#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);
double wstod(const wchar_t *nptr, wchar_t **endptr);
The wcstod(), wcstof(), and wcstold() functions convert the initial portion of the widecharacter string pointed to by nptr to double, float, and long double representation,
respectively. They first decompose the input widecharacter string into three parts:
1. An initial, possibly empty, sequence of whitespace widecharacter codes (as specified by iswspace(3C))
2. A subject sequence interpreted as a floatingpoint constant or representing infinity or NaN
3. A final widecharacter string of one or more unrecognized widecharacter codes, including the terminating null widecharacter code of the input widecharacter string.
Then they attempt to convert the subject sequence to a floatingpoint number, and return the result.
The expected form of the subject sequence is an optional plus or minus sign, then one of the following:
• A nonempty sequence of decimal digits optionally containing a radix character, then an optional exponent part
• A 0x or 0X, then a nonempty sequence of hexadecimal digits optionally containing a radix character, then an optional binary exponent part
• One of INF or INFINITY, or any other wide string equivalent except for case
• One of NAN or NAN(nwcharsequenceopt), or any other wide string ignoring case in the NAN part, where:
nwcharsequence: digit nondigit nwcharsequence digit nwcharsequence nondigit
In default mode for wcstod(), only decimal, INF/INFINITY, and NAN/NAN(ncharsequence) forms are recognized. In C99/SUSv3 mode, hexadecimal strings are also recognized.
In default mode for wcstod(), the ncharsequence in the NAN(ncharequence) form can contain any character except ')' (right parenthesis) or '\0' (null). In C99/SUSv3 mode, the ncharsequence can contain only upper and lower case letters, digits, and '_' (underscore).
The wcstof() and wcstold() functions always function in C99/SUSv3conformant mode.
The subject sequence is defined as the longest initial subsequence of the input wide string, starting with the first nonwhitespace 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 floatingpoint number, the sequence of wide characters starting with the first digit or the radix character (whichever occurs first) is interpreted as a floating constant according to the rules of the C language, except that the radix character is used in place of a period, and that if neither an exponent part nor a radix character appears in a decimal floatingpoint number, or if a binary exponent part does not appear in a hexadecimal floatingpoint number, an exponent part of the appropriate type with value zero is assumed to follow the last digit in the string. If the subject sequence begins with a minus sign, the sequence is interpreted as negated. A widecharacter sequence INF or INFINITY is interpreted as an infinity. A widecharacter sequence NAN or NAN(nwcharsequenceopt) is interpreted as a quiet NaN. A pointer to the final wide string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
If the subject sequence has either the decimal or hexadecimal form, the value resulting from the conversion is rounded correctly according to the prevailing floating point rounding direction mode. The conversion also raises floating point inexact, underflow, or overflow exceptions as appropriate.
The radix character is defined in the program's locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character defaults to a period ('.').
If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
The wcstod() function does not change the setting of errno if successful.
The wstod() function is identical to wcstod().
The watof(str) function is equivalent to wstod(nptr, (wchar_t **)NULL).
Upon successful completion, these functions return the converted value. If no conversion could be performed, 0 is returned.
If the correct value is outside the range of representable values, ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL is returned (according to the sign of the value), a floating point overflow exception is raised, and errno is set to ERANGE.
If the correct value would cause an underflow, the correctly rounded result (which may be normal, subnormal, or zero) is returned, a floating point underflow exception is raised, and errno is set to ERANGE.
The wcstod() and wstod() functions will fail if:
ERANGE The value to be returned would cause overflow or underflow.
The wcstod() and wcstod() functions may fail if:
EINVAL No conversion could be performed.
Because 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 call wcstod(), wcstof(), wcstold(), or wstod(), then check
errno and if it is nonzero, assume an error has occurred.
See attributes(5) for descriptions of the following attributes:
ATTRIBUTE TYPE  ATTRIBUTE VALUE 
Interface Stability 
wcstod(), wcstof(), and wcstold() are Standard.

MTLevel  MTSafe 
iswspace(3C), localeconv(3C), scanf(3C), setlocale(3C), wcstol(3C), attributes(5), standards(5)