#include <termios.h>

       int ioctl(int fd, int cmd, ...);

       The  ioctl(2) call for terminals and serial ports accepts many possible
       command arguments.  Most require a third  argument,  of  varying  type,
       here called argp or arg.

       Use  of  ioctl makes for nonportable programs.  Use the POSIX interface
       described in termios(3) whenever possible.

   Get and set terminal attributes
       TCGETS    struct termios *argp
              Equivalent to tcgetattr(fd, argp).
              Get the current serial port settings.

       TCSETS    const struct termios *argp
              Equivalent to tcsetattr(fd, TCSANOW, argp).
              Set the current serial port settings.

       TCSETSW   const struct termios *argp
              Equivalent to tcsetattr(fd, TCSADRAIN, argp).
              Allow the output buffer to drain, and  set  the  current  serial
              port settings.

       TCSETSF   const struct termios *argp
              Equivalent to tcsetattr(fd, TCSAFLUSH, argp).
              Allow the output buffer to drain, discard pending input, and set
              the current serial port settings.

       The following four  ioctls  are  just  like  TCGETS,  TCSETS,  TCSETSW,
       TCSETSF,  except  that  they take a struct termio * instead of a struct
       termios *.

              TCGETA    struct termio *argp

              TCSETA    const struct termio *argp

              TCSETAW   const struct termio *argp

              TCSETAF   const struct termio *argp

   Locking the termios structure
       The termios structure of a terminal can be locked.  The lock is  itself
       a  termios  structure,  with nonzero bits or fields indicating a locked

       TIOCGLCKTRMIOS struct termios *argp
              Gets the locking status of the termios structure of  the  termi-

       TIOCGWINSZ     struct winsize *argp
              Get window size.

       TIOCSWINSZ     const struct winsize *argp
              Set window size.

       The struct used by these ioctls is defined as

           struct winsize {
               unsigned short ws_row;
               unsigned short ws_col;
               unsigned short ws_xpixel;   /* unused */
               unsigned short ws_ypixel;   /* unused */

       When the window size changes, a SIGWINCH signal is sent  to  the  fore-
       ground process group.

   Sending a break
       TCSBRK    int arg
              Equivalent to tcsendbreak(fd, arg).
              If  the terminal is using asynchronous serial data transmission,
              and arg is zero, then send a break (a stream of zero  bits)  for
              between  0.25  and  0.5  seconds.   If the terminal is not using
              asynchronous serial data transmission, then either  a  break  is
              sent,  or the function returns without doing anything.  When arg
              is nonzero, nobody knows what will happen.

              (SVr4, UnixWare, Solaris, Linux treat  tcsendbreak(fd,arg)  with
              nonzero arg like tcdrain(fd).  SunOS treats arg as a multiplier,
              and sends a stream of bits arg times as long as  done  for  zero
              arg.   DG/UX and AIX treat arg (when nonzero) as a time interval
              measured in milliseconds.  HP-UX ignores arg.)

       TCSBRKP   int arg
              So-called "POSIX version" of TCSBRK.  It treats nonzero arg as a
              timeinterval  measured in deciseconds, and does nothing when the
              driver does not support breaks.

       TIOCSBRK  void
              Turn break on, that is, start sending zero bits.

       TIOCCBRK  void
              Turn break off, that is, stop sending zero bits.

   Software flow control
       TCXONC    int arg
              Equivalent to tcflow(fd, arg).
              See tcflow(3) for the argument  values  TCOOFF,  TCOON,  TCIOFF,

   Buffer count and flushing
       FIONREAD  int *argp

   Faking input
       TIOCSTI   const char *argp
              Insert the given byte in the input queue.

   Redirecting console output
       TIOCCONS  void
              Redirect  output  that  would  have  gone  to  /dev/console   or
              /dev/tty0  to  the given terminal.  If that was a pseudoterminal
              master, send it to the slave.  In Linux before  version  2.6.10,
              anybody  can  do  this  as long as the output was not redirected
              yet; since  version  2.6.10,  only  root  (a  process  with  the
              CAP_SYS_ADMIN capability) may do this.  If output was redirected
              already EBUSY is returned, but redirection  can  be  stopped  by
              using this ioctl with fd pointing at /dev/console or /dev/tty0.

   Controlling terminal
       TIOCSCTTY int arg
              Make  the given terminal the controlling terminal of the calling
              process.  The calling process must be a session leader  and  not
              have  a  controlling  terminal  already.   If  this  terminal is
              already the controlling terminal of a  different  session  group
              then the ioctl fails with EPERM, unless the caller is root (more
              precisely: has the CAP_SYS_ADMIN capability) and arg  equals  1,
              in which case the terminal is stolen, and all processes that had
              it as controlling terminal lose it.

       TIOCNOTTY void
              If the given terminal was the controlling terminal of the  call-
              ing  process, give up this controlling terminal.  If the process
              was session leader, then send SIGHUP and SIGCONT  to  the  fore-
              ground  process  group  and all processes in the current session
              lose their controlling terminal.

   Process group and session ID
       TIOCGPGRP pid_t *argp
              When successful, equivalent to *argp = tcgetpgrp(fd).
              Get the process group ID of the foreground process group on this

       TIOCSPGRP const pid_t *argp
              Equivalent to tcsetpgrp(fd, *argp).
              Set the foreground process group ID of this terminal.

       TIOCGSID  pid_t *argp
              Get  the  session ID of the given terminal.  This will fail with
              ENOTTY in case the terminal is not a master  pseudoterminal  and
              not our controlling terminal.  Strange.

   Exclusive mode
       TIOCEXCL  void
              Put the terminal into exclusive mode.  No further open(2) opera-
              tions on the terminal  are  permitted.   (They  will  fail  with
              EBUSY,   except   for   root,   that  is,  a  process  with  the

   Pseudoterminal ioctls
       TIOCPKT   const int *argp
              Enable (when *argp is nonzero) or disable packet mode.   Can  be
              applied  to  the  master side of a pseudoterminal only (and will
              return ENOTTY  otherwise).   In  packet  mode,  each  subsequent
              read(2)  will  return  a  packet  that  either contains a single
              nonzero control byte, or has a single byte containing zero (' ')
              followed  by  data written on the slave side of the pseudotermi-
              nal.  If the first byte is not TIOCPKT_DATA (0), it is an OR  of
              one or more of the following bits:

              TIOCPKT_FLUSHREAD   The read queue for the terminal is flushed.
              TIOCPKT_FLUSHWRITE  The write queue for the terminal is flushed.
              TIOCPKT_STOP        Output to the terminal is stopped.
              TIOCPKT_START       Output to the terminal is restarted.
              TIOCPKT_DOSTOP      The start and stop characters are ^S/^Q.
              TIOCPKT_NOSTOP      The start and stop characters are not ^S/^Q.

              While this mode is in use, the presence of control status infor-
              mation to be read from the master side  may  be  detected  by  a
              select(2) for exceptional conditions.

              This  mode  is  used  by rlogin(1) and rlogind(8) to implement a
              remote-echoed, locally ^S/^Q flow-controlled remote login.

              not been implemented under Linux.

   Modem control
       TIOCMGET  int *argp
              get the status of modem bits.

       TIOCMSET  const int *argp
              set the status of modem bits.

       TIOCMBIC  const int *argp
              clear the indicated modem bits.

       TIOCMBIS  const int *argp
              set the indicated modem bits.

       Bits used by these four ioctls:

       TIOCM_LE        DSR (data set ready/line enable)
       TIOCM_DTR       DTR (data terminal ready)
       TIOCM_RTS       RTS (request to send)
       TIOCM_ST        Secondary TXD (transmit)
       TIOCM_SR        Secondary RXD (receive)
       TIOCM_CTS       CTS (clear to send)
       TIOCM_CAR       DCD (data carrier detect)
       TIOCM_CD         see TIOCM_CAR
       TIOCM_RNG       RNG (ring)
       TIOCM_RI         see TIOCM_RNG
       TIOCM_DSR       DSR (data set ready)
       will  block until DCD is asserted, unless the O_NONBLOCK flag is given.
       If CLOCAL is set, the line behaves as if DCD is always  asserted.   The
       software  carrier  flag  is usually turned on for local devices, and is
       off for lines with modems.

       For the TIOCLINUX ioctl, see console_ioctl(4).

   Kernel debugging
       #include <linux/tty.h>

       TIOCTTYGSTRUCT struct tty_struct *argp
              Get the tty_struct corresponding to fd.

       The ioctl(2) system call returns 0 on success.  On error it returns  -1
       and sets errno appropriately.

       EINVAL Invalid command parameter.

              Unknown command.

       ENOTTY Inappropriate fd.

       EPERM  Insufficient permission.

       Check the condition of DTR on the serial port.

       #include <termios.h>
       #include <fcntl.h>
       #include <sys/ioctl.h>

           int fd, serial;

           fd = open("/dev/ttyS0", O_RDONLY);
           ioctl(fd, TIOCMGET, &serial);
           if (serial & TIOCM_DTR)
               puts("TIOCM_DTR is not set");
               puts("TIOCM_DTR is set");

       ioctl(2), termios(3), console_ioctl(4), pty(7)

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