unable to write /proc/sys/kernel/ns_last_pid file - c++

Question

I want to edit the ns_last_pid file present in /proc/sys/kernel, but i'm getting the error of Read-only file system. How to resolve this?
This is what i've written to open the file.
int fd = open("/proc/sys/kernel/ns_last_pid", O_RDWR | O_CREAT, 0644);
if (fd < 0) {
cout<<strerror(errno)<<"\n";
return 1;
}
I've to write this file, change it's value. This file contains a single number represnting the last pid allocated to any process. I've to edit this so that i can get desired pid number for a process. like these guys are doing for their project CRIU(see first link).
Pid_restore(criu.org),
How to set process ID in Linux for a specific program(stackoverflow answer)
EDIT 1: Smallest reproducible example
#include <fstream>
#include <bits/stdc++.h>
#include <sys/types.h>
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/syscall.h>
using namespace std;
int main(){
printf("Opening ns_last_pid...\n");
int fd = open("/proc/sys/kernel/ns_last_pid", O_RDWR | O_CREAT, 0644);
if (fd < 0) {
cout<<strerror(errno)<<"\n";
return 1;
}
printf("Locking ns_last_pid...\n");
if (flock(fd, LOCK_EX)) {
close(fd);
printf("Can't lock ns_last_pid\n");
return 1;
}
printf("Done\n");
char buf[100];
int pid_max = 30000;
snprintf(buf, sizeof(buf), "%d", pid_max-1);
printf("Writing pid-1 to ns_last_pid...\n");
cout<<fd<<"\n";
if (write(fd, buf, strlen(buf)) != strlen(buf)) {
cout<<strerror(errno)<<"\n";
printf("Can't write to buf\n");
return 1;
}
printf("Done\n");
printf("Cleaning up...");
if (flock(fd, LOCK_UN)) {
printf("Can't unlock");
}
close(fd);
printf("Done\n");
return 0;
}

Answer 1

For a program to change kernel files, it should be owned by root
sudo chown root program // program is the executable(the binary)
set the setuid bit on the executable to execute a program with superuser access.
with this it will run as root even if we execute it as any user on our machine.
sudo chmod u+s program
Compile the source code and run the program with sudo to prevent other permission access errors.
Thanks to TedLyngmo for suggesting this solution.