With the register_printf_type/register_printf_modifier/register_printf_specifier APIs the C library is just told the size of the argument and is provided with a callback to fetch the argument from va_list using va_arg into C library provided memory. The C library isn't told what alignment requirement it has, but we were using direct load of a __float128 value from that memory which assumes __alignof (__float128) alignment. The following patch fixes that by using memcpy instead. I haven't been able to reproduce an actual crash, tried #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); } printf ("%+-#*.20Qe\n", width, r); printf ("%Qa\n", r); printf ("%+-#46.*Qe\n", prec, r); printf ("%d %Qe %d %Qe %d %Qe\n", 1, r, 2, r, 3, r); return 0; } In any case, I think memcpy for loading from it is right. 2024-04-03 Simon Chopin <simon.chopin@canonical.com> Jakub Jelinek <jakub@redhat.com> PR libquadmath/114533 * printf/printf_fp.c (__quadmath_printf_fp): Use memcpy to copy __float128 out of args. * printf/printf_fphex.c (__quadmath_printf_fphex): Likewise. Signed-off-by: Simon Chopin <simon.chopin@canonical.com>