NAME

wcstod, wcstof, wcstold — convert a wide-character string to a double-precision number

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);


DESCRIPTION

^[CX] 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.1-2024 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 characters

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-sequence_opt), 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-sequence_opt) 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.

^[CX] 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 ^[CX]  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.

RETURN VALUE

Upon successful completion, these functions shall return the converted value. If no conversion could be performed, 0 shall be returned ^[CX]  and errno may be set to [EINVAL].

If the correct value would cause an overflow and default rounding is in effect, ±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 ^[CX]  and errno set to [ERANGE].

ERRORS

The wcstod() function shall fail if:

[ERANGE]

The value to be returned would cause overflow and default rounding is in effect ^[CX]  or the value to be returned would cause underflow.

The wcstod() function may fail if:

[EINVAL]

^[CX] No conversion could be performed.

EXAMPLES

None.

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

fscanf, iswspace, localeconv, setlocale, wcstol

XBD 7. Locale, <float.h>, <wchar.h>

CHANGE HISTORY

First released in Issue 4. Derived from the MSE working draft.

Issue 5

The DESCRIPTION is updated to indicate that errno is not changed if the function is successful.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• In the RETURN VALUE and ERRORS sections, the [EINVAL] optional error condition is added if no conversion could be performed.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

• The wcstod() prototype is updated.

• The wcstof() and wcstold() functions are added.

• If the correct value for wcstod() would cause underflow, the return value changed from 0 (as specified in Issue 5) to the smallest normalized positive number.

• The DESCRIPTION, RETURN VALUE, and APPLICATION USAGE sections are extensively updated.

ISO/IEC 9899:1999 standard, Technical Corrigendum 1 is incorporated.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/66 is applied, correcting the second paragraph in the RETURN VALUE section.

Issue 7

Austin Group Interpretation 1003.1-2001 #015 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0723 [302] and XSH/TC1-2008/0724 [105] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0391 [584] and XSH/TC2-2008/0392 [796] are applied.

Issue 8

Austin Group Defect 1163 is applied, clarifying the handling of white space in the input string.

Austin Group Defect 1686 is applied, addressing some inconsistencies with strtod().