#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().

   Multithreading (see pthreads(7))
       The ecvt() and fcvt() functions are not thread-safe.

       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").
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