https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81141
Bug ID: 81141
Summary: missing warning using sizeof a/sizeof *a with a
zero-length array
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
GCC detects the common mistake of using the (sizeof P / sizeof *P) expression
to attempt to compute the length of an array when P is a pointer. A similar
mistake is using the same expression when P is a zero-length array because the
result is zero, or a one-element array as the last element of a struct because
such an array is often used as a substitute for a flexible array member. This
is a enhancement request to add such a warning (either under
-Wsizeof-pointer-div or under a new option).
If this enhancement should be made available under a new warning option, since
the programming error it detects boils down to essentially the same problem
with the same conseequence, I would suggest to have the new option subsume the
-Wsizeof-pointer-div functionality as well. A possible name for the new option
might be -Wsizeof-division: "warn about uses of sizeof that appear as if they
attempt to compute the size of an array but do not actually do so, or that may
yield the wrong number of elements."
$ cat x.c && gcc -O2 -S -Wall x.c
struct S { char n, *s, a[0]; };
void h (void *d, const struct S *s)
{
__builtin_memcpy (d, s->s, sizeof s->s / sizeof *s->s); // warning (good)
__builtin_memcpy (d, s->a, sizeof s->a / sizeof *s->a); // missing warning
extern char a[0];
__builtin_memcpy (d, s->a, sizeof a / sizeof *a); // missing warning
}
x.c: In function ‘h’:
x.c:5:42: warning: division ‘sizeof (char * const) / sizeof (char)’ does not
compute the number of array elements [-Wsizeof-pointer-div]
__builtin_memcpy (d, s->s, sizeof s->s / sizeof *s->s); // warning (good)
^
