Shared memory ignores read only flag in Linux c

I'm using shared memory with shmget and shmat for educational purpose.
I'm trying to make a memory chunk to be mutable only by it's creator and all other processes can read only.
But the reader processes can somehow write without any error.

This is my code for the creator of the shared memory:

#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

int main(){
    int shmid = shmget((key_t)56666, 1, IPC_CREAT | O_RDONLY);
     if (shmid ==-1) {
        perror("Err0:");
        exit(EXIT_FAILURE);
    }
    void* shmaddr = shmat(shmid, (void *)0,0);
    if (shmaddr == (void *)-1) {
        perror("Err:");
        exit(EXIT_FAILURE);
    }
    *(char*)shmaddr = 'a'; 
    putchar(*(char*)shmaddr);
    while(1);
    return 0;
}

And this is my code for the reader:

#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
int main(){
    int shmid = shmget((key_t)56666, 4, O_RDONLY);
     if (shmid ==-1) {
        perror("Err0:");
        exit(EXIT_FAILURE);
    }
    void* shmaddr = shmat(shmid, (void *)0,0);
    if (shmaddr == (void *)-1) {
        perror("Err:");
        exit(EXIT_FAILURE);
    }
    *(char*)shmaddr = 'b'; 
    putchar(*(char*)shmaddr);
    return 0;
}

As you can see the reader can edit the memory but no error occures even though I open the memory as read only in the reader and created it with read only flag in the creator of the shared memory.