C(++) & CLI: Reading input generated by the terminal itself

Question

When you write \e[6m (the "tell-me-where-I-am" code) to a compliant terminal, it will write back \e[<ROW>;<COLUMN>R, eg \e75;1R.

NOTE: all the \es here are the escape code, not literal "slash ee"s, so what's actually written back is garbled if not captured.

In bash, you can read & output this via:

#!/bin/bash

echo -en "\E[6n";
read -sdR CURTEXT;
CURPOS=${CURTEXT#*[};
echo $CURPOS;

Or the one-line CLI command echo -en "\E[6n"; read -sdR ROW_COL_RAW; ROW_COL=${ROW_COL_RAW#*[}; printf "%b\n" $ROW_COL;.

NOTE: I got this from https://stackoverflow.com/a/6003016/3969962.

While bash can obtain this text via read, I can't figure out how to obtain it from C++. The escaped coordinate text is still being written by the terminal itself, but I can't read these characters from std::cin, even by fread().

---

1. How, and where, is this text actually being written (by the terminal iteslf)?
2. Is there some way I can read this data in C++ (eg, to a std::string)? (hopefully portably and without race conditions).

---

My goal is to determine the current tab width so that output can be nicely displayed. I got the terminal dimensions via ioctl(0, TIOCGWINSZ, &win_size).

Answer 1

1. How, and where, is this text actually being written (by the terminal iteslf)?

I believe it's written by the terminal mimicking user input, exactly in the same place where it goes if input by the user. If you press enter after it's shown in the terminal, this should appear in the fread()/cin of your program.

2. Is there some way I can read this data in C++ (eg, to a std::string)? (hopefully portably and without race conditions).

According to the other question you posted, you can avoid writing to the terminal by setting it to raw mode. Do this from your program and it should work. Just be careful when handling the received data because it has non printable characters. I think the following code does what you want. set_raw_tty() and its helper functions are based on this reference and are posted at the end of this answer to make it clearer:

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

#include <iostream>
#include <string>

void tty_atexit(void);

static struct termios orig_termios;
static int ttyfd = STDIN_FILENO;
void fatal(char *mess);
void tty_atexit(void);
int tty_reset(void);
int tty_raw(void);
void set_raw_tty();

int main()
{
    set_raw_tty();

    std::cout << "\E[6n";

    std::string s;

    char c = 0;
    int ret = -1;

    ret = fread(&c, 1, 1, stdin);
    while (ret == 1) {
        ret = fread(&c, 1, 1, stdin);
        if (ret != 1 || c == 'R')
            break;
        s += c;
    }

    std::cout << "s = "<< s << "\r\n" << std::flush;
}

Testing:

$ ./a.out 
s = [2;1
$ ./a.out 
s = [4;1
$ 
$ 
$ 
$ ./a.out 
s = [9;1

The rest of the code to be added after main function:

void set_raw_tty()
{
    if (tcgetattr(ttyfd,&orig_termios) < 0) {
        printf ("%s %d: error here\n", __FUNCTION__, __LINE__);
        exit(1);
    }
    if (atexit(tty_atexit) != 0) {
        printf ("%s %d: error here\n", __FUNCTION__, __LINE__);
        exit(1);
    }
    if (tty_raw() != 0) {
        printf ("%s %d: error here\n", __FUNCTION__, __LINE__);
        exit(1);
    }
}

void tty_atexit(void)
{
    tty_reset();
}

int tty_raw(void)
{
    struct termios raw;
    raw = orig_termios;
    raw.c_iflag &= ~(BRKINT | ICRNL | INPCK | ISTRIP | IXON);
    raw.c_oflag &= ~(OPOST);
    raw.c_cflag |= (CS8);
    raw.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
    raw.c_cc[VMIN] = 0; raw.c_cc[VTIME] = 8;
    return tcsetattr(ttyfd,TCSAFLUSH,&raw) < 0;
}

int tty_reset(void)
{
    if (tcsetattr(ttyfd,TCSAFLUSH,&orig_termios) < 0)
        return -1;
    return 0;
}