Linux man pages

Section (3) strfromd

Linux manual pages	Section 3

Name

strfromd, strfromf, strfroml — convert a floating-point value into a string

Synopsis

#include <stdlib.h>

`int strfromd(`	char *restrict `str`,
	size_t `n`,
	const char *restrict `format`,
	double `fp)`;

`int strfromf(`	char *restrict `str`,
	size_t `n`,
	const char *restrict `format`,
	float `fp)`;

`int strfroml(`	char *restrict `str`,
	size_t `n`,
	const char *restrict `format`,
	long double `fp)`;

Note

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
`strfromd`(), `strfromf`(), `strfroml`():
__STDC_WANT_IEC_60559_BFP_EXT__

DESCRIPTION

These functions convert a floating-point value, fp, into a string of characters, str, with a configurable format string. At most n characters are stored into str.

The terminating null character (_zsingle_quotesz__zsingle_quotesz_) is written if and only if n is sufficiently large, otherwise the written string is truncated at n characters.

The strfromd(), strfromf(), and strfroml() functions are equivalent to

snprintf(str, n, format, fp);

except for the format string.

Format of the format string

The format string must start with the character _zsingle_quotesz_%_zsingle_quotesz_. This is followed by an optional precision which starts with the period character (.), followed by an optional decimal integer. If no integer is specified after the period character, a precision of zero is used. Finally, the format string should have one of the conversion specifiers a, A, e, E, f, F, g, or G.

The conversion specifier is applied based on the floating-point type indicated by the function suffix. Therefore, unlike snprintf(), the format string does not have a length modifier character. See snprintf(3) for a detailed description of these conversion specifiers.

The implementation conforms to the C99 standard on conversion of NaN and infinity values:

If fp is a NaN, +NaN, or -NaN, and f (or a, e, g) is the conversion specifier, the conversion is to nan, nan, or -nan, respectively. If F (or A, E, G) is the conversion specifier, the conversion is to NAN or -NAN.

Likewise if fp is infinity, it is converted to [-]inf or [-]INF.

A malformed format string results in undefined behavior.

RETURN VALUE

The strfromd(), strfromf(), and strfroml() functions return the number of characters that would have been written in str if n had enough space, not counting the terminating null character. Thus, a return value of n or greater means that the output was truncated.

VERSIONS

The strfromd(), strfromf(), and strfroml() functions are available in glibc since version 2.25.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7) and the POSIX Safety Concepts section in GNU C Library manual.

Interface	Attribute	Value
`strfromd`(), `strfromf`(), `strfroml`()	Thread safety	MT-Safe locale
	Asynchronous signal safety	AS-Unsafe heap
	Asynchronous cancellation safety	AC-Unsafe mem

	Note
	These attributes are preliminary.

CONFORMING TO

C99, ISO/IEC TS 18661-1.

NOTES

The strfromd(), strfromf(), and strfroml() functions take account of the LC_NUMERIC category of the current locale.

EXAMPLES

To convert the value 12.1 as a float type to a string using decimal notation, resulting in 12.100000:

#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdlib.h>
int ssize = 10;
char s[ssize];
strfromf(s, ssize, %f, 12.1);

To convert the value 12.3456 as a float type to a string using decimal notation with two digits of precision, resulting in 12.35:

#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdlib.h>
int ssize = 10;
char s[ssize];
strfromf(s, ssize, %.2f, 12.3456);

To convert the value 12.345e19 as a double type to a string using scientific notation with zero digits of precision, resulting in 1E+20:

#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdlib.h>
int ssize = 10;
char s[ssize];
strfromd(s, ssize, %.E, 12.345e19);

COLOPHON

This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

  Copyright (c) 2016, IBM Corporation.
Written by Wainer dos Santos Moschetta <wainersmlinux.vnet.ibm.com>

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References consulted:
  Glibc 2.25 source code and manual.
  C99 standard document.
  ISO/IEC TS 18661-1 technical specification.
  snprintf and other man.3 pages.