https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90404
Bug ID: 90404
Summary: No warning on attempts to modify a const
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: david at westcontrol dot com
Target Milestone: ---
The compiler knows that you can't change an object declared "const", even via a
pointer cast. And it (rightfully and helpfully) uses that knowledge for
optimisation. But it should also be able to warn about it.
const int c = 20;
int foo(void) {
*((int*) &c) = 10;
return c;
}
Current gcc, with -O2, compiles this to:
foo:
mov DWORD PTR c[rip], 10
mov eax, 20
ret
The write to c is unnecessary, but could trigger a helpful fault message on
many target systems (and who cares about the efficiency of incorrect code?).
And the compiler correctly optimises the return value to 20 as it knows the
write to "c" cannot succeed.
However, there is no warning or error message here - even with -Wall -Wextra.
With -Wcast-qual, you get a warning - but that will have false positives in
cases like :
int v = 20;
int foo2(void) {
const int* p = &v;
*((int*) p) = 10;
return v;
}
foo2:
mov DWORD PTR v[rip], 10
mov eax, 10
ret
The optimiser can see the difference here - I would like warnings to show the
difference too.
