double strtod(const char *nptr, char **endptr);
#define _XOPEN_SOURCE=600 /* or #define _ISOC99_SOURCE */
#include <stdlib.h>
float strtof(const char *nptr, char **endptr);
long double strtold(const char *nptr, char **endptr);
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.
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 endptr.
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 errno.