символьная строка в число с двойной точностью (character string to a double-precision number)
Пролог (Prolog)
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.
Имя (Name)
wcstod, wcstof, wcstold — convert a wide-character string to a
double-precision number
Синопсис (Synopsis)
#include <wchar.h>
double wcstod(const wchar_t *restrict nptr, wchar_t **restrict endptr);
float wcstof(const wchar_t *restrict nptr, wchar_t **restrict endptr);
long double wcstold(const wchar_t *restrict nptr,
wchar_t **restrict endptr);
Описание (Description)
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.1‐2017 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 decimal digits
* 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:
digit
nondigit
n-wchar-sequence digit
n-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 expected form.
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 <hyphen-minus>, 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-defined manner.
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 locale, additional locale-specific
subject sequence forms 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 null pointer.
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.
Возвращаемое значение (Return value)
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].
Ошибки (Error)
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.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
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 direction.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
fscanf(3p), iswspace(3p), localeconv(3p), setlocale(3p),
wcstol(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale,
float.h(0p), wchar.h(0p)
