How to create pty that is connectable by Screen app in Linux - c++

Question

I wanted to create C/C++ application, that creates new (virtual) device in /dev/xxx and will be able to connect with 'screen' application.
For example program running in loop, that creates new /dev/ttyABC. Then I'll use 'screen /dev/ttyABC', and when I send there some chars, then app send it back to the 'screen'.
I really don't know where start. I found some referencies on pty library but I don't even know, if I have right direction.
Could you help me? Where to look? Post example?
Thanks

Answer 1

You could use a Pseudoterminal via openpty to achieve this. openpty returns a pair of file descriptors (master and slave pty devices) that are connected to each other via their stdout / stdin. The output of one will appear at the input of another and vice-versa.
Using this (rough!) example...
#include <fcntl.h>
#include <cstdio>
#include <errno.h>
#include <pty.h>
#include <string.h>
#include <unistd.h>
int main(int, char const *[])
{
int master, slave;
char name[256];
auto e = openpty(&master, &slave, &name[0], nullptr, nullptr);
if(0 > e) {
std::printf("Error: %s\n", strerror(errno));
return -1;
}
std::printf("Slave PTY: %s\n", name);
int r;
while((r = read(master, &name[0], sizeof(name)-1)) > 0) {
name[r] = '\0';
std::printf("%s", &name[0]);
}
close(slave);
close(master);
return 0;
}
... Echoing some text (in another terminal session) to the slave pty sends it to master's input. E.g. echo "Hello" > /dev/pts/2

Answer 2

Based on the answer provided by #gmbeard , I was able to create an echo PTY device and connect to it with screen and minicom. What made the difference was using a raw PTY device by initializing a termios struct.
Here is the code
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <cstdio>
#include <pty.h>
#include <termios.h>
#define BUF_SIZE (256)
int main(int, char const *[])
{
int master, slave;
char buf[BUF_SIZE];
struct termios tty;
tty.c_iflag = (tcflag_t) 0;
tty.c_lflag = (tcflag_t) 0;
tty.c_cflag = CS8;
tty.c_oflag = (tcflag_t) 0;
auto e = openpty(&master, &slave, buf, &tty, nullptr);
if(0 > e) {
std::printf("Error: %s\n", strerror(errno));
return -1;
}
std::printf("Slave PTY: %s\n", buf);
int r;
while ( (r = read(master, buf, BUF_SIZE)) > 0 )
{
write(master, buf, r);
}
close(slave);
close(master);
return 0;
}