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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  double,  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 isspace())

        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  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) 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  interpreted 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 strtold(), then check errno.

RETURN VALUE

       Upon successful completion, these functions shall return the  converted
       value.  If  no  conversion 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
       current 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
       digits, 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 behavior 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("Its a float with value %g\n", d);
                  else
                  {
                      l = strtol(s, &endp, 0);
                      if (s != endp && *endp ==\0)
                          printf("Its 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:

              Its an integer with value 16

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

              Its 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 original Standard can be  obtained  online
       at http://www.opengroup.org/unix/online.html .