Am 13.06.2019 21:39, schrieb Sami Kerola:
> On Wed, 12 Jun 2019 at 23:20, Michael Kerrisk (man-pages)
> <mtk.manpages@xxxxxxxxx> wrote:
>> On Tue, 7 May 2019 at 23:17, Sami Kerola <kerolasa@xxxxxx> wrote:
>>>
>>> Example tries to clarify one should not refer to variables that are not in
>>> on_exit() scope. Such variables include heap in main(). In short only
>>
>> I must confess I never thought about this one at length. Why are heap
>> variable not in scope for fcuntions registered with
>> on_exit()/atexit()?
>
> Hello Michael,
>
> To be honest I do not know why heap variables do not work. If I must guess
> I would say probably something to do with when during execution main() heap
> is returned back to free memory - that is before on_exit() runs.
>
The memory of i in brocken.c is allocated in main() and freed when you
leave main. Because on_exit() is running after main(), game over.
hope that helps,
re,
wh
> Any way the following shell script will print 42 when on_exit() refers
> variable that is allocated. At least with up to date (2019-06-13) arch
> linux (glibc 2.29-1) compiled either gcc or clang will print 0 when
> executing broken version. I suppose zero is just co-incident. More
> probably value does not have any guarantees.
>
> Whether on_exit() update should example code showing how to use function
> properly, or a warning in BUGS section is matter of taste. Either way it
> would be nice if others would not need wonder what is this function doing.
>
> -- snip
> #!/bin/sh
>
> cat > ./broken.c <<EOF
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
>
> static void run_on_exit(int exit_val, void *arg)
> {
> int *i = (int *)arg;
>
> printf("%d\n", *i);
> _exit(exit_val);
> }
>
> int main(int argc, char **argv)
> {
> int i = 42;
>
> on_exit(run_on_exit, &i);
> return 0;
> }
> EOF
>
> cat > ./works.c <<EOF
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
>
> static void run_on_exit(int exit_val, void *arg)
> {
> int *i = (int *)arg;
>
> printf("%d\n", *i);
> free(i);
> _exit(exit_val);
> }
>
> int main(int argc, char **argv)
> {
> int *i = malloc(sizeof(int));
>
> *i = 42;
> on_exit(run_on_exit, i);
> return 0;
> }
> EOF
>
> gcc -Wall -pedantic -o broken broken.c
> gcc -Wall -pedantic -o works works.c
> set -x
> ./broken
> ./works
> -- snip
>
> $ ./snip.sh
> + ./broken
> 0
> + ./works
> 42
>
