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.
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
--
Sami Kerola
http://www.iki.fi/kerolasa/
