> Will it change malloc alignment for i386? No, since SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_16) will be true for any configuration that existed before 2.16. > X32 MALLOC_ALIGNMENT == 16 is specified in x32 psABI. So is __alignof__ (long double) == 16. > Malloc doesn't need to guess it. It's not a guess. It's an algorithm.