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The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group

 NAME

fprintf, printf, snprintf, sprintf - print formatted output

 SYNOPSIS



#include <stdio.h>

int fprintf(FILE *stream, const char *format, ...);
int printf(const char *format, ...);
int snprintf(char *s, size_t n, const char *format, ...);
int sprintf(char *s, const char *format, ...);

 DESCRIPTION

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

snprintf() is identical to sprintf() with the addition of the n argument, which states the size of the buffer referred to by s.

Each of these functions converts, formats and prints its arguments under control of the format. The format is a character string, beginning and ending in its initial shift state, if any. The format is composed of zero or more directives: ordinary characters, which are simply copied to the output stream and conversion specifications, each of which results in the fetching of zero or more arguments. The results are undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments are evaluated but are otherwise ignored.

Conversions can be applied to the nth argument after the format in the argument list, rather than to the next unused argument. In this case, the conversion character % (see below) is replaced by the sequence %n$ where n is a decimal integer in the range [1, {NL_ARGMAX}], 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 (see the EXAMPLES section).

In format strings containing the %n$ form of conversion specifications, numbered arguments in the argument list can be referenced from the format string as many times as required.

In format strings containing the % form of conversion specifications, each argument in the argument list is used exactly once.

All forms of the fprintf() functions allow for the insertion of a language-dependent radix character in the output string. The radix character is defined in the program's locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character defaults to a period (.).

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

A field width, or precision, or both, may be indicated by an asterisk (*). In this case an argument of type int supplies the field width or precision. Arguments specifying field width, or precision, or both must appear in that order before the argument, if any, to be converted. A negative field width is taken as a - flag followed by a positive field width. A negative precision is taken as if the precision were omitted.  In format strings containing the %n$ form of a conversion specification, a field width or precision may be indicated by the sequence *m$, where m is a decimal integer in the range [1, {NL_ARGMAX}] giving the position in the argument list (after the format argument) of an integer argument containing the field width or precision, for example:


printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

The format can contain either numbered argument specifications (that is, %n$ and *m$), or unnumbered argument specifications (that is, % and *), but normally not both. The only exception to this is that %% can be mixed with the %n$ form. The results of mixing numbered and unnumbered argument specifications in a format string are 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, are specified in the format string.

The flag characters and their meanings are:

'
The integer portion of the result of a decimal conversion (%i, %d, %u, %f, %g or %G) will be formatted with thousands' grouping characters. For other conversions the behaviour is undefined. The non-monetary grouping character is used.
-
The result of the conversion will be left-justified within the field. The conversion will be right-justified if this flag is not specified.
+
The result of a signed conversion will always begin with a sign (+ or -). The conversion will begin with a sign only when a negative value is converted if this flag is not specified.
space
If the first character of a signed conversion is not a sign or if a signed conversion results in no characters, a space will be prefixed to the result. This means that if the space and + flags both appear, the space flag will be ignored.
#
This flag specifies that the value is to be converted to an alternative form. For o conversion, it increases the precision (if necessary) to force the first digit of the result to be 0. For x or X conversions, a non-zero result will have 0x (or 0X) prefixed to it. For e, E, f, g or G conversions, the result will always contain a radix character, even if no digits follow the radix character. Without this flag, a radix character appears in the result of these conversions only if a digit follows it. For g and G conversions, trailing zeros will not be removed from the result as they normally are. For other conversions, the behaviour is undefined.
0
For d, i, o, u, x, X, e, E, f, g and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the 0 and - flags both appear, the 0 flag will be ignored. For d, i, o, u, x and X conversions, if a precision is specified, the 0 flag will be ignored. If the 0 and ' flags both appear, the grouping characters are inserted before zero padding. For other conversions, the behaviour is undefined.

The conversion characters and their meanings are:

d, i
The int argument is converted to a signed decimal in the style [-]dddd. 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 zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.
o
The unsigned int argument is converted to unsigned octal format in the style dddd. 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 zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.
u
The unsigned int argument is converted to unsigned decimal format in the style dddd. 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 zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.
x
The unsigned int argument is converted to unsigned hexadecimal format in the style dddd; the letters abcdef are used. 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 zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.
X
Behaves the same as the x conversion character except that letters ABCDEF are used instead of abcdef.
f
The double argument is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the radix character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly 0 and no # flag is present, no radix character appears. If a radix character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits. The fprintf() family of functions may make available character string representations for infinity and NaN.
e, E
The double argument is converted in the style [-]d.ddde±dd, where there is one digit before the radix character (which is non-zero if the argument is non-zero) 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 0 and no # flag is present, no radix character appears. The value is rounded to the appropriate number of digits. The E conversion character will produce a number with E instead of e introducing the exponent. The exponent always contains at least two digits. If the value is 0, the exponent is 0. The fprintf() family of functions may make available character string representations for infinity and NaN.
g, G
The double argument is converted in the style f or e (or in the style E in the case of a G conversion character), with the precision specifying the number of significant digits. If an explicit precision is 0, it is taken as 1. The style used depends on the value converted; style e (or E) will be used only if the exponent resulting from such a conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional portion of the result; a radix character appears only if it is followed by a digit. The fprintf() family of functions may make available character string representations for infinity and NaN.
c
The int argument is converted to an unsigned char, and the resulting byte is written. If an l (ell) qualifier is present, the wint_t argument is converted as if by an ls conversion specification with no precision and an argument that points to a two-element array of type wchar_t, the first element of which contains the wint_t argument to the ls conversion specification and the second element contains a null wide-character.
s
The argument must be a pointer to an array of char, Bytes from the array are written up to (but not including) any terminating null byte. If the precision is specified, no more than that many bytes are written. If the precision is not specified or is greater than the size of the array, the array must contain a null byte. If an l (ell) qualifier is present, the argument must be a pointer to an array of type wchar_t. Wide-characters from the array are converted to characters (each as if by a call to the wcrtomb() function, with the conversion state described by an mbstate_t object initialised to zero before the first wide-character is converted) up to and including a terminating null wide-character. The resulting characters are written up to (but not including) the terminating null character (byte). If no precision is specified, the array must contain a null wide-character. If a precision is specified, no more than that many characters (bytes) are written (including shift sequences, if any), and the array must contain a null wide-character if, to equal the character sequence length given by the precision, the function would need to access a wide-character one past the end of the array. In no case is a partial character written.
p
The argument must be a pointer to void. The value of the pointer is converted to a sequence of printable characters, in an implementation-dependent manner.
n
The argument must be a pointer to an integer into which is written the number of bytes written to the output so far by this call to one of the fprintf() functions. No argument is converted.
C
Same as lc.
S
Same as ls.
%
Print a %; no argument is converted. The entire conversion specification must be %%.

If a conversion specification does not match one of the above forms, the behaviour is undefined.

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 fprintf() and printf() are printed as if fputc() had been called.

The st_ctime and st_mtime fields of the file will be marked for update between the call to a successful execution of fprintf() or printf() and the next successful completion of a call to fflush() or fclose() on the same stream or a call to exit() or abort().

 RETURN VALUE

Upon successful completion, these functions return the number of bytes transmitted excluding the terminating null in the case of sprintf() or snprintf() or a negative value if an output error was encountered.

If the value of n is zero on a call to snprintf(), an unspecified value less than 1 is returned.

 ERRORS

For the conditions under which fprintf() and printf() will fail and may fail, refer to fputc() or fputwc().

In addition, all forms of fprintf() may fail if:

[EILSEQ]
A wide-character code that does not correspond to a valid character has been detected.
[EINVAL]
There are insufficient arguments.

In addition, printf() and fprintf() may fail if:

[ENOMEM]
Insufficient storage space is available.

 EXAMPLES

To print the language-independent date and time format, the following statement could be used:

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

For 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

 APPLICATION USAGE

If the application calling fprintf() has any objects of type wint_t or wchar_t, it must also include the header <wchar.h> to have these objects defined.

 FUTURE DIRECTIONS

None.

 SEE ALSO

fputc(), fscanf(), setlocale(), wcrtomb(), <stdio.h>, <wchar.h>, the XBD specification, Locale , Base Working Group Resolution # bwg98-006.

DERIVATION

Derived from Issue 1 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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