Re: [Tinycc-devel] An interesting bug with clock() in debug mode

[Rob]

Your code has undefined behaviour due to the buffer overflow of dst. Try this:

clang -x c -std=c11 -DDEBUG demo.tinyc -fsanitize=address -o dt && ./dt


On Wed, 23 Nov 2022 at 20:50, Stefanos via Tinycc-devel
<tinycc-devel@nongnu.org> wrote:
>
> Greetings everyone,
>
> I hope I find you well.
>
> I was playing with C and discovered an interesting TCC bug by mistake!
>
> Here's the code I used:
>
> ```
> cat demo.tinyc
> #if 0
> set -e;
> exec tcc -Wall -std=c11 -x c -run "$0"
> #endif
>
> /*
>  * Original example taken from
>  * https://riptutorial.com/c/example/10946/copying-strings
>  * but of course I have expanded it a bit further.
>  */
> #include <stdio.h>
> #include <string.h>
> #include <assert.h>
>
> #ifdef DEBUG
> #include <time.h>
> #endif
>
> int main(void)
> {
> #ifdef DEBUG
>     clock_t begin = clock();
>     printf("DEBUG MODE ON (current clock time (begin) is %ld)\n", begin);
> #endif
>
>     char dst[24];
>     printf("sizeof(dst): %3zu bytes\n", sizeof(dst));
>     memset(dst, '\0', sizeof(dst));
>     char src[] = "Marvin and Nemo";
>     size_t len = strlen(dst);
>     printf("strlen(dst): %3zu characters\n", len);
>
>     strcpy(dst, "Clownfish: ");
>     assert(len < sizeof(dst) - 1);
>     strncat(dst + len, src, sizeof(dst) - len - 1);
>     putchar('\n');
>     printf("sizeof(dst): %3zu bytes\n", sizeof(dst));
>     printf("dst(\"%s\") [%zu characters]\n", dst, strlen(dst));
>
> #ifdef DEBUG
>     printf("Current time via clock(): %ld, initial time (begin): %ld\n", 
> clock(), begin);
>     double timespent = (double)((clock() - begin)) / CLOCKS_PER_SEC;
>
>     printf("Total time: %f secs, %.3f ms, %.2f μs\n",
>             timespent, timespent*1000.0, timespent*1000.0*1000.0);
> #endif
>     return 0;
> }
>
> /* vim: set ft=c: */
> ```
>
> Here is the command and output of GCC:
>
> gcc -x c -Wall -Wextra -Wpedantic -std=c11 -DDEBUG demo.tinyc -o dt && ./dt 
> && rm dt
> DEBUG MODE ON (current clock time (begin) is 514)
> sizeof(dst):  24 bytes
> strlen(dst):   0 characters
>
> sizeof(dst):  24 bytes
> dst("Clownfish: Marvin and Nemo") [26 characters]
> Current time via clock(): 549, initial time (begin): 514
> Total time: 0.000037 secs, 0.037 ms, 37.00 μs
>
>
> Here's also clang's:
>
> clang -x c -Wall -Wextra -Wpedantic -std=c11 -DDEBUG demo.tinyc -o dt && ./dt 
> && rm dt
> DEBUG MODE ON (current clock time (begin) is 615)
> sizeof(dst):  24 bytes
> strlen(dst):   0 characters
>
> sizeof(dst):  24 bytes
> dst("Clownfish: Marvin and Nemo") [26 characters]
> Current time via clock(): 659, initial time (begin): 615
> Total time: 0.000046 secs, 0.046 ms, 46.00 μs
>
> Now here's TCC's output:
>
> tcc -x c -Wall -std=c11 -DDEBUG demo.tinyc -o dt && ./dt && rm dt
> DEBUG MODE ON (current clock time (begin) is 521)
> sizeof(dst):  24 bytes
> strlen(dst):   0 characters
>
> sizeof(dst):  24 bytes
> dst("Clownfish: Marvin and Nemo") [26 characters]
> Current time via clock(): 557, initial time (begin): 28525
> Total time: -0.027966 secs, -27.966 ms, -27966.00 μs
>
> No matter how many times I execute it, initial time (begin) remains 28525.
>
> I have no idea where to look for a possible fix.
>
> Please advice.
>
> Regards,
>
> Stefanos
>
> P.S.: the #if 0 trick is to create a portable way of scripting my code so
> I can test with all C compilers that exist out there; without it, it
> complains about the #! character on first line of code and does not
> let me compile it.
>
> As you can see, I have already demonstrated that way with 3 different
> compilers, whereas in reality I use it as `./demo.tinyc`.
>
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