ioctl_tty
Section: System Calls (2)
Updated: 2023-02-05
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NAME
ioctl_tty - ioctls for terminals and serial lines
LIBRARY
Standard C library
(libc, -lc)
SYNOPSIS
#include <sys/ioctl.h>
#include <asm/termbits.h> /* Definition of struct termios,
struct termios2, and
Bnnn, BOTHER, CBAUD, CLOCAL,
TC*{FLUSH,ON,OFF} and other constants */
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.
Please note that
struct termios
from
<asm/termbits.h>
is different and incompatible with
struct termios
from
<termios.h>.
These ioctl calls require
struct termios
from
<asm/termbits.h>.
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.
- TCSETSW
-
Argument:
const struct termios~*argp
-
Equivalent to
tcsetattr(fd, TCSADRAIN, argp).
-
Allow the output buffer to drain, and
set the current serial port settings.
- TCSETSF
-
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.
- TIOCGLCKTRMIOS
-
Argument:
struct termios~*argp
-
Gets the locking status of the
termios
structure of the terminal.
- TIOCSLCKTRMIOS
-
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).
- TIOCGWINSZ
-
Argument:
struct winsize~*argp
-
Get window size.
- TIOCSWINSZ
-
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.)
- TCSBRKP
-
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.
- TIOCSBRK
-
Argument:
void
-
Turn break on, that is, start sending zero bits.
- TIOCCBRK
-
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
- FIONREAD
-
Argument:
int~*argp
-
Get the number of bytes in the input buffer.
- TIOCINQ
-
Argument:
int~*argp
-
Same as
FIONREAD.
- TIOCOUTQ
-
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.
- TIOCSERGETLSR
-
Argument:
int~*argp
-
Get line status register.
Status register has
TIOCSER_TEMT
bit set when
output buffer is empty and also hardware transmitter is physically empty.
-
Does not have to be supported by all serial tty drivers.
-
tcdrain(3)
does not wait and returns immediately when
TIOCSER_TEMT
bit is set.
Faking input
- TIOCSTI
-
Argument:
const char~*argp
-
Insert the given byte in the input queue.
Redirecting console output
- TIOCCONS
-
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.
Before Linux 2.6.10,
anybody can do this as long as the output was not redirected yet;
since Linux 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
-
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.
- TIOCNOTTY
-
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
- TIOCGPGRP
-
Argument:
pid_t~*argp
-
When successful, equivalent to
*argp = tcgetpgrp(fd).
-
Get the process group ID of the foreground process group on this terminal.
- TIOCSPGRP
-
Argument:
const pid_t~*argp
-
Equivalent to
tcsetpgrp(fd, *argp).
-
Set the foreground process group ID of this terminal.
- TIOCGSID
-
Argument:
pid_t~*argp
-
When successful, equivalent to
*argp = tcgetsid(fd).
-
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
-
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.)
- TIOCGEXCL
-
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.
- TIOCNXCL
-
Argument:
void
-
Disable exclusive mode.
Line discipline
- TIOCGETD
-
Argument:
int~*argp
-
Get the line discipline of the terminal.
- TIOCSETD
-
Argument:
const int~*argp
-
Set the line discipline of the terminal.
Pseudoterminal ioctls
- TIOCPKT
-
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 haS/haQ.
|
TIOCPKT_NOSTOP |
The start and stop characters are not haS/haQ.
|
-
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 haS/haQ flow-controlled remote login.
- TIOCGPKT
-
Argument:
const int~*argp
-
(since Linux 3.8)
Return the current packet mode setting in the integer pointed to by
argp.
- TIOCSPTLCK
-
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).)
- TIOCGPTLCK
-
Argument:
int~*argp
-
(since Linux 3.8)
Place the current lock state of the pseudoterminal slave device
in the location pointed to by
argp.
- TIOCGPTPEER
-
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
- TIOCMGET
-
Argument:
int~*argp
-
Get the status of modem bits.
- TIOCMSET
-
Argument:
const int~*argp
-
Set the status of modem bits.
- TIOCMBIC
-
Argument:
const int~*argp
-
Clear the indicated modem bits.
- TIOCMBIS
-
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)
|
- TIOCMIWAIT
-
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.
- TIOCGICOUNT
-
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
- TIOCGSOFTCAR
-
Argument:
int~*argp
-
("Get software carrier flag")
Get the status of the CLOCAL flag in the c_cflag field of the
termios
structure.
- TIOCSSOFTCAR
-
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.
Linux-specific
For the
TIOCLINUX
ioctl, see
ioctl_console(2).
Kernel debugging
#include <linux/tty.h>
- TIOCTTYGSTRUCT
-
Argument:
struct tty_struct~*argp
-
Get the
tty_struct
corresponding to
fd.
This command was removed in Linux 2.5.67.
RETURN VALUE
The
ioctl(2)
system call returns 0 on success.
On error, it returns -1 and sets
errno
to indicate the error.
ERRORS
- EINVAL
-
Invalid command parameter.
- ENOIOCTLCMD
-
Unknown command.
- ENOTTY
-
Inappropriate
fd.
- EPERM
-
Insufficient permission.
EXAMPLES
Check the condition of DTR on the serial port.
#include <fcntl.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <unistd.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 set");
else
puts("TIOCM_DTR is not set");
close(fd);
}
Get or set arbitrary baudrate on the serial port.
/* SPDX-License-Identifier: GPL-2.0-or-later */
#include <asm/termbits.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
#if !defined BOTHER
fprintf(stderr, "BOTHER is unsupported\n");
/* Program may fallback to TCGETS/TCSETS with Bnnn constants */
exit(EXIT_FAILURE);
#else
/* Declare tio structure, its type depends on supported ioctl */
# if defined TCGETS2
struct termios2 tio;
# else
struct termios tio;
# endif
int fd, rc;
if (argc != 2 && argc != 3 && argc != 4) {
fprintf(stderr, "Usage: %s device [output [input] ]\n", argv[0]);
exit(EXIT_FAILURE);
}
fd = open(argv[1], O_RDWR | O_NONBLOCK | O_NOCTTY);
if (fd < 0) {
perror("open");
exit(EXIT_FAILURE);
}
/* Get the current serial port settings via supported ioctl */
# if defined TCGETS2
rc = ioctl(fd, TCGETS2, &tio);
# else
rc = ioctl(fd, TCGETS, &tio);
# endif
if (rc) {
perror("TCGETS");
close(fd);
exit(EXIT_FAILURE);
}
/* Change baud rate when more arguments were provided */
if (argc == 3 || argc == 4) {
/* Clear the current output baud rate and fill a new value */
tio.c_cflag &= ~CBAUD;
tio.c_cflag |= BOTHER;
tio.c_ospeed = atoi(argv[2]);
/* Clear the current input baud rate and fill a new value */
tio.c_cflag &= ~(CBAUD << IBSHIFT);
tio.c_cflag |= BOTHER << IBSHIFT;
/* When 4th argument is not provided reuse output baud rate */
tio.c_ispeed = (argc == 4) ? atoi(argv[3]) : atoi(argv[2]);
/* Set new serial port settings via supported ioctl */
# if defined TCSETS2
rc = ioctl(fd, TCSETS2, &tio);
# else
rc = ioctl(fd, TCSETS, &tio);
# endif
if (rc) {
perror("TCSETS");
close(fd);
exit(EXIT_FAILURE);
}
/* And get new values which were really configured */
# if defined TCGETS2
rc = ioctl(fd, TCGETS2, &tio);
# else
rc = ioctl(fd, TCGETS, &tio);
# endif
if (rc) {
perror("TCGETS");
close(fd);
exit(EXIT_FAILURE);
}
}
close(fd);
printf("output baud rate: %u\n", tio.c_ospeed);
printf("input baud rate: %u\n", tio.c_ispeed);
exit(EXIT_SUCCESS);
#endif
}
SEE ALSO
ldattach(8),
ioctl(2),
ioctl_console(2),
termios(3),
pty(7)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- Get and set terminal attributes
-
- Locking the termios structure
-
- Get and set window size
-
- Sending a break
-
- Software flow control
-
- Buffer count and flushing
-
- Faking input
-
- Redirecting console output
-
- Controlling terminal
-
- Process group and session ID
-
- Exclusive mode
-
- Line discipline
-
- Pseudoterminal ioctls
-
- Modem control
-
- Marking a line as local
-
- Linux-specific
-
- Kernel debugging
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- SEE ALSO
-
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