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Linux 2.6 - man page for strtod (linux section 3posix)

STRTOD(P)			    POSIX Programmer's Manual				STRTOD(P)

NAME
       strtod, strtof, strtold - convert a string to a double-precision number

SYNOPSIS
       #include <stdlib.h>

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

DESCRIPTION
       These functions shall convert the initial portion of the string pointed to by nptr to dou-
       ble, float, and long double representation, respectively. First, they decompose the  input
       string into three parts:

	1. An  initial,  possibly empty, sequence of white-space characters (as specified by iss-
	   pace())

	2. A subject sequence interpreted as a floating-point constant or  representing  infinity
	   or NaN

	3. A  final string of one or more unrecognized characters, including the terminating null
	   byte of the input string

       Then they shall 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, then one of
       the following:

	* A non-empty sequence of decimal digits optionally containing a radix character, then an
	  optional exponent part

	* A  0x  or  0X,  then a non-empty sequence of hexadecimal digits optionally containing a
	  radix character, then 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 sub-
       ject 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) shall be interpreted as a floating constant of the  C  language,	except	that  the
       radix  character  shall be used in place of a period, and that 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 shall be interpreted as negated. A
       character sequence INF or INFINITY shall be interpreted as an infinity,	if  representable
       in the return type, 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) shall  be  inter-
       preted  as  a  quiet  NaN,  if  supported in the return type, 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 endptr, provided that endptr is not a null pointer.

       If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, 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
       shall default 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() function shall not change the setting of errno 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, then call strtod(), strtof(), or str-
       told(), then check errno.

RETURN VALUE
       Upon successful completion, these functions shall return the converted value. If  no  con-
       version could be performed, 0 shall be returned, and errno may be set to [EINVAL].

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

ERRORS
       These functions shall fail if:

       ERANGE The value to be returned would cause overflow    or underflow.

       These functions may fail if:

       EINVAL No conversion could be performed.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       If  the	subject  sequence has the hexadecimal form and FLT_RADIX is not a power of 2, and
       the result is not exactly representable, the result should be one of the  two  numbers  in
       the  appropriate  internal  format  that  are  adjacent to the hexadecimal floating source
       value, with the extra stipulation that the error should have a correct sign for	the  cur-
       rent rounding direction.

       If  the	subject  sequence  has	the  decimal  form  and  at  most DECIMAL_DIG (defined in
       <float.h>) significant digits, the result should be  correctly  rounded.  If  the  subject
       sequence D has the decimal form and more than DECIMAL_DIG significant digits, consider the
       two bounding, adjacent decimal strings L and U, both having DECIMAL_DIG	significant  dig-
       its,  such that the values of L, D, and U satisfy L <= D <= U. The result should be one of
       the (equal or adjacent) values that would be  obtained  by  correctly  rounding	L  and	U
       according  to  the  current  rounding direction, with the extra stipulation that the error
       with respect to D should have a correct sign for the current rounding direction.

       The changes to strtod() introduced by the ISO/IEC 9899:1999 standard can alter the  behav-
       ior  of	well-formed  applications  complying with the ISO/IEC 9899:1990 standard and thus
       earlier versions of the base documents. One such example would be:

	      int
	      what_kind_of_number (char *s)
	      {
		  char *endp;
		  double d;
		  long l;

		  d = strtod(s, &endp);
		  if (s != endp && *endp == `\0')
		      printf("It's a float with value %g\n", d);
		  else
		  {
		      l = strtol(s, &endp, 0);
		      if (s != endp && *endp == `\0')
			  printf("It's an integer with value %ld\n", 1);
		      else
			  return 1;
		  }
		  return 0;
	      }

       If the function is called with:

	      what_kind_of_number ("0x10")

       an ISO/IEC 9899:1990 standard-compliant library will result in the function printing:

	      It's an integer with value 16

       With the ISO/IEC 9899:1999 standard, the result is:

	      It's a float with value 16

       The change in behavior is due to the inclusion of floating-point  numbers  in  hexadecimal
       notation without requiring that either a decimal point or the binary exponent be present.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       isspace()  , localeconv() , scanf() , setlocale() , strtol() , the Base Definitions volume
       of IEEE Std 1003.1-2001, Chapter 7, Locale, <float.h>, <stdlib.h>

COPYRIGHT
       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					STRTOD(P)


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