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SubjectRe: Observe wrapping variables in while loops



On Sun, 8 Mar 2009, Roel Kluin wrote:

> Inspired by your definition of 'if', I came up with something to test variable
> wrapping in while loops, and was wondering if this was useful for the linux
> kernel:
>
> The source below is adapted to illustrate how it works. If you run it, this
> will print:
>
> 8 6 4 2 0 variable wraps at ../test.c:88:main(): -1!
> ...-2
> 9 7 5 3 1 variable wraps at ../test.c:94:main(): -1!
> ...-1
> 8 7 6 5 4 3 2 1 0 ...-1
> 8 7 6 5 4 3 2 1 ...0
> 21 22 23 24 25 26 27 28 29 30 ...31
> 19 18 17 16 15 14 13 12 11 10 ...9
> 4 3 2 1 0 -1 -2 -3 -4 -5 ...-6
> -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 ...11
> 4 2 0 variable wraps at ../test.c:129:main(): -1!
> ...-2
>
> In the first two loops and the last, a wrap occurs due to the double decrement.
> Tests like 'while(i++ < 30)' and 'while(j-- > -5)' are boolean.
>
> However, this may not work for a construction like this:

Interesting variation. Now the question is, how many bugs are we aware of
that were caused by wraps? If there are a few, then I would say this is
definitely something to look at. Otherwise, it may not be worth putting
this in mainline.

-- Steve



>
> while ((ret = foo())) {
> do_stuff();
> }
> ------------------------------>8-------------8<---------------------------------
> #include <stdio.h>
>
> struct while_data {
> const char *func;
> const char *file;
> unsigned line;
> unsigned long last;
> };
>
> int is_while_wrap(struct while_data *w, unsigned long val)
> {
> if (w->last < val) {
> printf("variable wraps at %s:%u:%s(): %d!\n", w->file, w->line, w->func, val);
> return 0;
> }
> w->last = val;
> return val;
> }
>
> #define __while_check__(x) ({ \
> static struct while_data \
> ______w = { \
> .func = __func__, \
> .file = __FILE__, \
> .line = __LINE__, \
> .last = ULONG_MAX, \
> }; \
> printf(" "); \
> is_while_wrap(&______w, (x)); \
> })
>
> #define while(x) while(__while_check__(x))
>
>
> int main(int argc, char* argv[])
> {
> unsigned int i=9;
> while(i--) {
> printf("%d", i);
> --i;
> }
> printf("...%d\n", i);
> i=10;
> while(--i) {
> printf("%d", i);
> --i;
> }
> printf("...%d\n", i);
>
> i=9;
> while(i--)
> printf("%d", i);
> printf("...%d\n", i);
>
> i=9;
> while(--i)
> printf("%d", i);
> printf("...%d\n", i);
>
> i=20;
> while(i++ < 30)
> printf("%d", i);
> printf("...%d\n", i);
>
> i=20;
> while(i-- > 10)
> printf("%d", i);
> printf("...%d\n", i);
>
> int j=5;
> while(j-- > -5)
> printf("%d", j);
> printf("...%d\n", j);
> while(j++ < 10)
> printf("%d", j);
> printf("...%d\n", j);
>
> j=5;
> while(j--) {
> printf("%d", j);
> j--;
> }
> printf("...%d\n", j);
> }
>


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