Can't get LibC termios VTIME to work with Crystal

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1

I’m building an interactive file explorer/manager and thus I’m putting the terminal in “raw” mode. I also set VMIN to 0 effectively changing the minimum number of required bytes to zero before C’s read function can return. In addition to that, I also set VTIME to 1. It ensures that read will wait at least 1/10 of second (100ms) before returning. While VMIN seems to work fine, VTIME does not seem to have any effect.

Here’s the code:

lib LibC
  VTIME = 17
end

def raw_mode!
  LibC.tcgetattr(STDIN.fd, out orig_termios)

  at_exit do
    LibC.tcsetattr(STDIN.fd, LibC::TCSAFLUSH, pointerof(orig_termios))
  end

  raw = orig_termios
  raw.c_iflag &= ~(LibC::BRKINT | LibC::ICRNL | LibC::INPCK | LibC::ISTRIP | LibC::IXON)
  raw.c_oflag &= ~(LibC::OPOST)
  raw.c_cflag |= (LibC::CS8)
  raw.c_lflag &= ~(LibC::ECHO | LibC::ICANON | LibC::IEXTEN | LibC::ISIG)
  raw.c_cc[LibC::VMIN] = 0
  raw.c_cc[LibC::VTIME] = 1

  LibC.tcsetattr(STDIN.fd, LibC::TCSAFLUSH, pointerof(raw))

  yield
end

raw_mode! do
  loop do
    b = STDIN.read_byte

    if b.nil?
      puts "read timeout\r\n"
      next
    end

    ch = b.chr
        
    if ch.ascii_control?
      puts "#{b}\r\n"
    else
      puts "#{b} (#{ch})\r\n"
    end
    
    exit if ch == 'q'
  end
end

For comparsion purposes, here’s the equivalent C code that does respect VTIME:

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

struct termios orig_termios;

void disableRawMode() {
  tcsetattr(STDIN_FILENO, TCSAFLUSH, &orig_termios);
}

void enableRawMode() {
  tcgetattr(STDIN_FILENO, &orig_termios);
  atexit(disableRawMode);
  struct termios 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);

  printf("VMIN is %d\n", VMIN);
  printf("VTIME is %d\n", VTIME);
  
  raw.c_cc[VMIN] = 0;
  raw.c_cc[VTIME] = 1;
  tcsetattr(STDIN_FILENO, TCSAFLUSH, &raw);
}
int main() {
  enableRawMode();
  while (1) {
    char c = '\0';
    read(STDIN_FILENO, &c, 1);
    if (iscntrl(c)) {
      printf("%d\r\n", c);
    } else {
      printf("%d ('%c')\r\n", c, c);
    }
    if (c == 'q') break;
  }
  return 0;
}

and here’s a man page referencing both VMIN and VTIME.

2

I did some small terminal apps in Crystal few years ago, to avoid dealing with raw terminal stuff I just used a termbox binding https://github.com/hugopl/textui/blob/master/src/textui/terminal.cr

Unfortunately termbox isn’t being developed anymore… but it still works and IIRC this binding compiles the termbox itself in a static library so there’s no runtime dependencies on termbox.

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