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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, ...)

s output string
size byte size of the string, including tailing NUL
format conversion 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;