c - Is this a portable way to make a stack buffer eight-byte aligned? -

struct {   uint64_t a; char z[120]; } b;  ...  struct buffer_with_alignment_requirement* c = (struct buffer_w*)&b; c->start_using_it; 

without first member a, might crash when accessing fields in buffer. portable , correct add member force aligment whole struct strict enough?

this not question pointer aliasing, whether address of b on stack eight-byte aligned.

it depends on mean aligned. if want (uintptr_t)&b % 8 == 0, there no portable way obtain this, conversion pointer integer implementation-defined , need not sane, natural mapping.

if want buffer sufficiently aligned access type uint64_t, solution works well. why don't use (possibly array of) type struct buffer_with_alignment_requirement, rather ugly union hackery? in other words, give buffer right type intend access begin with. can pass pointer type read, fread, revc, etc. , other functions might using write buffer, , if you'll passing function expects buffer pointer of type char * or unsigned char *, can cast when passing it; cast well-defined , valid.