man3/strfromd.3

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.\" References consulted:
.\"   Glibc 2.25 source code and manual.
.\"   C99 standard document.
.\"   ISO/IEC TS 18661-1 technical specification.
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.\"
.TH STRFROMD 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
strfromd, strfromf, strfroml \- convert a floating-point value into
a string
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.BI "int strfromd(char *restrict " str ", size_t " n ,
.BI "             const char *restrict " format ", double " fp ");"
.BI "int strfromf(char *restrict " str ", size_t " n ,
.BI "             const char *restrict " format ", float "fp ");"
.BI "int strfroml(char *restrict " str ", size_t " n ,
.BI "             const char *restrict " format ", long double " fp ");"
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR strfromd (),
.BR strfromf (),
.BR strfroml ():
.nf
    __STDC_WANT_IEC_60559_BFP_EXT__
.fi
.SH DESCRIPTION
These functions convert a floating-point value,
.IR fp ,
into a string of characters,
.IR str ,
with a configurable
.IR format
string.
At most
.I n
characters are stored into
.IR str .
.PP
The terminating null byte ('\e0') is written if and only if
.I n
is sufficiently large, otherwise the written string is truncated at
.I n
characters.
.PP
The
.BR strfromd (),
.BR strfromf (),
and
.BR strfroml ()
functions are equivalent to
.PP
.in +4n
.EX
snprintf(str, n, format, fp);
.EE
.in
.PP
except for the
.I format
string.
.SS Format of the format string
The
.I format
string must start with the character \(aq%\(aq.
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
.BR a ,
.BR A ,
.BR e ,
.BR E ,
.BR f ,
.BR F ,
.BR g ,
or
.BR G .
.PP
The conversion specifier is applied based on the floating-point type
indicated by the function suffix.
Therefore, unlike
.BR snprintf (),
the format string does not have a length modifier character.
See
.BR snprintf (3)
for a detailed description of these conversion specifiers.
.PP
The implementation conforms to the C99 standard on conversion of NaN and
infinity values:
.PP
.RS
If
.I fp
is a NaN, +NaN, or \-NaN, and
.BR f
(or
.BR a ,
.BR e ,
.BR g )
is the conversion specifier, the conversion is to "nan", "nan", or "\-nan",
respectively.
If
.B F
(or
.BR A ,
.BR E ,
.BR G )
is the conversion specifier, the conversion is to "NAN" or "\-NAN".
.PP
Likewise if
.I fp
is infinity, it is converted to [\-]inf or [\-]INF.
.RE
.PP
A malformed
.I format
string results in undefined behavior.
.SH RETURN VALUE
The
.BR strfromd (),
.BR strfromf (),
and
.BR strfroml ()
functions return the number of characters that would have been written in
.I str
if
.I n
had enough space,
not counting the terminating null byte.
Thus, a return value of
.I n
or greater means that the output was truncated.
.SH VERSIONS
The
.BR strfromd (),
.BR strfromf (),
and
.BR strfroml ()
functions are available in glibc since version 2.25.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7)
and the
.B POSIX Safety Concepts
section in GNU C Library manual.
.PP
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR strfromd (),
.BR strfromf (),
.BR strfroml ()
T}	Thread safety	MT-Safe locale
\^	Async-signal safety	AS-Unsafe heap
\^	Async-cancel safety	AC-Unsafe mem
.TE
.hy
.ad
.sp 1
Note: these attributes are preliminary.
.SH CONFORMING TO
C99, ISO/IEC TS 18661-1.
.SH NOTES
The
.BR strfromd (),
.BR strfromf (),
and
.BR strfroml ()
functions take account of the
.B LC_NUMERIC
category of the current locale.
.SH EXAMPLES
To convert the value 12.1 as a float type to a string using decimal
notation, resulting in "12.100000":
.PP
.in +4n
.EX
#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdlib.h>
int ssize = 10;
char s[ssize];
strfromf(s, ssize, "%f", 12.1);
.EE
.in
.PP
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":
.PP
.in +4n
.EX
#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdlib.h>
int ssize = 10;
char s[ssize];
strfromf(s, ssize, "%.2f", 12.3456);
.EE
.in
.PP
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":
.PP
.in +4n
.EX
#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdlib.h>
int ssize = 10;
char s[ssize];
strfromd(s, ssize, "%.E", 12.345e19);
.EE
.in
.SH SEE ALSO
.BR atof (3),
.BR snprintf (3),
.BR strtod (3)