Returns true when running under a TTY, false under a VT

is_real_tty.c

#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdbool.h>
#include <linux/kd.h>
#include <sys/ioctl.h>

static int is_a_console(int fd){
	char arg = 0;
	return ((ioctl(fd, KDGKBTYPE, &arg) == 0) && ((arg == KB_101) || (arg == KB_84)));
}

static int open_a_console(const char *fnam){
	int fd = open(fnam, O_RDWR);
	if(fd < 0 && errno == EACCES)
		fd = open(fnam, O_WRONLY);
	if(fd < 0 && errno == EACCES)
		fd = open(fnam, O_RDONLY);
	if(fd < 0)
		return -1;
	if(!is_a_console(fd)){
		close(fd);
		return -1;
	}
	return fd;
}

int getfd(const char *fnam){
	int fd;
	if(fnam){
		fd = open_a_console(fnam);
		if(fd >= 0)
			return fd;
		return -1;
	}
	fd = open_a_console("/dev/tty");
	if(fd >= 0)
		return fd;
	fd = open_a_console("/dev/tty0");
	if(fd >= 0)
		return fd;
	fd = open_a_console("/dev/vc/0");
	if(fd >= 0)
		return fd;
	fd = open_a_console("/dev/console");
	if(fd >= 0)
		return fd;
	for(fd = 0; fd < 3; ++fd)
		if(is_a_console(fd))
			return fd;
	return -1;
}

bool is_real_tty(void){
	if(getfd(NULL) == -1)
		return false;
	else
		return true;
}

int main(){
	puts(is_real_tty()?"Outputting to a TTY":"Outputting to a VT");
}