NAME¶
tty_ioctl - ioctls for terminals and serial lines
SYNOPSIS¶
#include <termios.h>
int ioctl(int fd, int cmd, ...);
DESCRIPTION¶
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 terminal.
- TIOCSLCKTRMIOS const struct termios
*argp
- Sets the locking status of the termios structure of
the terminal. Only root (more precisely: 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
<sys/ioctl.h>.
- 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 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, 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, TCIOFLUSH.
- 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
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 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) operations on the terminal are permitted. (They will fail
with EBUSY, except for root, that is, a process with the
CAP_SYS_ADMIN capability.)
- 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 (' ') 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 this 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.
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)
TIOCM_RI see TIOCM_RNG
TIOCM_DSR DSR (data set ready)
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
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.
Linux-specific¶
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.
RETURN VALUE¶
The
ioctl(2) system call returns 0 on success. On error it returns -1 and
sets
errno appropriately.
ERRORS¶
- EINVAL
- Invalid command parameter.
- ENOIOCTLCMD
- Unknown command.
- ENOTTY
- Inappropriate fd.
- EPERM
- Insufficient permission.
EXAMPLE¶
Check the condition of DTR on the serial port.
#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>
int
main(void)
{
int fd, serial;
fd = open("/dev/ttyS0", O_RDONLY);
ioctl(fd, TIOCMGET, &serial);
if (serial & TIOCM_DTR)
puts("TIOCM_DTR is not set");
else
puts("TIOCM_DTR is set");
close(fd);
}
SEE ALSO¶
ioctl(2),
termios(3),
console_ioctl(4),
pty(7)
COLOPHON¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.