Signed overflow / unsigned wrap-around
I've recently read this:
https://lwn.net/ml/linux-kernel/20240122235208.work.748-kees@kernel.org/
Trying to experiment a bit with what I learned:
$ cat wrapme.c
#include <stdint.h>
#include <err.h>
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
int main(int argc, char **argv)
{
int val, ival;
if (argc < 2)
errx(1, "Usage: %s VALUE", argv[0]);
val = INT_MAX;
ival = atoi(argv[1]); // feeling lazy today.
printf("will it overflow?\n");
if (val + ival < val)
printf("It will overflow\n");
else
printf("It won't overflow\n");
val += ival;
printf("%d\n", val);
return 0;
}
Compiled with different options...
$ make
gcc -o ftrapv -ftrapv wrapme.c
gcc -o fwrapv -fwrapv wrapme.c
gcc -o default wrapme.c
How will it behave upon signed integer overflow?
$ ./fwrapv 0
will it overflow?
It won't overflow
2147483647
$ ./fwrapv 1
will it overflow?
It will overflow
-2147483648
$ ./ftrapv 0
will it overflow?
It won't overflow
2147483647
$ ./ftrapv 1
will it overflow?
Aborted (core dumped)
$ ./default 0
will it overflow?
It won't overflow
2147483647
$ ./default 1
will it overflow?
It won't overflow
-2147483648
