

n The arg is taken to be an integer pointer into which is written the number of characters output so far to the output stream by the current printf call. 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. Characters generated by printf and fprintf are printed as if putc(3S) had been called.

RETURN VALUE

In the att universe, each function returns the number of characters transmitted (not including the \0 in the case of sprintf), or a negative value if an output error was encountered.

In the ucb universe, printf and fprintf return 0 upon successful completion, or a negative value if an output error was encountered. Sprintf returns its first argument: s.

EXAMPLES

To print a date and time in the form ”Sunday, July 3, 10:02,” where weekday and month are pointers to null-terminated strings:

printf("%s, %s %d, %d:%.2d", weekday, month, day, hour, min);

To print π to 5 decimal places:

printf("pi = %.5f", 4 ∗ atan(1.0));

NOTES

The f, e, and g conversion specifiers cause the corresponding argument to be accessed as if it were a double precision floating point argument. When floating point arguments are passed as single precision values, they may print out incorrectly under one of these specifiers. It is advised that floating point arguments be passed as double precision values in such cases.

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