N_TTY

The default (and fallback) TTY line discipline. It tries to handle characters as per POSIX.

External Functions

void n_tty_inherit_ops(struct tty_ldisc_ops *ops)

inherit N_TTY methods

Parameters

struct tty_ldisc_ops *ops

struct tty_ldisc_ops where to save N_TTY methods

Enables a ‘subclass’ line discipline to ‘inherit’ N_TTY methods.

Internal Functions

void n_tty_kick_worker(struct tty_struct *tty)

start input worker (if required)

Parameters

struct tty_struct *tty

terminal

Description

Re-schedules the flip buffer work if it may have stopped.

Locking:

• Caller holds exclusive termios_rwsem, or

• n_tty_read()/consumer path:

  holds non-exclusive termios_rwsem

void n_tty_write_wakeup(struct tty_struct *tty)

asynchronous I/O notifier

Parameters

struct tty_struct *tty

tty device

Description

Required for the ptys, serial driver etc. since processes that attach themselves to the master and rely on ASYNC IO must be woken up.

void put_tty_queue(unsigned char c, struct n_tty_data *ldata)

add character to tty

Parameters

unsigned char c

character

struct n_tty_data *ldata

n_tty data

Description

Add a character to the tty read_buf queue.

Locking:

• n_tty_receive_buf()/producer path:

  caller holds non-exclusive termios_rwsem

void reset_buffer_flags(struct n_tty_data *ldata)

reset buffer state

Parameters

struct n_tty_data *ldata

line disc data to reset

Description

Reset the read buffer counters and clear the flags. Called from n_tty_open() and n_tty_flush_buffer().

Locking:

• caller holds exclusive termios_rwsem, or
• (locking is not required)

void n_tty_flush_buffer(struct tty_struct *tty)

clean input queue

Parameters

struct tty_struct *tty

terminal device

Description

Flush the input buffer. Called when the tty layer wants the buffer flushed (eg at hangup) or when the N_TTY line discipline internally has to clean the pending queue (for example some signals).

Holds termios_rwsem to exclude producer/consumer while buffer indices are reset.

Locking: ctrl.lock, exclusive termios_rwsem

int is_utf8_continuation(unsigned char c)

utf8 multibyte check

Parameters

unsigned char c

byte to check

Return

true if the utf8 character c is a multibyte continuation character. We use this to correctly compute the on-screen size of the character when printing.

int is_continuation(unsigned char c, struct tty_struct *tty)

multibyte check

Parameters

unsigned char c

byte to check

struct tty_struct *tty

terminal device

Return

true if the utf8 character c is a multibyte continuation character and the terminal is in unicode mode.

int do_output_char(unsigned char c, struct tty_struct *tty, int space)

output one character

Parameters

unsigned char c

character (or partial unicode symbol)

struct tty_struct *tty

terminal device

int space

space available in tty driver write buffer

Description

This is a helper function that handles one output character (including special characters like TAB, CR, LF, etc.), doing OPOST processing and putting the results in the tty driver’s write buffer.

Note that Linux currently ignores TABDLY, CRDLY, VTDLY, FFDLY and NLDLY. They simply aren’t relevant in the world today. If you ever need them, add them here.

Locking: should be called under the output_lock to protect the column state and space left in the buffer.

Return

the number of bytes of buffer space used or -1 if no space left.

int process_output(unsigned char c, struct tty_struct *tty)

output post processor

Parameters

unsigned char c

character (or partial unicode symbol)

struct tty_struct *tty

terminal device

Description

Output one character with OPOST processing.

Locking: output_lock to protect column state and space left (also, this is called from n_tty_write() under the tty layer write lock).

Return

-1 when the output device is full and the character must be retried.

ssize_t process_output_block(struct tty_struct *tty, const unsigned char *buf, unsigned int nr)

block post processor

Parameters

struct tty_struct *tty

terminal device

const unsigned char *buf

character buffer

unsigned int nr

number of bytes to output

Description

Output a block of characters with OPOST processing.

This path is used to speed up block console writes, among other things when processing blocks of output data. It handles only the simple cases normally found and helps to generate blocks of symbols for the console driver and thus improve performance.

Locking: output_lock to protect column state and space left (also, this is called from n_tty_write() under the tty layer write lock).

Return

the number of characters output.

size_t __process_echoes(struct tty_struct *tty)

write pending echo characters

Parameters

struct tty_struct *tty

terminal device

Description

Write previously buffered echo (and other ldisc-generated) characters to the tty.

Characters generated by the ldisc (including echoes) need to be buffered because the driver’s write buffer can fill during heavy program output. Echoing straight to the driver will often fail under these conditions, causing lost characters and resulting mismatches of ldisc state information.

Since the ldisc state must represent the characters actually sent to the driver at the time of the write, operations like certain changes in column state are also saved in the buffer and executed here.

A circular fifo buffer is used so that the most recent characters are prioritized. Also, when control characters are echoed with a prefixed “^”, the pair is treated atomically and thus not separated.

Locking: callers must hold output_lock.

void add_echo_byte(unsigned char c, struct n_tty_data *ldata)

add a byte to the echo buffer

Parameters

unsigned char c

unicode byte to echo

struct n_tty_data *ldata

n_tty data

Description

Add a character or operation byte to the echo buffer.

void echo_move_back_col(struct n_tty_data *ldata)

add operation to move back a column

Parameters

struct n_tty_data *ldata

n_tty data

Description

Add an operation to the echo buffer to move back one column.

void echo_set_canon_col(struct n_tty_data *ldata)

add operation to set the canon column

Parameters

struct n_tty_data *ldata

n_tty data

Description

Add an operation to the echo buffer to set the canon column to the current column.

void echo_erase_tab(unsigned int num_chars, int after_tab, struct n_tty_data *ldata)

add operation to erase a tab

Parameters

unsigned int num_chars

number of character columns already used

int after_tab

true if num_chars starts after a previous tab

struct n_tty_data *ldata

n_tty data

Description

Add an operation to the echo buffer to erase a tab.

Called by the eraser function, which knows how many character columns have been used since either a previous tab or the start of input. This information will be used later, along with canon column (if applicable), to go back the correct number of columns.

void echo_char_raw(unsigned char c, struct n_tty_data *ldata)

echo a character raw

Parameters

unsigned char c

unicode byte to echo

struct n_tty_data *ldata

line disc data

Description

Echo user input back onto the screen. This must be called only when L_ECHO(tty) is true. Called from the tty_driver.receive_buf() path.

This variant does not treat control characters specially.

void echo_char(unsigned char c, struct tty_struct *tty)

echo a character

Parameters

unsigned char c

unicode byte to echo

struct tty_struct *tty

terminal device

Description

Echo user input back onto the screen. This must be called only when L_ECHO(tty) is true. Called from the tty_driver.receive_buf() path.

This variant tags control characters to be echoed as “^X” (where X is the letter representing the control char).

void finish_erasing(struct n_tty_data *ldata)

complete erase

Parameters

struct n_tty_data *ldata

n_tty data

void eraser(unsigned char c, struct tty_struct *tty)

handle erase function

Parameters

unsigned char c

character input

struct tty_struct *tty

terminal device

Description

Perform erase and necessary output when an erase character is present in the stream from the driver layer. Handles the complexities of UTF-8 multibyte symbols.

Locking: n_tty_receive_buf()/producer path:

caller holds non-exclusive termios_rwsem

void isig(int sig, struct tty_struct *tty)

handle the ISIG optio

Parameters

int sig

signal

struct tty_struct *tty

terminal

Description

Called when a signal is being sent due to terminal input. Called from the tty_driver.receive_buf() path, so serialized.

Performs input and output flush if !NOFLSH. In this context, the echo buffer is ‘output’. The signal is processed first to alert any current readers or writers to discontinue and exit their i/o loops.

Locking: ctrl.lock

void n_tty_receive_break(struct tty_struct *tty)

handle break

Parameters

struct tty_struct *tty

terminal

Description

An RS232 break event has been hit in the incoming bitstream. This can cause a variety of events depending upon the termios settings.

Locking: n_tty_receive_buf()/producer path:

caller holds non-exclusive termios_rwsem

Note

may get exclusive termios_rwsem if flushing input buffer

void n_tty_receive_overrun(struct tty_struct *tty)

handle overrun reporting

Parameters

struct tty_struct *tty

terminal

Description

Data arrived faster than we could process it. While the tty driver has flagged this the bits that were missed are gone forever.

Called from the receive_buf path so single threaded. Does not need locking as num_overrun and overrun_time are function private.

void n_tty_receive_parity_error(struct tty_struct *tty, unsigned char c)

error notifier

Parameters

struct tty_struct *tty

terminal device

unsigned char c

character

Description

Process a parity error and queue the right data to indicate the error case if necessary.

Locking: n_tty_receive_buf()/producer path:

caller holds non-exclusive termios_rwsem

void n_tty_receive_char(struct tty_struct *tty, unsigned char c)

perform processing

Parameters

struct tty_struct *tty

terminal device

unsigned char c

character

Description

Process an individual character of input received from the driver. This is serialized with respect to itself by the rules for the driver above.

Locking: n_tty_receive_buf()/producer path:

caller holds non-exclusive termios_rwsem publishes canon_head if canonical mode is active

int n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp, const char *fp, int count, int flow)

process input

Parameters

struct tty_struct *tty

device to receive input

const unsigned char *cp

input chars

const char *fp

flags for each char (if NULL, all chars are TTY_NORMAL)

int count

number of input chars in cp

int flow

enable flow control

Description

Called by the terminal driver when a block of characters has been received. This function must be called from soft contexts not from interrupt context. The driver is responsible for making calls one at a time and in order (or using flush_to_ldisc()).

In canonical mode, the maximum line length is 4096 chars (including the line termination char); lines longer than 4096 chars are truncated. After 4095 chars, input data is still processed but not stored. Overflow processing ensures the tty can always receive more input until at least one line can be read.

In non-canonical mode, the read buffer will only accept 4095 chars; this provides the necessary space for a newline char if the input mode is switched to canonical.

Note it is possible for the read buffer to _contain_ 4096 chars in non-canonical mode: the read buffer could already contain the maximum canon line of 4096 chars when the mode is switched to non-canonical.

Locking: n_tty_receive_buf()/producer path:

claims non-exclusive termios_rwsem publishes commit_head or canon_head

Return

the # of input chars from cp which were processed.

void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)

termios data changed

Parameters

struct tty_struct *tty

terminal

struct ktermios *old

previous data

Description

Called by the tty layer when the user changes termios flags so that the line discipline can plan ahead. This function cannot sleep and is protected from re-entry by the tty layer. The user is guaranteed that this function will not be re-entered or in progress when the ldisc is closed.

Locking: Caller holds tty->termios_rwsem

void n_tty_close(struct tty_struct *tty)

close the ldisc for this tty

Parameters

struct tty_struct *tty

device

Description

Called from the terminal layer when this line discipline is being shut down, either because of a close or becsuse of a discipline change. The function will not be called while other ldisc methods are in progress.

int n_tty_open(struct tty_struct *tty)

open an ldisc

Parameters

struct tty_struct *tty

terminal to open

Description

Called when this line discipline is being attached to the terminal device. Can sleep. Called serialized so that no other events will occur in parallel. No further open will occur until a close.

bool copy_from_read_buf(struct tty_struct *tty, unsigned char **kbp, size_t *nr)

copy read data directly

Parameters

struct tty_struct *tty

terminal device

unsigned char **kbp

data

size_t *nr

size of data

Description

Helper function to speed up n_tty_read(). It is only called when ICANON is off; it copies characters straight from the tty queue.

Locking:

• called under the ldata->atomic_read_lock sem

• n_tty_read()/consumer path:

  caller holds non-exclusive termios_rwsem; read_tail published

Return

true if it successfully copied data, but there is still more data to be had.

bool canon_copy_from_read_buf(struct tty_struct *tty, unsigned char **kbp, size_t *nr)

copy read data in canonical mode

Parameters

struct tty_struct *tty

terminal device

unsigned char **kbp

data

size_t *nr

size of data

Description

Helper function for n_tty_read(). It is only called when ICANON is on; it copies one line of input up to and including the line-delimiting character into the result buffer.

Locking:

• called under the atomic_read_lock mutex

• n_tty_read()/consumer path:

  caller holds non-exclusive termios_rwsem; read_tail published

Note

When termios is changed from non-canonical to canonical mode and the read buffer contains data, n_tty_set_termios() simulates an EOF push (as if C-d were input) _without_ the DISABLED_CHAR in the buffer. This causes data already processed as input to be immediately available as input although a newline has not been received.

int job_control(struct tty_struct *tty, struct file *file)

check job control

Parameters

struct tty_struct *tty

tty

struct file *file

file handle

Description

Perform job control management checks on this file/tty descriptor and if appropriate send any needed signals and return a negative error code if action should be taken.

Locking:

• redirected write test is safe
• current->signal->tty check is safe
• ctrl.lock to safely reference tty->ctrl.pgrp

ssize_t n_tty_read(struct tty_struct *tty, struct file *file, unsigned char *kbuf, size_t nr, void **cookie, unsigned long offset)

read function for tty

Parameters

struct tty_struct *tty

tty device

struct file *file

file object

unsigned char *kbuf

kernelspace buffer pointer

size_t nr

size of I/O

void **cookie

if non-NULL, this is a continuation read

unsigned long offset

where to continue reading from (unused in n_tty)

Description

Perform reads for the line discipline. We are guaranteed that the line discipline will not be closed under us but we may get multiple parallel readers and must handle this ourselves. We may also get a hangup. Always called in user context, may sleep.

This code must be sure never to sleep through a hangup.

Locking: n_tty_read()/consumer path:

claims non-exclusive termios_rwsem; publishes read_tail

ssize_t n_tty_write(struct tty_struct *tty, struct file *file, const unsigned char *buf, size_t nr)

write function for tty

Parameters

struct tty_struct *tty

tty device

struct file *file

file object

const unsigned char *buf

userspace buffer pointer

size_t nr

size of I/O

Description

Write function of the terminal device. This is serialized with respect to other write callers but not to termios changes, reads and other such events. Since the receive code will echo characters, thus calling driver write methods, the output_lock is used in the output processing functions called here as well as in the echo processing function to protect the column state and space left in the buffer.

This code must be sure never to sleep through a hangup.

Locking: output_lock to protect column state and space left

(note that the process_output*() functions take this lock themselves)

__poll_t n_tty_poll(struct tty_struct *tty, struct file *file, poll_table *wait)

poll method for N_TTY

Parameters

struct tty_struct *tty

terminal device

struct file *file

file accessing it

poll_table *wait

poll table

Description

Called when the line discipline is asked to poll() for data or for special events. This code is not serialized with respect to other events save open/close.

This code must be sure never to sleep through a hangup.

Locking: called without the kernel lock held – fine.