TTY Struct¶

Initialization
Name
Reference counting
Install
Read & Write
Start & Stop
Wakeup
Hangup
Misc
TTY Struct Flags
TTY Struct Reference

struct tty_struct is allocated by the TTY layer upon the first open of the TTY device and released after the last close. The TTY layer passes this structure to most of struct tty_operation’s hooks. Members of tty_struct are documented in TTY Struct Reference at the bottom.

Initialization ¶

void tty_init_termios(struct tty_struct *tty)¶: helper for termios setup

Parameters

struct tty_struct *tty: the tty to set up

Description

Initialise the termios structure for this tty. This runs under the tty_mutex currently so we can be relaxed about ordering.

Name ¶

const char * tty_name(const struct tty_struct *tty)¶: return tty naming

Parameters

const struct tty_struct *tty: tty structure

Description

Convert a tty structure into a name. The name reflects the kernel naming policy and if udev is in use may not reflect user space

Locking: none

Reference counting ¶

struct tty_struct * tty_kref_get(struct tty_struct *tty)¶: get a tty reference

Parameters

struct tty_struct *tty

tty device

Return a new reference to a tty object. The caller must hold sufficient locks/counts to ensure that their existing reference cannot go away

void tty_kref_put(struct tty_struct *tty)¶: release a tty kref

Parameters

struct tty_struct *tty: tty device

Description

Release a reference to the tty device and if need be let the kref layer destruct the object for us.

Install ¶

int tty_standard_install(struct tty_driver *driver, struct tty_struct *tty)¶: usual tty->ops->install

Parameters

struct tty_driver *driver: the driver for the tty
struct tty_struct *tty: the tty

Description

If the driver overrides tty->ops->install, it still can call this function to perform the standard install operations.

Read & Write ¶

int tty_put_char(struct tty_struct *tty, unsigned char ch)¶: write one character to a tty

Parameters

struct tty_struct *tty: tty
unsigned char ch: character to write

Description

Write one byte to the tty using the provided tty->ops->put_char() method if present.

Note

the specific put_char operation in the driver layer may go away soon. Don’t call it directly, use this method

Return

the number of characters successfully output.

Start & Stop ¶

void stop_tty(struct tty_struct *tty)¶: propagate flow control

Parameters

struct tty_struct *tty: tty to stop

Description

Perform flow control to the driver. May be called on an already stopped device and will not re-call the tty_driver->stop() method.

This functionality is used by both the line disciplines for halting incoming flow and by the driver. It may therefore be called from any context, may be under the tty atomic_write_lock but not always.

Locking:: flow.lock

void start_tty(struct tty_struct *tty)¶: propagate flow control

Parameters

struct tty_struct *tty: tty to start

Description

Start a tty that has been stopped if at all possible. If tty was previously stopped and is now being started, the tty_driver->start() method is invoked and the line discipline woken.

Locking:: flow.lock

Wakeup ¶

void tty_wakeup(struct tty_struct *tty)¶: request more data

Parameters

struct tty_struct *tty: terminal

Description

Internal and external helper for wakeups of tty. This function informs the line discipline if present that the driver is ready to receive more output data.

Hangup ¶

void tty_hangup(struct tty_struct *tty)¶: trigger a hangup event

Parameters

struct tty_struct *tty: tty to hangup

Description

A carrier loss (virtual or otherwise) has occurred on tty. Schedule a hangup sequence to run after this event.

void tty_vhangup(struct tty_struct *tty)¶: process vhangup

Parameters

struct tty_struct *tty: tty to hangup

Description

The user has asked via system call for the terminal to be hung up. We do this synchronously so that when the syscall returns the process is complete. That guarantee is necessary for security reasons.

int tty_hung_up_p(struct file *filp)¶: was tty hung up

Parameters

struct file *filp: file pointer of tty

Return

true if the tty has been subject to a vhangup or a carrier loss

Misc ¶

int tty_do_resize(struct tty_struct *tty, struct winsize *ws)¶: resize event

Parameters

struct tty_struct *tty: tty being resized
struct winsize *ws: new dimensions

Description

Update the termios variables and send the necessary signals to peform a terminal resize correctly.

TTY Struct Flags ¶

These bits are used in the tty_struct.flags field.

So that interrupts won’t be able to mess up the queues, copy_to_cooked must be atomic with respect to itself, as must tty->write. Thus, you must use the inline functions set_bit() and clear_bit() to make things atomic.

TTY_THROTTLED: Driver input is throttled. The ldisc should call tty_driver.unthrottle() in order to resume reception when it is ready to process more data (at threshold min).
TTY_IO_ERROR: If set, causes all subsequent userspace read/write calls on the tty to fail, returning -EIO. (May be no ldisc too.)
TTY_OTHER_CLOSED: Device is a pty and the other side has closed.
TTY_EXCLUSIVE: Exclusive open mode (a single opener).
TTY_DO_WRITE_WAKEUP: If set, causes the driver to call the tty_ldisc_ops.write_wakeup() method in order to resume transmission when it can accept more data to transmit.
TTY_LDISC_OPEN: Indicates that a line discipline is open. For debugging purposes only.
TTY_PTY_LOCK: A flag private to pty code to implement TIOCSPTLCK/TIOCGPTLCK logic.
TTY_NO_WRITE_SPLIT: Prevent driver from splitting up writes into smaller chunks (preserve write boundaries to driver).
TTY_HUPPED: The TTY was hung up. This is set post tty_driver.hangup().
TTY_HUPPING: The TTY is in the process of hanging up to abort potential readers.
TTY_LDISC_CHANGING: Line discipline for this TTY is being changed. I/O should not block when this is set. Use tty_io_nonblock() to check.
TTY_LDISC_HALTED: Line discipline for this TTY was stopped. No work should be queued to this ldisc.

TTY Struct Reference ¶

struct tty_struct¶: state associated with a tty while open

Definition

struct tty_struct {
  int magic;
  struct kref kref;
  struct device *dev;
  struct tty_driver *driver;
  const struct tty_operations *ops;
  int index;
  struct ld_semaphore ldisc_sem;
  struct tty_ldisc *ldisc;
  struct mutex atomic_write_lock;
  struct mutex legacy_mutex;
  struct mutex throttle_mutex;
  struct rw_semaphore termios_rwsem;
  struct mutex winsize_mutex;
  struct ktermios termios, termios_locked;
  char name[64];
  unsigned long flags;
  int count;
  struct winsize winsize;
  struct {
    spinlock_t lock;
    bool stopped;
    bool tco_stopped;
    unsigned long unused[0];
  } flow;
  struct {
    spinlock_t lock;
    struct pid *pgrp;
    struct pid *session;
    unsigned char pktstatus;
    bool packet;
    unsigned long unused[0];
  } ctrl;
  int hw_stopped;
  unsigned int receive_room;
  int flow_change;
  struct tty_struct *link;
  struct fasync_struct *fasync;
  wait_queue_head_t write_wait;
  wait_queue_head_t read_wait;
  struct work_struct hangup_work;
  void *disc_data;
  void *driver_data;
  spinlock_t files_lock;
  struct list_head tty_files;
#define N_TTY_BUF_SIZE 4096;
  int closing;
  unsigned char *write_buf;
  int write_cnt;
  struct work_struct SAK_work;
  struct tty_port *port;
};

Members

magic: magic value set early in alloc_tty_struct to TTY_MAGIC, for debugging purposes
kref: reference counting by tty_kref_get() and tty_kref_put(), reaching zero frees the structure
dev: class device or NULL (e.g. ptys, serdev)
driver: struct tty_driver operating this tty
ops: struct tty_operations of driver for this tty (open, close, etc.)
index: index of this tty (e.g. to construct name like tty12)
ldisc_sem: protects line discipline changes (ldisc) – lock tty not pty
ldisc: the current line discipline for this tty (n_tty by default)
atomic_write_lock: protects against concurrent writers, i.e. locks write_cnt, write_buf and similar
legacy_mutex: leftover from history (BKL -> BTM -> legacy_mutex), protecting several operations on this tty
throttle_mutex: protects against concurrent tty_throttle_safe() and tty_unthrottle_safe() (but not tty_unthrottle())
termios_rwsem: protects termios and termios_locked
winsize_mutex: protects winsize
termios: termios for the current tty, copied from/to driver.termios
termios_locked: locked termios (by TIOCGLCKTRMIOS and TIOCSLCKTRMIOS ioctls)
name: name of the tty constructed by tty_line_name() (e.g. ttyS3)
flags: bitwise OR of TTY_THROTTLED, TTY_IO_ERROR, …
count: count of open processes, reaching zero cancels all the work for this tty and drops a kref too (but does not free this tty)
winsize: size of the terminal “window” (cf. winsize_mutex)
flow: flow settings grouped together, see also flow.unused
flow.lock: lock for flow members
flow.stopped: tty stopped/started by stop_tty()/start_tty()
flow.tco_stopped: tty stopped/started by TCOOFF/TCOON ioctls (it has precedence over flow.stopped)
flow.unused: alignment for Alpha, so that no members other than flow.* are modified by the same 64b word store. The flow’s __aligned is there for the very same reason.
ctrl: control settings grouped together, see also ctrl.unused
ctrl.lock: lock for ctrl members
ctrl.pgrp: process group of this tty (setpgrp(2))
ctrl.session: session of this tty (setsid(2)). Writes are protected by both ctrl.lock and legacy_mutex, readers must use at least one of them.
ctrl.pktstatus: packet mode status (bitwise OR of TIOCPKT_ constants)
ctrl.packet: packet mode enabled
ctrl.unused: alignment for Alpha, see flow.unused for explanation
hw_stopped: not controlled by the tty layer, under driver’s control for CTS handling
receive_room: bytes permitted to feed to ldisc without any being lost
flow_change: controls behavior of throttling, see tty_throttle_safe() and tty_unthrottle_safe()
link: link to another pty (master -> slave and vice versa)
fasync: state for O_ASYNC (for SIGIO); managed by fasync_helper()
write_wait: concurrent writers are waiting in this queue until they are allowed to write
read_wait: readers wait for data in this queue
hangup_work: normally a work to perform a hangup (do_tty_hangup()); while freeing the tty, (re)used to release_one_tty()
disc_data: pointer to ldisc’s private data (e.g. to struct n_tty_data)
driver_data: pointer to driver’s private data (e.g. struct uart_state)
files_lock: protects tty_files list
tty_files: list of (re)openers of this tty (i.e. linked struct tty_file_private)
closing: when set during close, n_tty processes only START & STOP chars
write_buf: temporary buffer used during tty_write() to copy user data to
write_cnt: count of bytes written in tty_write() to write_buf
SAK_work: if the tty has a pending do_SAK, it is queued here
port: persistent storage for this device (i.e. struct tty_port)

Description

All of the state associated with a tty while the tty is open. Persistent storage for tty devices is referenced here as port and is documented in struct tty_port.