termios(3C)

termios: tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow, cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed, tcgetpgrp, tcsetpgrp, tcgetsid -- general terminal interface

Synopsis

#include <termios.h>

int tcgetattr(int fildes, struct termios *termios_p);

int tcsetattr(int fildes, int optional_actions,
	const struct termios *termios_p);

int tcsendbreak(int fildes, int duration);

int tcdrain(int fildes);

int tcflush(int fildes, int queue_selector);

int tcflow(int fildes, int action);

speed_t cfgetospeed(const struct termios *termios_p);

int cfsetospeed(struct termios *termios_p, speed_t speed);

speed_t cfgetispeed(const struct termios *termios_p);

int cfsetispeed(struct termios *termios_p, speed_t speed);

#include <sys/types.h>
#include <termios.h>

pid_t tcgetpgrp(int fildes);

int tcsetpgrp(int fildes, pid_t pgid);

pid_t tcgetsid(int fildes);

Description

These functions describe a general terminal interface for controlling asynchronous communications ports. A more detailed overview of the terminal interface can be found in ioctl(2) interface to termio(7) that provides the same functionality. However, the function interface described here is the preferred user interface.

Many of the functions described here have a termios_p argument that is a pointer to a termios structure. This structure contains the following members:

   tcflag_t   c_iflag;        /* input modes */
   tcflag_t   c_oflag;        /* output modes */
   tcflag_t   c_cflag;        /* control modes */
   tcflag_t   c_lflag;        /* local modes */
   cc_t       c_cc[NCCS];     /* control chars */

These structure members are described in detail in termio(7).

Get and set terminal attributes

The tcgetattr function gets the parameters associated with the object referred by fildes and stores them in the termios structure referenced by termios_p. This

function may be invoked from a background process; however, the terminal attributes may be subsequently changed by a foreground process.

The tcsetattr function sets the parameters associated with the terminal (unless support is required from the underlying hardware that is not available) from the termios structure referenced by termios_p as follows:

If optional_actions is TCSANOW, the change occurs immediately.
If optional_actions is TCSADRAIN, the change occurs after all output written to fildes has been transmitted. This function should be used when changing parameters that affect output.
If optional_actions is TCSAFLUSH, the change occurs after all output written to the object referred by fildes has been transmitted, and all input that has been received but not read is discarded before the change is made.

The symbolic constants for the values of optional_actions are defined in termios.h.

Line control

If the terminal is using asynchronous serial data transmission, the tcsendbreak function causes transmission of a continuous stream of zero-valued bits for a specific duration. If duration is zero, it causes transmission of zero-valued bits for at least 0.25 seconds, and not more than 0.5 seconds. If duration is not zero, it behaves in a way similar to tcdrain.

If the terminal is not using asynchronous serial data transmission, the tcsendbreak function sends data to generate a break condition or returns without taking any action.

The tcdrain function waits until all output written to the object referred to by fildes has been transmitted.

The tcflush function discards data written to the object referred to by fildes but not transmitted, or data received but not read, depending on the value of queue_selector:

If queue_selector is TCIFLUSH, it flushes data received but not read.
If queue_selector is TCOFLUSH, it flushes data written but not transmitted.
If queue_selector is TCIOFLUSH, it flushes both data received but not read, and data written but not transmitted.

The tcflow function suspends transmission or reception of data on the object referred to by fildes, depending on the value of action:

If action is TCOOFF, it suspends output.
If action is TCOON, it restarts suspended output.
If action if TCIOFF, the system transmits a STOP character, which causes the terminal device to stop transmitting data to the system.
If action is TCION, the system transmits a START character, which causes the terminal device to start transmitting data to the system.

Get and set baud rate

The baud rate functions get and set the values of the input and output baud rates in the termios structure. The effects on the terminal device described below do not become effective until the tcsetattr function is successfully called.

The input and output baud rates are stored in the termios structure. The values shown in the table are supported. The names in this table are defined in termios.h.

Name Description Name Description

B0 Hang up B600 600 baud

B50 50 baud B1200 1200 baud

B75 75 baud B1800 1800 baud

B110 110 baud B2400 2400 baud

B134 134.5 baud B4800 4800 baud

B150 150 baud B9600 9600 baud

B200 200 baud B19200 19200 baud

B300 300 baud B38400 38400 baud

Name	Description	Name	Description
`B0`	Hang up	`B600`	600 baud
`B50`	50 baud	`B1200`	1200 baud
`B75`	75 baud	`B1800`	1800 baud
`B110`	110 baud	`B2400`	2400 baud
`B134`	134.5 baud	`B4800`	4800 baud
`B150`	150 baud	`B9600`	9600 baud
`B200`	200 baud	`B19200`	19200 baud
`B300`	300 baud	`B38400`	38400 baud

cfgetospeed gets the output baud rate stored in the termios structure pointed to by termios_p.

cfsetospeed sets the output baud rate stored in the termios structure pointed to by termios_p to speed. The zero baud rate, B0, is used to terminate the connection. If B0 is specified, the modem control lines are no longer asserted. Normally, this disconnects the line.

cfgetispeed returns the input baud rate stored in the termios structure pointed to by termios_p.

cfsetispeed sets the input baud rate stored in the termios structure pointed to by termios_p to speed. If the input baud rate is set to zero, the input baud rate is specified by the value of the output baud rate. Both cfsetispeed and cfsetospeed return a value of zero if successful and -1 to indicate an error. Attempts to set unsupported baud rates are ignored. This refers both to changes to baud rates not supported by the hardware, and to changes setting the input and output baud rates to different values if the hardware does not support this.

For speeds higher than 38400 baud, use the functions tcgetspeed(3C) and tcsetspeed(3C).

Get and set terminal foreground process group ID

tcsetpgrp sets the foreground process group ID of the terminal specified by fildes to pgid. The file associated with fildes must be the controlling terminal of the calling process and the controlling terminal must be currently associated with the session of the calling process. pgid must match a process group ID of a process in the same session as the calling process.

tcgetpgrp returns the foreground process group ID of the terminal specified by fildes. tcgetpgrp is allowed from a process that is a member of a background process group; however, the information may be subsequently changed by a process that is a member of a foreground process group.

If there is no foreground process group, tcgetpgrp returns a value greater than 1 that does not match the process group ID of any existing process group.

Get terminal session ID

tcgetsid returns the session ID of the terminal specified by fildes.

Return values

On success, tcgetpgrp returns the process group ID of the foreground process group associated with the specified terminal. On failure, tcgetpgrp returns -1 and sets errno to identify the error.

On success, tcgetsid returns the session ID associated with the specified terminal. On failure, tcgetsid returns -1 and sets errno to identify the error.

On success, cfgetispeed returns the input baud rate from the termios structure.

On success, cfgetospeed returns the output baud rate from the termios structure.

On success, all other functions return 0. On failure, they return -1 and set errno to identify the error.

Errors

In the following conditions, all of the functions fail and set errno to:

EBADF: The fildes argument is not a valid file descriptor.
ENOTTY: The file associated with fildes is not a terminal.

tcsetattr also fails if the following is true:

EINVAL: The optional_actions argument is not a proper value, or an attempt was made to change an attribute represented in the termios structure to an unsupported value.

tcsendbreak also fails if the following is true:

EINVAL: The device does not support the tcsendbreak function.

tcdrain also fails if one or more of the following is true:

EINTR: A signal interrupted the tcdrain function.
EINVAL: The device does not support the tcdrain function.

tcflush also fails if the following is true:

EINVAL: The device does not support the tcflush function or the queue_selector argument is not a proper value.

tcflow also fails if the following is true:

EINVAL: The device does not support the tcflow function or the action argument is not a proper value.

tcgetpgrp also fails if the following is true:

ENOTTY: the calling process does not have a controlling terminal, or fildes does not refer to the controlling terminal.
EINTR: A signal interrupted the tcgetpgrp function.

tcsetpgrp also fails if the following is true:

EINVAL: pgid is not a valid process group ID.
ENOTTY: the calling process does not have a controlling terminal, or fildes does not refer to the controlling terminal, or the controlling terminal is no longer associated with the session of the calling process.
EPERM: pgid does not match the process group of an existing process in the same session as the calling process.

tcgetsid also fails if the following is true:

EACCES: fildes is a terminal that is not allocated to a session.
ENOTTY: the calling process does not have a controlling terminal, or fildes does not refer to the controlling terminal.

References

setpgid(2), setsid(2), termio(7)

Notices

Considerations for threads programming

Open file descriptors are a process resource and available to any sibling thread; if used concurrently, actions by one thread can interfere with those of a sibling.

While one thread is blocked, siblings might still be executing.