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

       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 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

              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,

   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

              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 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

              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

              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

              Argument: int *argp

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

              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.)

       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

       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.

RETURN VALUE         top

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

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 <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");

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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                          2021-08-27                   IOCTL_TTY(2)

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