This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements described
here and the ISO C standard is unintentional. This volume of
POSIX.1‐2008 defers to the ISO C standard.
These functions shall convert the initial portion of the wide-
character string pointed to by nptr to double, float, and long double
representation, respectively. First, they shall decompose the input
wide-character string into three parts:
1. An initial, possibly empty, sequence of white-space wide-
character codes (as specified by iswspace())
2. A subject sequence interpreted as a floating-point constant or
representing infinity or NaN
3. A final wide-character string of one or more unrecognized wide-
character codes, including the terminating null wide-character
code of the input wide-character string
Then they shall attempt to convert the subject sequence to a
floating-point number, and return the result.
The expected form of the subject sequence is an optional '+' or '−'
sign, then one of the following:
* A non-empty sequence of decimal digits optionally containing a
radix character; then an optional exponent part consisting of the
wide character 'e' or the wide character 'E', optionally followed
by a '+' or '−' wide character, and then followed by one or more
* A 0x or 0X, then a non-empty sequence of hexadecimal digits
optionally containing a radix character; then an optional binary
exponent part consisting of the wide character 'p' or the wide
character 'P', optionally followed by a '+' or '−' wide
character, and then followed by one or more decimal digits
* One of INF or INFINITY, or any other wide string equivalent
except for case
* One of NAN or NAN(n-wchar-sequenceopt), or any other wide string
ignoring case in the NAN part, where:
n-wchar-sequence:digitnondigitn-wchar-sequence digitn-wchar-sequence nondigit
The subject sequence is defined as the longest initial subsequence of
the input wide string, starting with the first non-white-space wide
character, that is of the expected form. The subject sequence
contains no wide characters if the input wide string is not of the
If the subject sequence has the expected form for a floating-point
number, the sequence of wide characters starting with the first digit
or the radix character (whichever occurs first) shall be interpreted
as a floating constant according to the rules of the C language,
except that the radix character shall be used in place of a period,
and that if neither an exponent part nor a radix character appears in
a decimal floating-point number, or if a binary exponent part does
not appear in a hexadecimal floating-point number, an exponent part
of the appropriate type with value zero shall be assumed to follow
the last digit in the string. If the subject sequence begins with a
minus-sign, the sequence shall be interpreted as negated. A wide-
character sequence INF or INFINITY shall be interpreted as an
infinity, if representable in the return type, else as if it were a
floating constant that is too large for the range of the return type.
A wide-character sequence NAN or NAN(n-wchar-sequenceopt) shall be
interpreted as a quiet NaN, if supported in the return type, else as
if it were a subject sequence part that does not have the expected
form; the meaning of the n-wchar sequences is implementation-defined.
A pointer to the final wide string shall be stored in the object
pointed to by endptr, provided that endptr is not a null pointer.
If the subject sequence has the hexadecimal form and FLT_RADIX is a
power of 2, the conversion shall be rounded in an implementation-
The radix character shall be as defined in the current locale
(category LC_NUMERIC). In the POSIX locale, or in a locale where the
radix character is not defined, the radix character shall default to
a <period> ('.').
In other than the C or POSIX locales, other implementation-defined
subject sequences may be accepted.
If the subject sequence is empty or does not have the expected form,
no conversion shall be performed; the value of nptr shall be stored
in the object pointed to by endptr, provided that endptr is not a
These functions shall not change the setting of errno if successful.
Since 0 is returned on error and is also a valid return on success,
an application wishing to check for error situations should set errno
to 0, then call wcstod(), wcstof(), or wcstold(), then check errno.
Upon successful completion, these functions shall return the
converted value. If no conversion could be performed, 0 shall be
returned and errno may be set to [EINVAL].
If the correct value is outside the range of representable values,
±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL shall be returned (according to
the sign of the value), and errno shall be set to [ERANGE].
If the correct value would cause underflow, a value whose magnitude
is no greater than the smallest normalized positive number in the
return type shall be returned and errno set to [ERANGE].
The wcstod() function shall fail if:
ERANGE The value to be returned would cause overflow or underflow.
The wcstod() function may fail if:
EINVAL No conversion could be performed.
The following sections are informative.
If the subject sequence has the hexadecimal form and FLT_RADIX is not
a power of 2, and the result is not exactly representable, the result
should be one of the two numbers in the appropriate internal format
that are adjacent to the hexadecimal floating source value, with the
extra stipulation that the error should have a correct sign for the
current rounding direction.
If the subject sequence has the decimal form and at most DECIMAL_DIG
(defined in <float.h>) significant digits, the result should be
correctly rounded. If the subject sequence D has the decimal form and
more than DECIMAL_DIG significant digits, consider the two bounding,
adjacent decimal strings L and U, both having DECIMAL_DIG significant
digits, such that the values of L, D, and U satisfy "L<=D<=U". The
result should be one of the (equal or adjacent) values that would be
obtained by correctly rounding L and U according to the current
rounding direction, with the extra stipulation that the error with
respect to D should have a correct sign for the current rounding
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 WCSTOD(3P)