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### 2.34 `strtod`, `strtof`—string to double or float

Synopsis

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

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

Description
The function `strtod` parses the character string str, producing a substring which can be converted to a double value. 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. `strtof` is identical to `strtod` except for its return type.

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.

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

Returns
`strtod` returns 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` is returned, and `ERANGE` is stored in errno. If the correct value would cause underflow, 0 is returned and `ERANGE` is stored in errno.

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