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3.2 quadmath_snprintf — Convert to string

The function quadmath_snprintf converts a __float128 floating-point number into a string. It is a specialized alternative to snprintf, where the format string is restricted to a single conversion specifier with Q modifier and conversion specifier e, E, f, F, g, G, a or A, with no extra characters before or after the conversion specifier. The %m$ or *m$ style must not be used in the format.


int quadmath_snprintf (char *s, size_t size, const char *format, ...)

soutput string
sizebyte size of the string, including tailing NUL
formatconversion specifier string

On some targets when supported by the C library hooks are installed for printf family of functions, so that printf ("%Qe", 1.2Q); etc. works too.

#include <quadmath.h>
#include <stdlib.h>
#include <stdio.h>

int main ()
  __float128 r;
  int prec = 20;
  int width = 46;
  char buf[128];

  r = 2.0q;
  r = sqrtq (r);
  int n = quadmath_snprintf (buf, sizeof buf, "%+-#*.20Qe", width, r);
  if ((size_t) n < sizeof buf)
    printf ("%s\n", buf);
    /* Prints: +1.41421356237309504880e+00 */
  quadmath_snprintf (buf, sizeof buf, "%Qa", r);
  if ((size_t) n < sizeof buf)
    printf ("%s\n", buf);
    /* Prints: 0x1.6a09e667f3bcc908b2fb1366ea96p+0 */
  n = quadmath_snprintf (NULL, 0, "%+-#46.*Qe", prec, r);
  if (n > -1)
      char *str = malloc (n + 1);
      if (str)
          quadmath_snprintf (str, n + 1, "%+-#46.*Qe", prec, r);
          printf ("%s\n", str);
          /* Prints: +1.41421356237309504880e+00 */
      free (str);
  return 0;