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The GNU C Library

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17.7 Noncanonical Mode Example

Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo.

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

/* Use this variable to remember original terminal attributes. */

struct termios saved_attributes;

reset_input_mode (void)
  tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);

set_input_mode (void)
  struct termios tattr;
  char *name;

  /* Make sure stdin is a terminal. */
  if (!isatty (STDIN_FILENO))
      fprintf (stderr, "Not a terminal.\n");
      exit (EXIT_FAILURE);

  /* Save the terminal attributes so we can restore them later. */
  tcgetattr (STDIN_FILENO, &saved_attributes);
  atexit (reset_input_mode);

  /* Set the funny terminal modes. */
  tcgetattr (STDIN_FILENO, &tattr);
  tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
  tattr.c_cc[VMIN] = 1;
  tattr.c_cc[VTIME] = 0;
  tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);

main (void)
  char c;

  set_input_mode ();

  while (1)
      read (STDIN_FILENO, &c, 1);
      if (c == '\004')          /* C-d */
        putchar (c);

  return EXIT_SUCCESS;

This program is careful to restore the original terminal modes before exiting or terminating with a signal. It uses the atexit function (see section 25.6.3 Cleanups on Exit) to make sure this is done by exit.

The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see 27. Job Control. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so.

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