strtof, strtod, or strtold Subroutine

Purpose

Converts a string to a double-precision number.

Syntax

#include <stdlib.h>

float strtof (nptr, endptr)
const char *restrict nptr;
char **restrict endptr;

double strtod ( nptr,  endptr)
const char *nptr
char**endptr;

long double strtold (nptr, endptr)
const char *restrict nptr;
char **restrict endptr;

Description

The strtof, strtod, and strtold subroutines convert the initial portion of the string pointed to by nptr to double, float, and long double representation, respectively. First, they decompose the input string into three parts:
  • An initial, possibly empty, sequence of white-space characters (as specified by isspace()).
  • A subject sequence interpreted as a floating-point constant or representing infinity or NaN.
  • A final string of one or more unrecognized characters, including the terminating null byte of the input string.

Then, they attempt to convert the subject sequence to a floating-point number, and return the result.

The expected form of the subject sequence is an optional plus or minus sign, and one of the following:
  • A non-empty sequence of decimal digits optionally containing a radix character, and an optional exponent part
  • A 0x or 0X, and a non-empty sequence of hexadecimal digits optionally containing a radix character, and an optional binary exponent part
  • One of INF or INFINITY, ignoring case
  • One of NAN or NAN(n-char-sequence opt ), ignoring case in the NAN part, where: 
    n-char-sequence:
	 	digit
 		nondigit
		n-char-sequence digit
		n-char-sequence nondigit

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is not of the expected form.

If the subject sequence has the expected form for a floating-point number, the sequence of characters starting with the first digit or the decimal-point character (whichever occurs first) are interpreted as a floating constant of the C language, except that the radix character is used in place of a period, and if neither an exponent part nor a radix character appears in a decimal floating-point number, or if a binary exponent part does not appear in a hexadecimal floating-point number, an exponent part of the appropriate type with value zero is assumed to follow the last digit in the string.

If the subject sequence begins with a minus sign, the sequence is interpreted as negated. A character sequence INF or INFINITY shall be interpreted as an infinity, if representable in the return type, or else as if it were a floating constant that is too large for the range of the return type. A character sequence NAN or NAN(n-char-sequence opt ) is interpreted as a quiet NaN, if supported in the return type, or else as if it were a subject sequence part that does not have the expected form. The meaning of the n-char sequences is implementation-defined. A pointer to the final string is stored in the object pointed to by the endptr parameter, provided that the endptr parameter is not a null pointer.

If the subject sequence has the hexadecimal form, the value resulting from the conversion is correctly rounded.

The radix character is defined in the program's locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character defaults to a period.

In other than the C or POSIX locales, other implementation-defined subject sequences may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion shall be performed; the value of str is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

The strtod subroutine does not change the setting of the errno global variable if successful.

Since 0 is returned on error and is also a valid return on success, an application wishing to check for error situations should set errno to 0, call the strtof or strtold subroutine, then check errno.

Parameters

Item Description
nptr Specifies the string to be converted.
endptr Points to the final string.

Return Values

Upon successful completion, the strtof and strtold subroutines return the converted value. If no conversion could be performed, 0 is returned, and the errno global variable may be set to EINVAL.

If the correct value is outside the range of representable values, HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the sign of the value), and errno is set to ERANGE.

If the correct value would cause an underflow, a value whose magnitude is no greater than the smallest normalized positive number in the return type is returned and the errno global variable is set to ERANGE.

Error Codes

Note: Because a value of 0 can indicate either an error or a valid result, an application that checks for errors with the strtod, strtof, and strtold subroutines should set the errno global variable equal to 0 prior to the subroutine call. The application can check the errno global variable after the subroutine call.

If the string pointed to by NumberPointer is empty or begins with an unrecognized character, a value of 0 is returned for the strtod, strtof, and strtold subroutines.

If the conversion cannot be performed, a value of 0 is returned, and the errno global variable is set to indicate the error.

If the conversion causes an overflow (that is, the value is outside the range of representable values), +/- HUGE_VAL is returned with the sign indicating the direction of the overflow, and the errno global variable is set to ERANGE.

If the conversion would cause an underflow, a properly signed value of 0 is returned and the errno global variable is set to ERANGE.

For the strtod, strtof, and strtold subroutines, if the value of the EndPointer parameter is not (char**) NULL, a pointer to the character that stopped the subroutine is stored in *EndPointer. If a floating-point value cannot be formed, *EndPointer is set to NumberPointer.

The strtof subroutine has only one rounding error. (If the strtod subroutine is used to create a double-precision floating-point number and then that double-precision number is converted to a floating-point number, two rounding errors could occur.)