Re: [PATCH] C undefined behavior fix

Petr Vandrovec
2002-01-02T17:45:58Z

On 2 Jan 02 at 11:45, Paul Koning wrote:
>
> It might be interesting for the compiler to warn about this coding
> error (since it presumably can detect it). But "fixing" the behavior
> of undefined code seems like a strange thing to do.

It is even worse (gcc 2.95.4 20011223 (Debian prerelease), i386).
Test code:

#include <string.h>
char* dst;
void main(void) {
strcpy(dst, "test"+CONSTANT);
}

# gcc -O2 -S test.c -DCONSTANT=10
test.c: In function `main':
test.c:4: warning: offset outside bounds of constant string
...
and compiler generated correct code (call to strcpy with "test"+10).

But:
# gcc -O2 -S test.c -DCONSTANT=0x80000000
test.c: In function `main':
test.c:4: warning: offset outside bounds of constant string
gcc: Internal compiler error: program cc1 got fatal signal 11

(and for CONSTANT < 5 it of course generated correct code to fill
dst with string contents; and yes, I know that code will sigsegv on
run because of dst is not initialized - but it should die at runtime,
not at compile time).

So we should definitely change RELOC(), or sooner or later gcc will
die on such code :-(

Debian's gcc 3.0.3-1 generates:
0 <= CONSTANT <= 4: fills dst directly with constant
5 <= CONSTANT <= 0x7FFFFFFF: emit warnings + use strcpy()
0x80000000U <= CONSTANT <= 0xFFFFFFFFU: use strcpy() silently
... and it does not die.
Best regards,
Petr Vandrovec
vandrove@vc.cvut.cz