### 2.37 `strtod`, `strtof`, `strtold`, `strtod_l`, `strtof_l`, `strtold_l`—string to double or float

Synopsis

         #include <stdlib.h>
         double strtod(const char *restrict str, char **restrict tail);
         float strtof(const char *restrict str, char **restrict tail);
         long double strtold(const char *restrict str,
             char **restrict tail);

         #include <stdlib.h>
         double strtod_l(const char *restrict str, char **restrict tail,
             locale_t locale);
         float strtof_l(const char *restrict str, char **restrict tail,
             locale_t locale);
         long double strtold_l(const char *restrict str,
             char **restrict tail,
             locale_t locale);

         double _strtod_r(void *reent,
             const char *restrict str, char **restrict tail);

Description
`strtod`, `strtof`, `strtold` parse the character string str, producing a substring which can be converted to a double, float, or long double value, respectively. The substring converted is the longest initial subsequence of str, beginning with the first non-whitespace character, that has one of these formats:

         [+|-]digits[.[digits]][(e|E)[+|-]digits]
         [+|-].digits[(e|E)[+|-]digits]
         [+|-](i|I)(n|N)(f|F)[(i|I)(n|N)(i|I)(t|T)(y|Y)]
         [+|-](n|N)(a|A)(n|N)[<(>[hexdigits]<)>]
         [+|-]0(x|X)hexdigits[.[hexdigits]][(p|P)[+|-]digits]
         [+|-]0(x|X).hexdigits[(p|P)[+|-]digits]

The substring contains no characters if str is empty, consists entirely of whitespace, or if the first non-whitespace character is something other than `+`, `-`, `.`, or a digit, and cannot be parsed as infinity or NaN. If the platform does not support NaN, then NaN is treated as an empty substring. If the substring is empty, no conversion is done, and the value of str is stored in `*`tail. Otherwise, the substring is converted, and a pointer to the final string (which will contain at least the terminating null character of str) is stored in `*`tail. If you want no assignment to `*`tail, pass a null pointer as tail.

This implementation returns the nearest machine number to the input decimal string. Ties are broken by using the IEEE round-even rule. However, `strtof` is currently subject to double rounding errors.

`strtod_l`, `strtof_l`, `strtold_l` are like `strtod`, `strtof`, `strtold` but perform the conversion based on the locale specified by the locale object locale. If locale is LC_GLOBAL_LOCALE or not a valid locale object, the behaviour is undefined.

The alternate function `_strtod_r` is a reentrant version. The extra argument reent is a pointer to a reentrancy structure.

Returns
These functions return the converted substring value, if any. If no conversion could be performed, 0 is returned. If the correct value is out of the range of representable values, plus or minus `HUGE_VAL` (`HUGE_VALF`, `HUGE_VALL`) is returned, and `ERANGE` is stored in errno. If the correct value would cause underflow, 0 is returned and `ERANGE` is stored in errno.

Portability
`strtod` is ANSI. `strtof`, `strtold` are C99. `strtod_l`, `strtof_l`, `strtold_l` are GNU extensions.

Supporting OS subroutines required: `close`, `fstat`, `isatty`, `lseek`, `read`, `sbrk`, `write`.