Why is effective UID still not 0? - c

Question

So I got this code below from here. However when I follow the commands:
sudo chown root:root a.out
sudo chmod u+s a.out
It still won't run with effective uid to 0.
This is the code:
#define _POSIX_C_SOURCE 200112L // Needed with glibc (e.g., linux).
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
void report (uid_t real) {
printf ("Real UID: %d Effective UID: %d\n", real, geteuid());
}
int main (void) {
uid_t real = getuid();
report(real);
seteuid(real);
report(real);
return 0;
}
The output is:
Real UID: 1000 Effective UID: 1000
Real UID: 1000 Effective UID: 1000
Where I supposed it should have been:
Real UID: 1000 Effective UID: 0
Real UID: 1000 Effective UID: 1000

Answer 1

The getuid function returns the real user id only. The real user id of user process is still 1000. The 1000 is stored in the real variable.
So, while calling of seteuid function you again sets the 1000 as the effective user id.But actually you have to set the 0 in that place. So only the effective user id doesn't changed.
Try the below it will works fine.
#define _POSIX_C_SOURCE 200112L // Needed with glibc (e.g., linux).
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
void report (uid_t real) {
printf ("Real UID: %d Effective UID: %d\n", real, geteuid());
}
int main (void) {
uid_t real = getuid();
report(real);
seteuid(0);
report(real);
return 0;
}

Answer 2

Apparently as #MarkPlotnick said.
cd to the directory where that a.out is. Run df . to get the mount
point. Run mount | grep themountpoint and see what the filesystem type
and mount options are.
If there is something saying nosuid then you need to remount that part of filesystem without the nosuid flag.
This website here gives a good explanation about how to do that.