Standard Line Discipline

The tty subsystem provides a standard line discipline for processing input and output.

This line discipline is implemented in os/drivers/tty/tty_ldisc_std.c

Implement the standard line discipline for a tty.

General Considerations

The line discipline of a tty is the component that is responsible for processing user input and then buffer it until it is read by a user process. Depending on the state of the line discipline, output may be echoed, i.e., printed to the screen. Furthermore, it takes output from a user process, processes it and passes it to the tty driver, e.g., for outputting it to the screen.

Initialization

This line discipline is initialized by calling

void tty_ldisc_std_init(void)

Initialize the standard line discipline.

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Registering the Line Discipline to a TTY

This line discipline is registered to a tty by calling

void tty_ldisc_std_register(tty_t *tty)

Register the line discipline to the tty.

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Implemented Line Discipline Operations

This line discipline implements the following line discipline operations:

Passing Input to the Line Discipline

Input can be passed by the receive_char() operation, implemented in

static void __tty_ldisc_std_ops_receive_char(tty_t *tty, char c)

Receive and handle an input character passed from lower levels.

  • pass downstream, or

  • handle, e.g.,

    • enter

    • end of transmission (^D)

    • backspace

    • end of text (^C)

    • form feed (^L)

This function uses the following helper:

static void __tty_ldisc_std_insert_printable(tty_t *tty, char c)

Insert a printable character c into the input buffer.

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Opening the Line Discipline

The line discipline is opened, i.e., initialized, by calling the open() operation, implemented in

static int __tty_ldisc_std_ops_open(tty_t *tty)

Open, i.e., initialize the line discipline for tty.

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Reading Input from the Line Discipline

Buffered input is read from the line discipline by calling the read() operation, implemented in

static ssize_t __tty_ldisc_std_ops_read(tty_t *tty, file_t *file, char *buffer, size_t buffer_size, size_t *offset)

Handle read issued by higher levels, i.e., user processes.

When a user process is attempting to read from a tty, that tty will pass the request on to the line discipline. The line discipline will then copy the content of its input buffer to the buffer of the reading process.

Note

This operation blocks until input becomes available.

Parameters:

  • tty – : the tty to read from

  • file – : the file to rad from

  • buffer – : the buffer to read the data into

  • buffer_size – : the size of that buffer

  • offset – : the offset in the file

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Writing Output to the Line Discipline

Output is written to the line discipline by calling the write() operation, implemented in

static ssize_t __tty_ldisc_std_ops_write(tty_t *tty, file_t *file, const char *buffer, size_t buffer_size, size_t *offset)

Handle write issued by higher levels, i.e., user processes.

When a user process is attempting to write to a tty, that tty will pass the request on the line discipline. The line discipline will then copy the content of the output buffer to the the lower level console.

Parameters:

  • tty – : the tty to write to

  • file – : the file to write to

  • buffer – : the buffer to write the data from

  • buffer_size – : the size of that buffer

  • offset – : the offset in the file

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Internal Data

The line discipline stores its data in the following data structure which is then stored in the field tty_t.ldisc_data

struct tty_ldisc_std_data_t

The standard line discipline’s private data.

Public Members

size_t input_pos

End of input already received from lower level.

size_t edit_pos

Position to place new input to, i.e., cursor position.

size_t read_pos

End of input already read by higher level.

char input_buffer[1024]

Buffer for input.

bool input_available

Denote that input has been received which is ready to be read.