libc.info: atof

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`atof', `atoff'--string to double or float

*Synopsis*
     #include <stdlib.h>
     double atof(const char *S);
     float atoff(const char *S);
   *Description*
`atof' converts the initial portion of a string to a `double'.  `atoff'
converts the initial portion of a string to a `float'.
   The functions parse the character string S, locating a substring
which can be converted to a floating point value. The substring must
match the format:
      [+|-]DIGITS[.][DIGITS][(e|E)[+|-]DIGITS]
   The substring converted is the longest initial fragment of S that
has the expected format, beginning with the first non-whitespace
character.  The substring is empty if `str' is empty, consists entirely
of whitespace, or if the first non-whitespace character is something
other than `+', `-', `.', or a digit.
   `atof(S)' is implemented as `strtod(S, NULL)'.  `atoff(S)' is
implemented as `strtof(S, NULL)'.
*Returns*
`atof' returns the converted substring value, if any, as a `double'; or
`0.0',  if no conversion could be performed.  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, `0.0' is returned and `ERANGE' is stored in
`errno'.
   `atoff' obeys the same rules as `atof', except that it returns a
`float'.
*Portability*
`atof' is ANSI C. `atof', `atoi', and `atol' are subsumed by `strod'
and `strol', but are used extensively in existing code. These functions
are less reliable, but may be faster if the argument is verified to be
in a valid range.
   Supporting OS subroutines required: `close', `fstat', `isatty',
`lseek', `read', `sbrk', `write'.