man3/strtod.3

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.\"     @(#)strtod.3	5.3 (Berkeley) 6/29/91
.\"
.\" Modified Sun Aug 21 17:16:22 1994 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sat May 04 19:34:31 MET DST 1996 by Michael Haardt
.\"   (michael@cantor.informatik.rwth-aachen.de)
.\" Added strof, strtold, aeb, 2001-06-07
.\"
.TH STRTOD 3 2020-06-09 "Linux" "Linux Programmer's Manual"
.SH NAME
strtod, strtof, strtold \- convert ASCII string to floating-point number
.SH SYNOPSIS
.B #include <stdlib.h>
.PP
.BI "double strtod(const char *" nptr ", char **" endptr );
.br
.BI "float strtof(const char *" nptr ", char **" endptr );
.br
.BI "long double strtold(const char *" nptr ", char **" endptr );
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.ad l
.PP
.BR strtof (),
.BR strtold ():
.RS 4
_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
.RE
.ad
.SH DESCRIPTION
The
.BR strtod (),
.BR strtof (),
and
.BR strtold ()
functions convert the initial portion of the string pointed to by
.I nptr
to
.IR double ,
.IR float ,
and
.I long double
representation, respectively.
.PP
The expected form of the (initial portion of the) string is
optional leading white space as recognized by
.BR isspace (3),
an optional plus (\(aq+\(aq) or minus sign (\(aq\-\(aq) and then either
(i) a decimal number, or (ii) a hexadecimal number,
or (iii) an infinity, or (iv) a NAN (not-a-number).
.PP
A
.I "decimal number"
consists of a nonempty sequence of decimal digits
possibly containing a radix character (decimal point, locale-dependent,
usually \(aq.\(aq), optionally followed by a decimal exponent.
A decimal exponent consists of an \(aqE\(aq or \(aqe\(aq, followed by an
optional plus or minus sign, followed by a nonempty sequence of
decimal digits, and indicates multiplication by a power of 10.
.PP
A
.I "hexadecimal number"
consists of a "0x" or "0X" followed by a nonempty sequence of
hexadecimal digits possibly containing a radix character,
optionally followed by a binary exponent.
A binary exponent
consists of a \(aqP\(aq or \(aqp\(aq, followed by an optional
plus or minus sign, followed by a nonempty sequence of
decimal digits, and indicates multiplication by a power of 2.
At least one of radix character and binary exponent must be present.
.PP
An
.I infinity
is either "INF" or "INFINITY", disregarding case.
.PP
A
.I NAN
is "NAN" (disregarding case) optionally followed by a string,
.IR (n-char-sequence) ,
where
.IR n-char-sequence
specifies in an implementation-dependent
way the type of NAN (see NOTES).
.SH RETURN VALUE
These functions return the converted value, if any.
.PP
If
.I endptr
is not NULL,
a pointer to the character after the last character used in the conversion
is stored in the location referenced by
.IR endptr .
.PP
If no conversion is performed, zero is returned and (unless
.I endptr
is null) the value of
.I nptr
is stored in the location referenced by
.IR endptr .
.PP
If the correct value would cause overflow, plus or minus
.B HUGE_VAL
.RB ( HUGE_VALF ,
.BR HUGE_VALL )
is returned (according to the sign of the value), and
.B ERANGE
is stored in
.IR errno .
If the correct value would cause underflow, zero is
returned and
.B ERANGE
is stored in
.IR errno .
.SH ERRORS
.TP
.B ERANGE
Overflow or underflow occurred.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw29 lb lb
l l l.
Interface	Attribute	Value
T{
.BR strtod (),
.BR strtof (),
.BR strtold ()
T}	Thread safety	MT-Safe locale
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C99.
.PP
.BR strtod ()
was also described in C89.
.SH NOTES
Since
0 can legitimately be returned
on both success and failure, the calling program should set
.I errno
to 0 before the call,
and then determine if an error occurred by checking whether
.I errno
has a nonzero value after the call.
.PP
In the glibc implementation, the
.IR n-char-sequence
that optionally follows "NAN"
is interpreted as an integer number
(with an optional '0' or '0x' prefix to select base 8 or 16)
that is to be placed in the
mantissa component of the returned value.
.\" From glibc 2.8's stdlib/strtod_l.c:
.\"     We expect it to be a number which is put in the
.\"     mantissa of the number.
.\" It looks as though at least FreeBSD (according to the manual) does
.\" something similar.
.\" C11 says: "An implementation may use the n-char sequence to determine
.\"	extra information to be represented in the NaN's significant."
.SH EXAMPLES
See the example on the
.BR strtol (3)
manual page;
the use of the functions described in this manual page is similar.
.SH SEE ALSO
.BR atof (3),
.BR atoi (3),
.BR atol (3),
.BR nan (3),
.BR nanf (3),
.BR nanl (3),
.BR strfromd (3),
.BR strtol (3),
.BR strtoul (3)