0 and 1 processes in Linux

Introduction

I have been reading source code for screen program in Linux and I stumbled upon code:

void Kill(pid_t pid, int sig)
{
	if (pid < 2)
		return;
	(void)kill(pid, sig);
}

That raised some questions.

0 process

Process with 0 pid is called swapper process or idle task. It is a special process that starts the Linux kernel and then becomes a waiting process that helps in planning tasks. This process is a part of kernel that is why it is not seen with ps .

1 process

Process with 1 pid is a special process called init that manipulates services (deamons) in Linux. It starts right after 0 process. init process have different implementations on different systems (systemd, SysVinit, launched, ruinit, OpenRC and etc.)

About kill()

int kill(pid_t pid, int sig);

The  kill()  system  call  can be used to send any  signal to any process
   group or process.

   If pid is positive, then signal sig is sent to the process with  the  ID
   specified by pid.

   If  pid equals 0, then sig is sent to every process in the process group
   of the calling process.

   If pid equals -1, then sig is sent to every process for which the  call‐
   ing process has permission to send signals, except for process 1 (init),
   but see below.

   If pid is less than -1, then sig is sent to every process in the process
   group whose ID is -pid.

So

#include <stdio.h>
#include <signal.h>
#include <errno.h>
#include <string.h>

int main(){
    if(kill(1, SIGKILL)){
        perror(strerror(errno));
        return 1;
    }
    return 0;
}

prints

Operation not permitted: Operation not permitted

And in theory you can’t send any signals to the 0 process.