EPERM error in changing nice value of a process through setpriority()

Question

I'm trying to change the nice value through a publicly available code but I'm getting a Permission Denied (EPERM) message.

The program fails when executed either as a normal user or with sudo.

#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/resource.h>

int
main(void)
{
    int pol;
    struct sched_param sp;
    struct rlimit rl;

    pol = sched_getscheduler(getpid());
    if (pol == -1) {
        perror("sched_getscheduler");
        exit(EXIT_FAILURE);
    }

    if (sched_getparam(0, &sp) == -1) {
        perror("sched_getparam");
        exit(EXIT_FAILURE);
    }
            
    printf("%ld: %-5s ", (long)getpid(),
            (pol == SCHED_OTHER) ? "OTHER" :
            (pol == SCHED_RR) ? "RR" :
            (pol == SCHED_FIFO) ? "FIFO" :
    #ifdef SCHED_BATCH              /* Linux-specific */
            (pol == SCHED_BATCH) ? "BATCH" :
    #endif
    #ifdef SCHED_IDLE               /* Linux-specific */
            (pol == SCHED_IDLE) ? "IDLE" :
    #endif
            "???");
    printf("%2d\n", sp.sched_priority);

    if (setpriority(PRIO_PROCESS, 0, -10) == -1) {
        perror("setpriority");
        exit(EXIT_FAILURE);
    }

    printf("New Nice value is %d\n", getpriority(PRIO_PROCESS, 0));

    #ifdef _POSIX_PRIORITY_SCHEDULING
        printf("Setting scheduler to real-time RR and current process priority to %d\n", sched_get_priority_max(SCHED_RR));
        sp.sched_priority=sched_get_priority_max(SCHED_RR);
        printf("sp.sched prio is %d\n", sp.sched_priority);
        if (sched_setscheduler(getpid(), SCHED_RR, &sp) == -1) {
            perror("sched_setscheduler");
            exit(EXIT_FAILURE);
        }
    #endif   

I'm using Ubuntu 20.04.

I have tried to modify /etc/security/limits.conf but with no success.

rlim_cur is 0. If i try to modify and set

rl.rlim_cur=(rlim_t) -20; I get an ivalid argument message.

If I write: rl.rlim_cur=(rlim_t) 20; I get a Permission denied message. (After this setting I invoke setrlimit).

Does this have to do with capabilities?

Any help/guidance is greatly appreciated.

(As you may have already imagined the code for changing the scheduling policy to SCHED_RR fails also, perhaps the answer to this post solves this too, otherwise it may be the subject of a different post...)