Based on sentsui by xrsec (https://gist.github.com/xrsec/da6c11a49ba883c11ba68e50f0a809de)

sentsui.c

#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <fcntl.h>

int main(int argc, char **argv) {
    if(argc != 3 || getuid()) {
        fprintf(stderr, "Run '%s <fd> <cmd>' as root\n", argv[0]);
        return 1;
    }
    int fd = open(argv[1], O_RDWR);
    if(!fd) {
        fprintf(stderr, "Can't open %s\n", argv[1]);
        return 1;
    }
    for(char *c = argv[2]; *c; c++)
        if(ioctl(fd, TIOCSTI, c))
            return 1;
    return ioctl(fd, TIOCSTI, "\n");
}