#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);
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:
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:
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.
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 may fail if:
The following sections are informative.
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.
The Base Definitions volume of POSIX.1-2017, Chapter 7, Locale, <float.h>, <wchar.h>
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