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

#include <stdlib.h>
int strfromd(char str[restrict .n], size_t n,
 const char *restrict format, double fp);
int strfromf(char str[restrict .n], size_t n,
 const char *restrict format, float fp);
int strfroml(char str[restrict .n], size_t n,
 const char *restrict format, long double fp);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    __STDC_WANT_IEC_60559_BFP_EXT__

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 byte ('\0') 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 '%'. 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.

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 byte. Thus, a return value of n or greater means that the output was truncated.

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);

atof(3), snprintf(3), strtod(3)