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       ioctl_tty - ioctls for terminals and serial lines


       #include <sys/ioctl.h>
       #include <termios.h>      /* Definition of CLOCAL, and
                                    TC*{FLUSH,ON,OFF} constants */

       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 Argument: struct termios *argp

              Equivalent to tcgetattr(fd, argp).

              Get the current serial port settings.

       TCSETS Argument: const struct termios *argp

              Equivalent to tcsetattr(fd, TCSANOW, argp).

              Set the current serial port settings.

              Argument: const struct termios *argp

              Equivalent to tcsetattr(fd, TCSADRAIN, argp).

              Allow the output buffer to drain, and set the current serial port settings.

              Argument: 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, added in Linux 2.6.20, are just like TCGETS,  TCSETS,  TCSETSW,
       TCSETSF,  except that they take a struct termios2 * instead of a struct termios *.  If the
       structure member c_cflag contains the flag BOTHER, then the baud rate  is  stored  in  the
       structure members c_ispeed and c_ospeed as integer values.  These ioctls are not supported
       on all architectures.

              TCGETS2    struct termios2 *argp
              TCSETS2    const struct termios2 *argp
              TCSETSW2   const struct termios2 *argp
              TCSETSF2   const struct termios2 *argp

       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 value.

              Argument: struct termios *argp

              Gets the locking status of the termios structure of the terminal.

              Argument: const struct termios *argp

              Sets the locking status of the termios structure of the terminal.  Only  a  process
              with the CAP_SYS_ADMIN capability can do this.

   Get and set window size
       Window  sizes  are  kept  in the kernel, but not used by the kernel (except in the case of
       virtual consoles, where the kernel will update the  window  size  when  the  size  of  the
       virtual console changes, for example, by loading a new font).

              Argument: struct winsize *argp

              Get window size.

              Argument: 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 foreground process group.

   Sending a break
       TCSBRK Argument: 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, and 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.)

              Argument: int arg

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

              Argument: void

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

              Argument: void

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

   Software flow control
       TCXONC Argument: int arg

              Equivalent to tcflow(fd, arg).

              See tcflow(3) for the argument values TCOOFF, TCOON, TCIOFF, TCION.

   Buffer count and flushing
              Argument: int *argp

              Get the number of bytes in the input buffer.

              Argument: int *argp

              Same as FIONREAD.

              Argument: int *argp

              Get the number of bytes in the output buffer.

       TCFLSH Argument: int arg

              Equivalent to tcflush(fd, arg).

              See tcflush(3) for the argument values TCIFLUSH, TCOFLUSH, TCIOFLUSH.

   Faking input
              Argument: const char *argp

              Insert the given byte in the input queue.

   Redirecting console output
              Argument: 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, then EBUSY is returned, but redirection
              can be stopped by using this ioctl with fd pointing at /dev/console or /dev/tty0.

   Controlling terminal
              Argument: 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 different 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.

              Argument: void

              If  the given terminal was the controlling terminal of the calling process, give up
              this controlling terminal.  If the process was session leader, then send SIGHUP and
              SIGCONT  to  the  foreground process group and all processes in the current session
              lose their controlling terminal.

   Process group and session ID
              Argument: pid_t *argp

              When successful, equivalent to *argp = tcgetpgrp(fd).

              Get the process group ID of the foreground process group on this terminal.

              Argument: const pid_t *argp

              Equivalent to tcsetpgrp(fd, *argp).

              Set the foreground process group ID of this terminal.

              Argument: pid_t *argp

              Get the session ID of the given terminal.  This fails with the error ENOTTY if  the
              terminal is not a master pseudoterminal and not our controlling terminal.  Strange.

   Exclusive mode
              Argument: void

              Put  the  terminal  into  exclusive  mode.   No  further  open(2) operations on the
              terminal are permitted.  (They fail with EBUSY,  except  for  a  process  with  the
              CAP_SYS_ADMIN capability.)

              Argument: int *argp

              (since  Linux  3.8) If the terminal is currently in exclusive mode, place a nonzero
              value in the location pointed to by argp; otherwise, place zero in *argp.

              Argument: void

              Disable exclusive mode.

   Line discipline
              Argument: int *argp

              Get the line discipline of the terminal.

              Argument: const int *argp

              Set the line discipline of the terminal.

   Pseudoterminal ioctls
              Argument: 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 ('\0') followed by data
              written on the slave side  of  the  pseudoterminal.   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
              TIOCPKT_START        Output to the terminal is
              TIOCPKT_DOSTOP       The start and stop characters
                                   are ^S/^Q.
              TIOCPKT_NOSTOP       The start and stop characters
                                   are not ^S/^Q.

              While  packet mode is in use, the presence of control status information to be read
              from the master side may be detected by a select(2) for exceptional conditions or a
              poll(2) for the POLLPRI event.

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

              Argument: const int *argp

              (since Linux 3.8) Return the current packet mode setting in the integer pointed  to
              by argp.

              Argument: int *argp

              Set  (if  *argp  is  nonzero)  or  remove  (if  *argp  is  zero)  the  lock  on the
              pseudoterminal slave device.  (See also unlockpt(3).)

              Argument: int *argp

              (since Linux 3.8) Place the current lock state of the pseudoterminal  slave  device
              in the location pointed to by argp.

              Argument: int flags

              (since  Linux  4.13)  Given a file descriptor in fd that refers to a pseudoterminal
              master, open (with the given open(2)-style flags) and return a new file  descriptor
              that  refers  to  the  peer  pseudoterminal  slave  device.   This operation can be
              performed regardless of whether the pathname of  the  slave  device  is  accessible
              through the calling process's mount namespace.

              Security-conscious  programs  interacting  with  namespaces  may  wish  to use this
              operation rather than open(2) with the pathname returned by ptsname(3), and similar
              library  functions  that  have insecure APIs.  (For example, confusion can occur in
              some cases using ptsname(3) with a pathname where  a  devpts  filesystem  has  been
              mounted in a different mount namespace.)

       The  BSD  ioctls  TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCREMOTE have not been implemented
       under Linux.

   Modem control
              Argument: int *argp

              Get the status of modem bits.

              Argument: const int *argp

              Set the status of modem bits.

              Argument: const int *argp

              Clear the indicated modem bits.

              Argument: const int *argp

              Set the indicated modem bits.

       The following bits are used by the above 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)

              Argument: int arg

              Wait for any of the 4 modem bits (DCD, RI,  DSR,  CTS)  to  change.   The  bits  of
              interest  are  specified  as  a  bit  mask in arg, by ORing together any of the bit
              values, TIOCM_RNG, TIOCM_DSR, TIOCM_CD,  and  TIOCM_CTS.   The  caller  should  use
              TIOCGICOUNT to see which bit has changed.

              Argument: struct serial_icounter_struct *argp

              Get  counts  of  input  serial line interrupts (DCD, RI, DSR, CTS).  The counts are
              written to the serial_icounter_struct structure pointed to by argp.

              Note: both 1->0 and 0->1 transitions are counted, except for RI,  where  only  0->1
              transitions are counted.

   Marking a line as local
              Argument: int *argp

              ("Get  software  carrier  flag")  Get  the status of the CLOCAL flag in the c_cflag
              field of the termios structure.

              Argument: const int *argp

              ("Set software carrier flag") Set the CLOCAL flag in  the  termios  structure  when
              *argp is nonzero, and clear it otherwise.

       If  the  CLOCAL  flag  for  a  line  is  off,  the hardware carrier detect (DCD) signal is
       significant, and an open(2)  of  the  corresponding  terminal  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 ioctl_console(2).

   Kernel debugging
       #include <linux/tty.h>

              Argument: struct tty_struct *argp

              Get the tty_struct corresponding to fd.  This command was removed in Linux 2.5.67.


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


       EINVAL Invalid command parameter.

              Unknown command.

       ENOTTY Inappropriate fd.

       EPERM  Insufficient permission.


       Check the condition of DTR on the serial port.

       #include <stdio.h>
       #include <unistd.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 set");
               puts("TIOCM_DTR is not set");


       ldattach(1), ioctl(2), ioctl_console(2), termios(3), pty(7)


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