Al Viro wrote:
> On Fri, Aug 03, 2007 at 12:51:16AM +0200, Guennadi Liakhovetski wrote:
>> On Fri, 3 Aug 2007, Stefan Richter wrote:
>>
>>> Guennadi Liakhovetski wrote:
>>>> with
>>>>
>>>> char c[4] = "012345";
>>>>
>>>> the compiler warns, but actually allocates a 6-byte long array...
>>> Off-topic here, but: sizeof c / sizeof *c == 4.
>> Don't think it is OT here - kernel depends on gcc. And, what I meant, is,
>> that gcc places all 7 (sorry, not 6 as I said above) characters in the
>> .rodata section of the compiled object file. Of course, it doesn't mean,
>> that c is 7 characters long.
>
> So gcc does that kind of recovery, after having warned you. Makes sense,
> as long as it's for ordinary variables (and not, say it, struct fields) -
> you get less likely runtime breakage on the undefined behaviour (e.g.
> passing c to string functions). So gcc has generated some padding between
> the global variables, that's all.
No, the fact that the full 012345\0 ends up in the object file is
apparently unrelated to what happens to the variable c...
> It doesn't change the fact that use of c[4] or strlen(c) or strcpy(..., c)
> means nasal demon country for you.
>
> Now, if gcc does that for similar situation with struct fields, you'd have
> a cause to complain.
...since only 0123 will get into c at runtime, i.e. a 4 bytes long array
without \0 appendix or other extraordinary padding.
#include <stdio.h>
#include <string.h>
int main()
{
char c[4] = "012345";
printf("%d %d _%s_\n", sizeof c / sizeof *c, strlen(c), c);
return 0;
}
$ ./a.out
4 8 _01230®¿_
$ strings a.out |grep 0123
012345
--
Stefan Richter
-=====-=-=== =--- ---==
http://arcgraph.de/sr/
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