9.c

#include <stdio.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
    int fd, ret;

    fd = open("./test", O_RDONLY);
    if (fd == -1)
    {
        perror("open");
        return -1;
    }

    ret = fchmod(fd, S_IRUSR | S_IRGRP | S_IROTH);
    if (ret == -1)
    {
        perror("fchmod");
        return -1;
    }

    close(fd);

    return 0;
}