#include <stdlib.h>

       char *ecvt(double number, int ndigits, int *decpt, int *sign);

       char *fcvt(double number, int ndigits, int *decpt, int *sign);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       ecvt(), fcvt():
           Since glibc 2.12:
               _SVID_SOURCE ||
                   (_XOPEN_SOURCE >= 500 ||
                       _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
                   !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
           Before glibc 2.12:
               _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||

       The ecvt() function converts number  to  a  null-terminated  string  of
       ndigits  digits  (where  ndigits  is reduced to a system-specific limit
       determined by the precision of a double), and returns a pointer to  the
       string.   The  high-order digit is nonzero, unless number is zero.  The
       low order digit is rounded.  The string itself does not contain a deci-
       mal  point;  however, the position of the decimal point relative to the
       start of the string is stored in *decpt.  A negative value  for  *decpt
       means that the decimal point is to the left of the start of the string.
       If the sign of number is negative, *sign is set  to  a  nonzero  value,
       otherwise it is set to 0.  If number is zero, it is unspecified whether
       *decpt is 0 or 1.

       The fcvt() function is identical to ecvt(), except that ndigits  speci-
       fies the number of digits after the decimal point.

       Both  the  ecvt()  and  fcvt()  functions  return a pointer to a static
       string containing the  ASCII  representation  of  number.   The  static
       string is overwritten by each call to ecvt() or fcvt().

       SVr2; marked as LEGACY in POSIX.1-2001.  POSIX.1-2008 removes the spec-
       ifications of ecvt() and fcvt(), recommending  the  use  of  sprintf(3)
       instead (though snprintf(3) may be preferable).

       Linux libc4 and libc5 specified the type of ndigits as size_t.  Not all
       locales use a point as the radix character ("decimal point").

       ecvt_r(3), gcvt(3), qecvt(3), setlocale(3), sprintf(3)

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