The strtod, strtof, and strtold functions convert the
initial portion of the string pointed to by nptr to double,
float, and long double representation, respectively.
The expected form of the (initial portion of the) string is optional leading
white space as recognized by isspace(3), an optional plus (``+'') or
minus sign (``-'') and then either (i) a decimal number, or (ii) a hexadecimal
number, or (iii) an infinity, or (iv) a NAN (not-a-number).
A decimal number consists of a nonempty sequence of decimal digits
possibly containing a radix character (decimal point, locale dependent,
usually ``.''), optionally followed by a decimal exponent. A decimal exponent
consists of an ``E'' or ``e'', followed by an optional plus or minus sign,
followed by a non-empty sequence of decimal digits, and indicates
multiplication by a power of 10.
A hexadecimal number consists of a ``0x'' or ``0X'' followed by a
nonempty sequence of hexadecimal digits possibly containing a radix character,
optionally followed by a binary exponent. A binary exponent consists of a
``P'' or ``p'', followed by an optional plus or minus sign, followed by a
non-empty sequence of decimal digits, and indicates multiplication by a power
of 2. At least one of radix character and binary exponent must be present.
An infinity is either ``INF'' or ``INFINITY'', disregarding case.
A NAN is ``NAN'' (disregarding case) optionally followed by `(', a
sequence of characters, followed by ')'. The character string specifies in an
implementation-dependent way the type of NAN.
These functions return the converted value, if any.
If endptr is not NULL, a pointer to the character after the last
character used in the conversion is stored in the location referenced by
If no conversion is performed, zero is returned and the value of nptr is
stored in the location referenced by endptr.
If the correct value would cause overflow, plus or minus HUGE_VAL
(HUGE_VALF, HUGE_VALL) is returned (according to the sign of the
value), and ERANGE is stored in errno. If the correct value
would cause underflow, zero is returned and ERANGE is stored in