strfromd, strfromf, strfroml - convert a floating-point value into a string
#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);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
strfromd(),
strfromf(),
strfroml():
__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 character ('\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.
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 character. Thus,
a return value of
n or greater means that the output was truncated.
The
strfromd(),
strfromf(), and
strfroml() functions are
available in glibc since version 2.25.
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.
C99, ISO/IEC TS 18661-1.
The
strfromd(),
strfromf(), and
strfroml() functions take
account of the
LC_NUMERIC category of the current locale.
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)