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           #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
       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 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.  For this case, arg should
                  be specified as zero.
                  If this terminal is already the controlling terminal of  a  dif-
                  ferent session group then the ioctl fails with EPERM, unless the
                  caller 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
       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.
                  The BSD ioctls TIOCSTOP, TIOCSTART, TIOCUCNTL,  TIOCREMOTE  have
                  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)
           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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