whatismytty.c

#include <unistd.h>
#include <stdio.h>

void print_tty(char *name,FILE *f){
        printf("%s (fileno %d): ",name,fileno(f));
        if(isatty(fileno(f)))
                printf("TTY %s\n",ttyname(fileno(f)));
        else
                printf("not a TTY\n");
}

int main(void){
        print_tty("stdin",stdin);
        print_tty("stdout",stdout);
        print_tty("stderr",stderr);
}