Hi Martin, On 2023-08-14 18:49, Martin Uecker wrote: > Am Montag, dem 14.08.2023 um 12:21 +0200 schrieb Alejandro Colomar: [...]
>> Would you mind chiming in to this question?: >> <https://software.codidact.com/posts/287754> > > Unclear. It is probably UB by omission. Agree. > But this is possibly > different from the FAM case. I don't think I agree on this. To me it really looks like the same thing. BTW, there was a mention there to the FAM case in a comment: <https://software.codidact.com/comments/thread/7169#comment-21254> > > I any case, I am not so concerned about the whether this UB, > but that a programmer might do: > > struct s = { .. } > struct s* p = malloc(...) > memcpy(p, &s, sizeof s); // copy header. That's (or could be) a bug, and just another manifestation of why sizeof(s) is wrong. Let's see a couple of ways how this can go wrong: $ cat memcpy.c #include <stdio.h> #include <stdlib.h> #include <string.h> struct s { int i; char c; char fam[]; }; struct h { int i; char c; }; int main(void) { char *f; struct h h = { .i = 42, .c = 3 }; struct s *p; p = malloc(sizeof(struct s) + sizeof("foobar")); strcpy(p->fam, "foobar"); /* * since we're copying the header, it shouldn't matter if we * copy it after copying the fam itself, no? They're at * different locations... or are they? */ memcpy(p, &h, sizeof(struct s)); puts(p->fam); free(p); p = malloc(sizeof(struct s) + sizeof("foobar")); f = mempcpy(p, &h, sizeof(struct s)); /* * We could reuse the pointer from mempcpy(3) to get the location * of just after the header, right? Heh. */ strcpy(f, "foobar"); puts(p->fam); free(p); } $ cc -Wall -Wextra memcpy.c -D_GNU_SOURCE $ ./a.out $ Cheers, Alex -- <http://www.alejandro-colomar.es/> GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
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