ioctl_tty(2) — Linux manual page


IOCTL_TTY(2)            Linux Programmer's Manual           IOCTL_TTY(2)

NAME         top

       ioctl_tty - ioctls for terminals and serial lines

SYNOPSIS         top

       #include <termios.h>

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

DESCRIPTION         top

       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 value.

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

       TIOCSLCKTRMIOS 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).

       The following constants and structure are defined in

       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
       foreground 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 time interval 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, TCION.

   Buffer count and flushing
       FIONREAD  int *argp
              Get the number of bytes in the input buffer.

       TIOCINQ   int *argp
              Same as FIONREAD.

       TIOCOUTQ  int *argp
              Get the number of bytes in the output buffer.

       TCFLSH    int arg
              Equivalent to tcflush(fd, arg).

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

   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, then 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
              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.

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

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

       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 fails with
              the error ENOTTY if 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)
              operations on the terminal are permitted.  (They fail with
              EBUSY, except for a process with the CAP_SYS_ADMIN

       TIOCGEXCL 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.

       TIOCNXCL  void
              Disable exclusive mode.

   Line discipline
       TIOCGETD  int *argp
              Get the line discipline of the terminal.

       TIOCSETD  const int *argp
              Set the line discipline of the terminal.

   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 ('\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 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 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

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

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

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

       TIOCGPTPEER       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.)

       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.

       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)

       TIOCMIWAIT                                  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.

       TIOCGICOUNT                                 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
       TIOCGSOFTCAR                                int *argp
              ("Get software carrier flag") Get the status of the CLOCAL
              flag in the c_cflag field of the termios structure.

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

       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>

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

RETURN VALUE         top

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

ERRORS         top

       EINVAL Invalid command parameter.

              Unknown command.

       ENOTTY Inappropriate fd.

       EPERM  Insufficient permission.

EXAMPLES         top

       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 set");
               puts("TIOCM_DTR is not set");

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

Linux                          2020-06-09                   IOCTL_TTY(2)

Pages that refer to this page: ioctl(2)ioctl_console(2)poll(2)termios(3)tty(4)pty(7)termio(7)