Minimal example usage of /dev/ptmx

tty_demo.c

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

int main() {
    int master_tty = open("/dev/ptmx", O_RDWR);
    if (master_tty < 0) {
        fprintf(stderr, "MASTER: could not open /dev/ptmx.\n");
        exit(1);
    }

    char* slave_name = ptsname(master_tty);
    if (slave_name == NULL) {
        fprintf(stderr, "MASTER: could not get name of slave device.\n");
        exit(1);
    }
    printf("MASTER: slave device %s created\n", slave_name);

    if (grantpt(master_tty) < 0 || unlockpt(master_tty) < 0) {
        fprintf(stderr, "MASTER: could not grant access to the slave device.\n");
        exit(1);
    }
    
    int slave_tty = open(slave_name, O_RDWR);
    if (slave_tty < 0) {
        fprintf(stderr, "MASTER: could not open %s\n", slave_name);
        exit(1);
    }

    int pid;
    if ((pid = fork()) > 0) {

        // Parent code

        close(slave_tty);       
 
        char c[2] = {'x', '\n'}; // Note that without the newline, the line buffer won't be flushed
        printf("MASTER: sending character %c\n", c[0]);
        if (write(master_tty, &c, 2) < 0) {
            fprintf(stderr, "MASTER: could not write to master end of TTY\n");
            exit(1);
        }

        waitpid(-1, NULL, 0);
        printf("MASTER: exiting\n");
        exit(0);

    } else {

        // Child code

        close(master_tty);

        char c;
        if (read(slave_tty, &c, 1) < 0) {
            fprintf(stderr, "SLAVE: could not read from %s\n", slave_name);
            exit(1);
        }

        printf("SLAVE: received character %c\n", c);
        printf("MASTER: exiting\n");
        exit(0);

    }

}