tty ioctl — ioctls for terminals and serial lines
#include <termios.h>
int
ioctl( |
int | fd, |
| int | cmd, | |
...); |
The ioctl() 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
non-portable programs. Use the POSIX interface described in
termios(3) whenever
possible.
argpEquivalent to tcgetattr(fd, argp).
Get the current serial port settings.
argpEquivalent to tcsetattr(fd, TCSANOW, argp).
Set the current serial port settings.
argpEquivalent to tcsetattr(fd, TCSADRAIN, argp).
Allow the output buffer to drain, and set the current serial port settings.
argpEquivalent 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 *.
argpargpargpargpThe termios structure of a tty can be locked. The lock is itself a termios structure, with non-zero bits or fields indicating a locked value.
argpGets the locking status of the termios structure of the terminal.
argpSets the locking status of the termios structure of the terminal. Only root can do this.
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, e.g. by loading a new font).
argpGet window size.
argpSet 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.
argEquivalent 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 non-zero, nobody
knows what will happen.
(SVr4, UnixWare, Solaris, Linux treat tcsendbreak(fd,arg) with
non-zero 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 non-zero) as a
timeinterval measured in milliseconds. HP-UX ignores
arg.)
argSo-called "POSIX version" of TCSBRK. It treats
non-zero arg as a timeinterval
measured in deciseconds, and does nothing when the
driver does not support breaks.
Turn break on, that is, start sending zero bits.
Turn break off, that is, stop sending zero bits.
argEquivalent to tcflow(fd, arg).
See tcflow(3) for the argument values TCOOFF, TCOON, TCIOFF, TCION.
argpGet the number of bytes in the input buffer.
argpSame as FIONREAD.
argpGet the number of bytes in the output buffer.
argEquivalent to tcflush(fd, arg).
See tcflush(3) for the argument values TCIFLUSH, TCOFLUSH, TCIOFLUSH.
Redirect output that would have gone to
/dev/console or
/dev/tty0 to the given
tty. If that was a pty master, send it to the slave.
Anybody can do this as long as the output was not
redirected yet. If it was redirected already EBUSY is
returned, but root may stop redirection by using this
ioctl with fd
pointing at /dev/console or /dev/tty0.
argMake the given tty the controlling tty of the
current process. The current process must be a
session leader and not have a controlling tty
already. If this tty is already the controlling tty
of a different session group then the ioctl fails
with EPERM, unless the caller is root and arg equals 1, in
which case the tty is stolen, and all processes that
had it as controlling tty lose it.
TIOCNOTTYvoidIf the given tty was the controlling tty of the current process, give up this controlling tty. 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 tty.
argpWhen successful, equivalent to *argp = tcgetpgrp(fd).
Get the process group ID of the foreground process group on this tty.
argpEquivalent to tcsetpgrp(fd, *argp).
Set the foreground process group ID of this tty.
argpGet the session ID of the given tty. This will fail with ENOTTY in case the tty is not a master pty and not our controlling tty. Strange.
Put the tty into exclusive mode. No further open(2) operations on the terminal are permitted. (They will fail with EBUSY, except for root.)
Disable exclusive mode.
argpGet the line discipline of the tty.
argpSet the line discipline of the tty.
argpEnable (when *argp is non-zero) or
disable packet mode. Can be applied to the master
side of a pseudo-terminal only (and will return
ENOTTY otherwise). In packet mode, each subsequent
read(2) will return
a packet that either contains a single non-zero
control byte, or has a single byte containing zero
('' ') followed by data written on the slave
side of the pty. 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 t_stopc is `^S' and t_startc is `^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.
argpget the status of modem bits.
argpset the status of modem bits.
argpclear the indicated modem bits.
argpset 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)
argp("Get software carrier flag") Get the status of the CLOCAL flag in the c_cflag field of the termios structure.
argp("Set software carrier flag") Set the CLOCAL flag
in the termios structure when *argp is non-zero, 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 tty 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.
The ioctl() system call
returns 0 on success. On error it returns −1 and sets
errno appropriately.
ENOIOCTLCMDUnknown command.
Invalid command parameter.
Insufficient permission.
Inappropriate fd.
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);
}
ioctl(2), termios(3), console_ioctl(4), pty(7)
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