terminal/freebsd/tty_ttydisc.c

/*
 * Copyright (c) 2006-2023, RT-Thread Development Team
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Change Logs:
 * Date           Author       Notes
 * 2023-11-13     Shell        init ver.
 */
#include "../bsd_porting.h"
#include "../tty_config.h"
#include "../tty_internal.h"
#include "../terminal.h"

#include <rtdef.h>

/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2008 Ed Schouten <ed@FreeBSD.org>
 * All rights reserved.
 *
 * Portions of this software were developed under sponsorship from Snow
 * B.V., the Netherlands.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Standard TTYDISC `termios' line discipline.
 */

/* Statistics. */
static rt_atomic_t tty_nin = 0;
#ifdef USING_BSD_SYSCTL
SYSCTL_ULONG(_kern, OID_AUTO, tty_nin, CTLFLAG_RD, &tty_nin, 0,
             "Total amount of bytes received");
#endif

static rt_atomic_t tty_nout = 0;
#ifdef USING_BSD_SYSCTL
SYSCTL_ULONG(_kern, OID_AUTO, tty_nout, CTLFLAG_RD, &tty_nout, 0,
             "Total amount of bytes transmitted");
#endif

/* termios comparison macro's. */
#define CMP_CC(v, c) \
    (tp->t_termios.c_cc[v] != _POSIX_VDISABLE && tp->t_termios.c_cc[v] == (c))
#define CMP_FLAG(field, opt) (tp->t_termios.c_##field##flag & (opt))

/* Characters that cannot be modified through c_cc. */
#define CTAB '\t'
#define CNL  '\n'
#define CCR  '\r'

/* Character is a control character. */
#define CTL_VALID(c) ((c) == 0x7f || (unsigned char)(c) < 0x20)
/* Control character should be processed on echo. */
#define CTL_ECHO(c, q) \
    (!(q) && ((c) == CERASE2 || (c) == CTAB || (c) == CNL || (c) == CCR))
/* Control character should be printed using ^X notation. */
#define CTL_PRINT(c, q) \
    ((c) == 0x7f ||     \
     ((unsigned char)(c) < 0x20 && ((q) || ((c) != CTAB && (c) != CNL))))
/* Character is whitespace. */
#define CTL_WHITE(c) ((c) == ' ' || (c) == CTAB)
/* Character is alphanumeric. */
#define CTL_ALNUM(c)                                             \
    (((c) >= '0' && (c) <= '9') || ((c) >= 'a' && (c) <= 'z') || \
     ((c) >= 'A' && (c) <= 'Z'))
/* Character is UTF8-encoded. */
#define CTL_UTF8(c) (!!((c)&0x80))
/* Character is a UTF8 continuation byte. */
#define CTL_UTF8_CONT(c) (((c)&0xc0) == 0x80)

#define TTY_STACKBUF  256
#define UTF8_STACKBUF 4

void ttydisc_open(struct lwp_tty *tp)
{
    ttydisc_optimize(tp);
}

void ttydisc_close(struct lwp_tty *tp)
{
    /* Clean up our flags when leaving the discipline. */
    tp->t_flags &= ~(TF_STOPPED | TF_HIWAT | TF_ZOMBIE);
    tp->t_termios.c_lflag &= ~FLUSHO;

    /*
     * POSIX states that we must drain output and flush input on
     * last close.  Draining has already been done if possible.
     */
    tty_flush(tp, FREAD | FWRITE);

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, close))
        ttyhook_close(tp);
#endif
}

static int ttydisc_read_canonical(struct lwp_tty *tp, struct uio *uio,
                                  int ioflag)
{
    char breakc[4] = {CNL}; /* enough to hold \n, VEOF and VEOL. */
    int error;
    size_t clen, flen = 0, n = 1;
    unsigned char lastc = _POSIX_VDISABLE;

#define BREAK_ADD(c)                                  \
    do                                                \
    {                                                 \
        if (tp->t_termios.c_cc[c] != _POSIX_VDISABLE) \
            breakc[n++] = tp->t_termios.c_cc[c];      \
    } while (0)
    /* Determine which characters we should trigger on. */
    BREAK_ADD(VEOF);
    BREAK_ADD(VEOL);
#undef BREAK_ADD
    breakc[n] = '\0';

    do
    {
        error = tty_wait_background(tp, curthread, SIGTTIN);
        if (error)
            return error;

        /*
         * Quite a tricky case: unlike the old TTY
         * implementation, this implementation copies data back
         * to userspace in large chunks. Unfortunately, we can't
         * calculate the line length on beforehand if it crosses
         * ttyinq_block boundaries, because multiple reads could
         * then make this code read beyond the newline.
         *
         * This is why we limit the read to:
         * - The size the user has requested
         * - The blocksize (done in tty_inq.c)
         * - The amount of bytes until the newline
         *
         * This causes the line length to be recalculated after
         * each block has been copied to userspace. This will
         * cause the TTY layer to return data in chunks using
         * the blocksize (except the first and last blocks).
         */
        clen =
            ttyinq_findchar(&tp->t_inq, breakc, uio->uio_resid, (char *)&lastc);

        /* No more data. */
        if (clen == 0)
        {
            if (tp->t_flags & TF_ZOMBIE)
                return 0;
            else if (ioflag & IO_NDELAY)
                return -EWOULDBLOCK;

            error = tty_wait(tp, &tp->t_inwait);
            if (error)
                return error;
            continue;
        }

        /* Don't send the EOF char back to userspace. */
        if (CMP_CC(VEOF, lastc))
            flen = 1;

        MPASS(flen <= clen);

        /* Read and throw away the EOF character. */
        error = ttyinq_read_uio(&tp->t_inq, tp, uio, clen, flen);
        if (error)
            return error;

        /** Notes: proceed when read buf frees and not reaching breaks */
    } while (uio->uio_resid > 0 && lastc == _POSIX_VDISABLE);

    return 0;
}

static int ttydisc_read_raw_no_timer(struct lwp_tty *tp, struct uio *uio,
                                     int ioflag)
{
    size_t vmin = tp->t_termios.c_cc[VMIN];
    ssize_t oresid = uio->uio_resid;
    int error;

    MPASS(tp->t_termios.c_cc[VTIME] == 0);

    /*
     * This routine implements the easy cases of read()s while in
     * non-canonical mode, namely case B and D, where we don't have
     * any timers at all.
     */

    for (;;)
    {
        error = tty_wait_background(tp, curthread, SIGTTIN);
        if (error)
            return error;

        error = ttyinq_read_uio(&tp->t_inq, tp, uio, uio->uio_resid, 0);
        if (error)
            return error;
        if (uio->uio_resid == 0 || (oresid - uio->uio_resid) >= vmin)
            return 0;

        /* We have to wait for more. */
        if (tp->t_flags & TF_ZOMBIE)
            return 0;
        else if (ioflag & IO_NDELAY)
            return -EWOULDBLOCK;

        error = tty_wait(tp, &tp->t_inwait);
        if (error)
            return error;
    }
}

static int ttydisc_read_raw_read_timer(struct lwp_tty *tp, struct uio *uio,
                                       int ioflag, int oresid)
{
    size_t vmin = MAX(tp->t_termios.c_cc[VMIN], 1);
    unsigned int vtime = tp->t_termios.c_cc[VTIME];
    struct timeval end, now, left;
    int error, hz;

    MPASS(tp->t_termios.c_cc[VTIME] != 0);

    /* Determine when the read should be expired. */
    end.tv_sec = vtime / 10;
    end.tv_usec = (vtime % 10) * 100000;
    getmicrotime(&now);
    timevaladd(&end, &now);

    for (;;)
    {
        error = tty_wait_background(tp, curthread, SIGTTIN);
        if (error)
            return error;

        error = ttyinq_read_uio(&tp->t_inq, tp, uio, uio->uio_resid, 0);
        if (error)
            return error;
        if (uio->uio_resid == 0 || (oresid - uio->uio_resid) >= vmin)
            return 0;

        /* Calculate how long we should wait. */
        getmicrotime(&now);
        if (timevalcmp(&now, &end, >))
            return 0;
        left = end;
        timevalsub(&left, &now);
        hz = tvtohz(&left);

        /*
         * We have to wait for more. If the timer expires, we
         * should return a 0-byte read.
         */
        if (tp->t_flags & TF_ZOMBIE)
            return 0;
        else if (ioflag & IO_NDELAY)
            return -EWOULDBLOCK;

        error = tty_timedwait(tp, &tp->t_inwait, hz);
        if (error)
            return (error == -EWOULDBLOCK ? 0 : error);
    }

    return 0;
}

static int ttydisc_read_raw_interbyte_timer(struct lwp_tty *tp, struct uio *uio,
                                            int ioflag)
{
    size_t vmin = tp->t_termios.c_cc[VMIN];
    ssize_t oresid = uio->uio_resid;
    int error;

    MPASS(tp->t_termios.c_cc[VMIN] != 0);
    MPASS(tp->t_termios.c_cc[VTIME] != 0);

    /*
     * When using the interbyte timer, the timer should be started
     * after the first byte has been received. We just call into the
     * generic read timer code after we've received the first byte.
     */

    for (;;)
    {
        error = tty_wait_background(tp, curthread, SIGTTIN);
        if (error)
            return error;

        error = ttyinq_read_uio(&tp->t_inq, tp, uio, uio->uio_resid, 0);
        if (error)
            return error;
        if (uio->uio_resid == 0 || (oresid - uio->uio_resid) >= vmin)
            return 0;

        /*
         * Not enough data, but we did receive some, which means
         * we'll now start using the interbyte timer.
         */
        if (oresid != uio->uio_resid)
            break;

        /* We have to wait for more. */
        if (tp->t_flags & TF_ZOMBIE)
            return 0;
        else if (ioflag & IO_NDELAY)
            return -EWOULDBLOCK;

        error = tty_wait(tp, &tp->t_inwait);
        if (error)
            return error;
    }

    return ttydisc_read_raw_read_timer(tp, uio, ioflag, oresid);
}

int ttydisc_read(struct lwp_tty *tp, struct uio *uio, int ioflag)
{
    int error;

    tty_assert_locked(tp);

    if (uio->uio_resid == 0)
        return 0;

    if (CMP_FLAG(l, ICANON))
        error = ttydisc_read_canonical(tp, uio, ioflag);
    else if (tp->t_termios.c_cc[VTIME] == 0)
        error = ttydisc_read_raw_no_timer(tp, uio, ioflag);
    else if (tp->t_termios.c_cc[VMIN] == 0)
        error = ttydisc_read_raw_read_timer(tp, uio, ioflag, uio->uio_resid);
    else
        error = ttydisc_read_raw_interbyte_timer(tp, uio, ioflag);

    if (ttyinq_bytesleft(&tp->t_inq) >= tp->t_inlow ||
        ttyinq_bytescanonicalized(&tp->t_inq) == 0)
    {
        /* Unset the input watermark when we've got enough space. */
        tty_hiwat_in_unblock(tp);
    }

    return error;
}

/* return the offset to special character from string start */
rt_inline unsigned int ttydisc_findchar(const char *obstart, unsigned int oblen)
{
    const char *c = obstart;

    while (oblen--)
    {
        if (CTL_VALID(*c))
            break;
        c++;
    }

    return (c - obstart);
}

static int ttydisc_write_oproc(struct lwp_tty *tp, char c)
{
    unsigned int scnt, error;

    MPASS(CMP_FLAG(o, OPOST));
    MPASS(CTL_VALID(c));

#define PRINT_NORMAL() ttyoutq_write_nofrag(&tp->t_outq, &c, 1)
    switch (c)
    {
        case CEOF:
            /* End-of-text dropping. */
            if (CMP_FLAG(o, ONOEOT))
                return 0;
            return PRINT_NORMAL();

        case CERASE2:
            /* Handle backspace to fix tab expansion. */
            if (PRINT_NORMAL() != 0)
                return (-1);
            if (tp->t_column > 0)
                tp->t_column--;
            return 0;

        case CTAB:
            /* Tab expansion. */
            scnt = 8 - (tp->t_column & 7);
            if (CMP_FLAG(o, TAB3))
            {
                error = ttyoutq_write_nofrag(&tp->t_outq, "        ", scnt);
            }
            else
            {
                error = PRINT_NORMAL();
            }
            if (error)
                return (-1);

            tp->t_column += scnt;
            MPASS((tp->t_column % 8) == 0);
            return 0;

        case CNL:
            /* Newline conversion. */
            if (CMP_FLAG(o, ONLCR))
            {
                /* Convert \n to \r\n. */
                error = ttyoutq_write_nofrag(&tp->t_outq, "\r\n", 2);
            }
            else
            {
                error = PRINT_NORMAL();
            }
            if (error)
                return (-1);

            if (CMP_FLAG(o, ONLCR | ONLRET))
            {
                tp->t_column = tp->t_writepos = 0;
                ttyinq_reprintpos_set(&tp->t_inq);
            }
            return 0;

        case CCR:
            /* Carriage return to newline conversion. */
            if (CMP_FLAG(o, OCRNL))
                c = CNL;
            /* Omit carriage returns on column 0. */
            if (CMP_FLAG(o, ONOCR) && tp->t_column == 0)
                return 0;
            if (PRINT_NORMAL() != 0)
                return (-1);

            tp->t_column = tp->t_writepos = 0;
            ttyinq_reprintpos_set(&tp->t_inq);
            return 0;
    }

    /*
     * Invisible control character. Print it, but don't
     * increase the column count.
     */
    return PRINT_NORMAL();
#undef PRINT_NORMAL
}

/*
 * Just like the old TTY implementation, we need to copy data in chunks
 * into a temporary buffer. One of the reasons why we need to do this,
 * is because output processing (only TAB3 though) may allow the buffer
 * to grow eight times.
 */
int ttydisc_write(struct lwp_tty *tp, struct uio *uio, int ioflag)
{
    char ob[TTY_STACKBUF];
    char *obstart;
    int error = 0;
    unsigned int oblen = 0;

    tty_assert_locked(tp);

    if (tp->t_flags & TF_ZOMBIE)
        return -EIO;

    /*
     * We don't need to check whether the process is the foreground
     * process group or if we have a carrier. This is already done
     * in ttydev_write().
     */

    while (uio->uio_resid > 0)
    {
        unsigned int nlen;

        MPASS(oblen == 0);

        if (CMP_FLAG(l, FLUSHO))
        {
            uio->uio_offset += uio->uio_resid;
            uio->uio_resid = 0;
            return 0;
        }

        /* Step 1: read data. */
        obstart = ob;
        nlen = MIN(uio->uio_resid, sizeof ob);
        tty_unlock(tp);
        error = uiomove(ob, nlen, uio);
        tty_lock(tp);
        if (error != 0)
            break;
        oblen = nlen;

        if (tty_gone(tp))
        {
            error = -ENXIO;
            break;
        }

        MPASS(oblen > 0);

        /* Step 2: process data. */
        do
        {
            unsigned int plen, wlen;

            if (CMP_FLAG(l, FLUSHO))
            {
                uio->uio_offset += uio->uio_resid;
                uio->uio_resid = 0;
                return 0;
            }

            /* Search for special characters for post processing. */
            if (CMP_FLAG(o, OPOST))
            {
                plen = ttydisc_findchar(obstart, oblen);
            }
            else
            {
                plen = oblen;
            }

            if (plen == 0)
            {
                /*
                 * We're going to process a character
                 * that needs processing
                 */
                if (ttydisc_write_oproc(tp, *obstart) == 0)
                {
                    obstart++;
                    oblen--;

                    tp->t_writepos = tp->t_column;
                    ttyinq_reprintpos_set(&tp->t_inq);
                    continue;
                }
            }
            else
            {
                /* We're going to write regular data. */
                wlen = ttyoutq_write(&tp->t_outq, obstart, plen);
                obstart += wlen;
                oblen -= wlen;
                tp->t_column += wlen;

                tp->t_writepos = tp->t_column;
                ttyinq_reprintpos_set(&tp->t_inq);

                if (wlen == plen)
                    continue;
            }

            /* Watermark reached. Try to sleep. */
            tp->t_flags |= TF_HIWAT_OUT;

            if (ioflag & IO_NDELAY)
            {
                error = EWOULDBLOCK;
                goto done;
            }

            /*
             * The driver may write back the data
             * synchronously. Be sure to check the high
             * water mark before going to sleep.
             */
            ttydevsw_outwakeup(tp);
            if ((tp->t_flags & TF_HIWAT_OUT) == 0)
                continue;

            error = tty_wait(tp, &tp->t_outwait);
            if (error)
                goto done;

            if (tp->t_flags & TF_ZOMBIE)
            {
                error = EIO;
                goto done;
            }
        } while (oblen > 0);
    }

done:
    if (!tty_gone(tp))
        ttydevsw_outwakeup(tp);

    /*
     * Add the amount of bytes that we didn't process back to the
     * uio counters. We need to do this to make sure write() doesn't
     * count the bytes we didn't store in the queue.
     */
    uio->uio_resid += oblen;
    return error;
}

/* set optimize flags if possible */
void ttydisc_optimize(struct lwp_tty *tp)
{
    tty_assert_locked(tp);

    if (ttyhook_hashook(tp, rint_bypass))
    {
        tp->t_flags |= TF_BYPASS;
    }
    else if (ttyhook_hashook(tp, rint))
    {
        tp->t_flags &= ~TF_BYPASS;
    }
    else if (!CMP_FLAG(i, ICRNL | IGNCR | IMAXBEL | INLCR | ISTRIP | IXON) &&
             (!CMP_FLAG(i, BRKINT) || CMP_FLAG(i, IGNBRK)) &&
             (!CMP_FLAG(i, PARMRK) ||
              CMP_FLAG(i, IGNPAR | IGNBRK) == (IGNPAR | IGNBRK)) &&
             !CMP_FLAG(l, ECHO | ICANON | IEXTEN | ISIG | PENDIN))
    {
        tp->t_flags |= TF_BYPASS;
    }
    else
    {
        tp->t_flags &= ~TF_BYPASS;
    }
}

void ttydisc_modem(struct lwp_tty *tp, int open)
{
    tty_assert_locked(tp);

    if (open)
        cv_broadcast(&tp->t_dcdwait);

    /*
     * Ignore modem status lines when CLOCAL is turned on, but don't
     * enter the zombie state when the TTY isn't opened, because
     * that would cause the TTY to be in zombie state after being
     * opened.
     */
    if (!tty_opened(tp) || CMP_FLAG(c, CLOCAL))
        return;

    if (open == 0)
    {
        /*
         * Lost carrier.
         */
        tp->t_flags |= TF_ZOMBIE;

        lwp_tty_signal_sessleader(tp, SIGHUP);
        tty_flush(tp, FREAD | FWRITE);
    }
    else
    {
        /*
         * Carrier is back again.
         */

        /* XXX: what should we do here? */
    }
}

static int ttydisc_echo_force(struct lwp_tty *tp, char c, int quote)
{
    if (CMP_FLAG(l, FLUSHO))
        return 0;

    if (CMP_FLAG(o, OPOST) && CTL_ECHO(c, quote))
    {
        /*
         * Only perform postprocessing when OPOST is turned on
         * and the character is an unquoted BS/TB/NL/CR.
         */
        return ttydisc_write_oproc(tp, c);
    }
    else if (CMP_FLAG(l, ECHOCTL) && CTL_PRINT(c, quote))
    {
        /*
         * Only use ^X notation when ECHOCTL is turned on and
         * we've got an quoted control character.
         *
         * Print backspaces when echoing an end-of-file.
         */
        char ob[4] = "^?\b\b";

        /* Print ^X notation. */
        if (c != 0x7f)
            ob[1] = c + 'A' - 1;

        if (!quote && CMP_CC(VEOF, c))
        {
            return ttyoutq_write_nofrag(&tp->t_outq, ob, 4);
        }
        else
        {
            tp->t_column += 2;
            return ttyoutq_write_nofrag(&tp->t_outq, ob, 2);
        }
    }
    else
    {
        /* Can just be printed. */
        tp->t_column++;
        return ttyoutq_write_nofrag(&tp->t_outq, &c, 1);
    }
}

static int ttydisc_echo(struct lwp_tty *tp, char c, int quote)
{
    /*
     * Only echo characters when ECHO is turned on, or ECHONL when
     * the character is an unquoted newline.
     */
    if (!CMP_FLAG(l, ECHO) && (!CMP_FLAG(l, ECHONL) || c != CNL || quote))
        return 0;

    return ttydisc_echo_force(tp, c, quote);
}

static void ttydisc_reprint_char(void *d, char c, int quote)
{
    struct lwp_tty *tp = d;

    ttydisc_echo(tp, c, quote);
}

static void ttydisc_reprint(struct lwp_tty *tp)
{
    cc_t c;

    /* Print  ^R\n, followed by the line. */
    c = tp->t_termios.c_cc[VREPRINT];
    if (c != _POSIX_VDISABLE)
        ttydisc_echo(tp, c, 0);
    ttydisc_echo(tp, CNL, 0);
    ttyinq_reprintpos_reset(&tp->t_inq);

    ttyinq_line_iterate_from_linestart(&tp->t_inq, ttydisc_reprint_char, tp);
}

struct ttydisc_recalc_length
{
    struct lwp_tty *tp;
    unsigned int curlen;
};

static void ttydisc_recalc_charlength(void *d, char c, int quote)
{
    struct ttydisc_recalc_length *data = d;
    struct lwp_tty *tp = data->tp;

    if (CTL_PRINT(c, quote))
    {
        if (CMP_FLAG(l, ECHOCTL))
            data->curlen += 2;
    }
    else if (c == CTAB)
    {
        data->curlen += 8 - (data->curlen & 7);
    }
    else
    {
        data->curlen++;
    }
}

static unsigned int ttydisc_recalc_linelength(struct lwp_tty *tp)
{
    struct ttydisc_recalc_length data = {tp, tp->t_writepos};

    ttyinq_line_iterate_from_reprintpos(&tp->t_inq, ttydisc_recalc_charlength,
                                        &data);
    return (data.curlen);
}

static int ttydisc_rubchar(struct lwp_tty *tp)
{
    char c;
    int quote;
    unsigned int prevpos, tablen;

    if (ttyinq_peekchar(&tp->t_inq, &c, &quote) != 0)
        return (-1);
    ttyinq_unputchar(&tp->t_inq);

    if (CMP_FLAG(l, ECHO))
    {
        /*
         * Remove the character from the screen. This is even
         * safe for characters that span multiple characters
         * (tabs, quoted, etc).
         */
        if (tp->t_writepos >= tp->t_column)
        {
            /* Retype the sentence. */
            ttydisc_reprint(tp);
        }
        else if (CMP_FLAG(l, ECHOE))
        {
            if (CTL_PRINT(c, quote))
            {
                /* Remove ^X formatted chars. */
                if (CMP_FLAG(l, ECHOCTL))
                {
                    tp->t_column -= 2;
                    ttyoutq_write_nofrag(&tp->t_outq, "\b\b  \b\b", 6);
                }
            }
            else if (c == ' ')
            {
                /* Space character needs no rubbing. */
                tp->t_column -= 1;
                ttyoutq_write_nofrag(&tp->t_outq, "\b", 1);
            }
            else if (c == CTAB)
            {
                /*
                 * Making backspace work with tabs is
                 * quite hard. Recalculate the length of
                 * this character and remove it.
                 *
                 * Because terminal settings could be
                 * changed while the line is being
                 * inserted, the calculations don't have
                 * to be correct. Make sure we keep the
                 * tab length within proper bounds.
                 */
                prevpos = ttydisc_recalc_linelength(tp);
                if (prevpos >= tp->t_column)
                    tablen = 1;
                else
                    tablen = tp->t_column - prevpos;
                if (tablen > 8)
                    tablen = 8;

                tp->t_column = prevpos;
                ttyoutq_write_nofrag(&tp->t_outq, "\b\b\b\b\b\b\b\b", tablen);
                return 0;
            }
#ifdef USING_BSD_UTF8
            else if ((tp->t_termios.c_iflag & IUTF8) != 0 && CTL_UTF8(c))
            {
                uint8_t bytes[UTF8_STACKBUF] = {0};
                int curidx = UTF8_STACKBUF - 1, cwidth = 1, nb = 0;
                teken_char_t codepoint;

                /* Save current byte. */
                bytes[curidx] = c;
                curidx--;
                nb++;
                /* Loop back through inq until we hit the
                 * leading byte. */
                while (CTL_UTF8_CONT(c) && nb < UTF8_STACKBUF)
                {
                    /*
                     * Check if we've reached the beginning
                     * of the line.
                     */
                    if (ttyinq_peekchar(&tp->t_inq, &c, &quote) != 0)
                        break;
                    ttyinq_unputchar(&tp->t_inq);
                    bytes[curidx] = c;
                    curidx--;
                    nb++;
                }
                /*
                 * Shift array so that the leading
                 * byte ends up at idx 0.
                 */
                if (nb < UTF8_STACKBUF)
                    memmove(&bytes[0], &bytes[curidx + 1],
                            nb * sizeof(uint8_t));
                /* Check for malformed UTF8 characters. */
                if (nb == UTF8_STACKBUF && CTL_UTF8_CONT(bytes[0]))
                {
                    /*
                     * Place all bytes back into the inq and
                     * delete the last byte only.
                     */
                    ttyinq_write(&tp->t_inq, bytes, UTF8_STACKBUF, 0);
                    ttyinq_unputchar(&tp->t_inq);
                }
                else
                {
                    /* Find codepoint and width. */
                    codepoint = teken_utf8_bytes_to_codepoint(bytes, nb);
                    if (codepoint == TEKEN_UTF8_INVALID_CODEPOINT ||
                        (cwidth = teken_wcwidth(codepoint)) == -1)
                    {
                        /*
                         * Place all bytes back into the
                         * inq and fall back to
                         * default behaviour.
                         */
                        cwidth = 1;
                        ttyinq_write(&tp->t_inq, bytes, nb, 0);
                        ttyinq_unputchar(&tp->t_inq);
                    }
                }
                tp->t_column -= cwidth;
                /*
                 * Delete character by punching
                 * 'cwidth' spaces over it.
                 */
                if (cwidth == 1)
                    ttyoutq_write_nofrag(&tp->t_outq, "\b \b", 3);
                else if (cwidth == 2)
                    ttyoutq_write_nofrag(&tp->t_outq, "\b\b  \b\b", 6);
            }
#endif
            else
            {
                /*
                 * Remove a regular character by
                 * punching a space over it.
                 */
                tp->t_column -= 1;
                ttyoutq_write_nofrag(&tp->t_outq, "\b \b", 3);
            }
        }
        else
        {
            /* Don't print spaces. */
            ttydisc_echo(tp, tp->t_termios.c_cc[VERASE], 0);
        }
    }

    return 0;
}

static void ttydisc_rubword(struct lwp_tty *tp)
{
    char c;
    int quote;
#ifdef ALTWERASE
    int alnum;
#endif

    /* Strip whitespace first. */
    for (;;)
    {
        if (ttyinq_peekchar(&tp->t_inq, &c, &quote) != 0)
            return;
        if (!CTL_WHITE(c))
            break;
        ttydisc_rubchar(tp);
    }

    /*
     * Record whether the last character from the previous iteration
     * was alphanumeric or not. We need this to implement ALTWERASE.
     */
#ifdef ALTWERASE
    alnum = CTL_ALNUM(c);
#endif
    for (;;)
    {
        ttydisc_rubchar(tp);

        if (ttyinq_peekchar(&tp->t_inq, &c, &quote) != 0)
            return;
        if (CTL_WHITE(c))
            return;
#ifdef ALTWERASE
        if (CMP_FLAG(l, ALTWERASE) && CTL_ALNUM(c) != alnum)
            return;
#endif
    }
}

int ttydisc_rint(struct lwp_tty *tp, char c, int flags)
{
    int signal, quote = 0;
    char ob[3] = {0xff, 0x00};
    size_t ol;

    tty_assert_locked(tp);

    rt_atomic_add(&tty_nin, 1);

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, rint))
        return ttyhook_rint(tp, c, flags);
#endif

    if (tp->t_flags & TF_BYPASS)
        goto processed;

    if (flags)
    {
        if (flags & TRE_BREAK)
        {
            if (CMP_FLAG(i, IGNBRK))
            {
                /* Ignore break characters. */
                return 0;
            }
            else if (CMP_FLAG(i, BRKINT))
            {
                /* Generate SIGINT on break. */
                tty_flush(tp, FREAD | FWRITE);
                lwp_tty_signal_pgrp(tp, SIGINT);
                return 0;
            }
            else
            {
                /* Just print it. */
                goto parmrk;
            }
        }
        else if (flags & TRE_FRAMING ||
                 (flags & TRE_PARITY && CMP_FLAG(i, INPCK)))
        {
            if (CMP_FLAG(i, IGNPAR))
            {
                /* Ignore bad characters. */
                return 0;
            }
            else
            {
                /* Just print it. */
                goto parmrk;
            }
        }
    }

    /* Allow any character to perform a wakeup. */
    if (CMP_FLAG(i, IXANY))
    {
        tp->t_flags &= ~TF_STOPPED;
        tp->t_termios.c_lflag &= ~FLUSHO; /* FLUSHO: Output is being flushed */
    }

    /* Remove the top bit. */
    if (CMP_FLAG(i, ISTRIP))
        c &= ~0x80;

    /* Skip input processing when we want to print it literally. */
    if (tp->t_flags & TF_LITERAL)
    {
        tp->t_flags &= ~TF_LITERAL;
        quote = 1;
        goto processed;
    }

    /* Special control characters that are implementation dependent. */
    if (CMP_FLAG(l, IEXTEN))
    {
        /* Accept the next character as literal. */
        if (CMP_CC(VLNEXT, c))
        {
            if (CMP_FLAG(l, ECHO))
            {
                if (CMP_FLAG(l, ECHOE))
                    ttyoutq_write_nofrag(&tp->t_outq, "^\b", 2);
                else
                    ttydisc_echo(tp, c, 0);
            }
            tp->t_flags |= TF_LITERAL;
            return 0;
        }
        /* Discard processing */
        if (CMP_CC(VDISCARD, c))
        {
            if (CMP_FLAG(l, FLUSHO))
            {
                tp->t_termios.c_lflag &= ~FLUSHO;
            }
            else
            {
                tty_flush(tp, FWRITE);
                ttydisc_echo(tp, c, 0);
                if (tp->t_inq.ti_end > 0)
                    ttydisc_reprint(tp);
                tp->t_termios.c_lflag |= FLUSHO;
            }
        }
    }

    /*
     * Handle signal processing.
     */
    if (CMP_FLAG(l, ISIG))
    {
#ifdef USING_BSD_SIGINFO
        if (CMP_FLAG(l, ICANON | IEXTEN) == (ICANON | IEXTEN))
        {
            if (CMP_CC(VSTATUS, c))
            {
                tty_signal_pgrp(tp, SIGINFO);
                return 0;
            }
        }
#endif /* USING_BSD_SIGINFO */

        /*
         * When compared to the old implementation, this
         * implementation also flushes the output queue. POSIX
         * is really brief about this, but does makes us assume
         * we have to do so.
         */
        signal = 0;
        if (CMP_CC(VINTR, c))
        {
            signal = SIGINT;
        }
        else if (CMP_CC(VQUIT, c))
        {
            signal = SIGQUIT;
        }
        else if (CMP_CC(VSUSP, c))
        {
            signal = SIGTSTP;
        }

        if (signal != 0)
        {
            /*
             * Echo the character before signalling the
             * processes.
             */
            if (!CMP_FLAG(l, NOFLSH))
                tty_flush(tp, FREAD | FWRITE);
            ttydisc_echo(tp, c, 0);
            lwp_tty_signal_pgrp(tp, signal);
            return 0;
        }
    }

    /*
     * Handle start/stop characters.
     */
    if (CMP_FLAG(i, IXON))
    {
        if (CMP_CC(VSTOP, c))
        {
            /* Stop it if we aren't stopped yet. */
            if ((tp->t_flags & TF_STOPPED) == 0)
            {
                tp->t_flags |= TF_STOPPED;
                return 0;
            }
            /*
             * Fallthrough:
             * When VSTART == VSTOP, we should make this key
             * toggle it.
             */
            if (!CMP_CC(VSTART, c))
                return 0;
        }
        if (CMP_CC(VSTART, c))
        {
            tp->t_flags &= ~TF_STOPPED;
            return 0;
        }
    }

    /* Conversion of CR and NL. */
    switch (c)
    {
        case CCR:
            if (CMP_FLAG(i, IGNCR))
                return 0;
            if (CMP_FLAG(i, ICRNL))
                c = CNL;
            break;
        case CNL:
            if (CMP_FLAG(i, INLCR))
                c = CCR;
            break;
    }

    /* Canonical line editing. */
    if (CMP_FLAG(l, ICANON))
    {
        if (CMP_CC(VERASE, c)
#ifdef VERASE2
            || CMP_CC(VERASE2, c)
#endif
        )
        {
            ttydisc_rubchar(tp);
            return 0;
        }
        else if (CMP_CC(VKILL, c))
        {
            while (ttydisc_rubchar(tp) == 0)
                ;
            return 0;
        }
        else if (CMP_FLAG(l, IEXTEN))
        {
            if (CMP_CC(VWERASE, c))
            {
                ttydisc_rubword(tp);
                return 0;
            }
            else if (CMP_CC(VREPRINT, c))
            {
                ttydisc_reprint(tp);
                return 0;
            }
        }
    }

processed:
    if (CMP_FLAG(i, PARMRK) && (unsigned char)c == 0xff)
    {
        /* Print 0xff 0xff. */
        ob[1] = 0xff;
        ol = 2;
        quote = 1;
    }
    else
    {
        ob[0] = c;
        ol = 1;
    }

    goto print;

parmrk:
    if (CMP_FLAG(i, PARMRK))
    {
        /* Prepend 0xff 0x00 0x.. */
        ob[2] = c;
        ol = 3;
        quote = 1;
    }
    else
    {
        ob[0] = c;
        ol = 1;
    }

print:
    /* See if we can store this on the input queue. */
    if (ttyinq_write_nofrag(&tp->t_inq, ob, ol, quote) != 0)
    {
        if (CMP_FLAG(i, IMAXBEL))
            ttyoutq_write_nofrag(&tp->t_outq, "\a", 1);

        /*
         * Prevent a deadlock here. It may be possible that a
         * user has entered so much data, there is no data
         * available to read(), but the buffers are full anyway.
         *
         * Only enter the high watermark if the device driver
         * can actually transmit something.
         */
        if (ttyinq_bytescanonicalized(&tp->t_inq) == 0)
            return 0;

        tty_hiwat_in_block(tp);
        return (-1);
    }

    /*
     * In raw mode, we canonicalize after receiving a single
     * character. Otherwise, we canonicalize when we receive a
     * newline, VEOL or VEOF, but only when it isn't quoted.
     */
    if (!CMP_FLAG(l, ICANON) ||
        (!quote && (c == CNL || CMP_CC(VEOL, c) || CMP_CC(VEOF, c))))
    {
        ttyinq_canonicalize(&tp->t_inq);
    }

    ttydisc_echo(tp, c, quote);

    return 0;
}

size_t ttydisc_rint_simple(struct lwp_tty *tp, const void *buf, size_t len)
{
    const char *cbuf;

    if (ttydisc_can_bypass(tp))
        return (ttydisc_rint_bypass(tp, buf, len));

    for (cbuf = buf; len-- > 0; cbuf++)
    {
        if (ttydisc_rint(tp, *cbuf, 0) != 0)
            break;
    }

    return (cbuf - (const char *)buf);
}

size_t ttydisc_rint_bypass(struct lwp_tty *tp, const void *buf, size_t len)
{
    size_t ret;

    tty_assert_locked(tp);

    MPASS(tp->t_flags & TF_BYPASS);

    rt_atomic_add(&tty_nin, len);

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, rint_bypass))
        return ttyhook_rint_bypass(tp, buf, len);
#endif

    ret = ttyinq_write(&tp->t_inq, buf, len, 0);
    ttyinq_canonicalize(&tp->t_inq);
    if (ret < len)
        tty_hiwat_in_block(tp);

    return (ret);
}

void ttydisc_rint_done(struct lwp_tty *tp)
{
    tty_assert_locked(tp);

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, rint_done))
        ttyhook_rint_done(tp);
#endif

    /* Wake up readers. */
    tty_wakeup(tp, FREAD);
    /* Wake up driver for echo. */
    ttydevsw_outwakeup(tp);
}

size_t ttydisc_rint_poll(struct lwp_tty *tp)
{
    size_t l;

    tty_assert_locked(tp);

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, rint_poll))
        return ttyhook_rint_poll(tp);
#endif

    /*
     * XXX: Still allow character input when there's no space in the
     * buffers, but we haven't entered the high watermark. This is
     * to allow backspace characters to be inserted when in
     * canonical mode.
     */
    l = ttyinq_bytesleft(&tp->t_inq);
    if (l == 0 && (tp->t_flags & TF_HIWAT_IN) == 0)
        return (1);

    return (l);
}

static void ttydisc_wakeup_watermark(struct lwp_tty *tp)
{
    size_t c;

    c = ttyoutq_bytesleft(&tp->t_outq);
    if (tp->t_flags & TF_HIWAT_OUT)
    {
        /* Only allow us to run when we're below the watermark. */
        if (c < tp->t_outlow)
            return;

        /* Reset the watermark. */
        tp->t_flags &= ~TF_HIWAT_OUT;
    }
    else
    {
        /* Only run when we have data at all. */
        if (c == 0)
            return;
    }
    tty_wakeup(tp, FWRITE);
}

size_t ttydisc_getc(struct lwp_tty *tp, void *buf, size_t len)
{
    tty_assert_locked(tp);

    if (tp->t_flags & TF_STOPPED)
        return 0;

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, getc_inject))
        return ttyhook_getc_inject(tp, buf, len);
#endif

    len = ttyoutq_read(&tp->t_outq, buf, len);

#ifdef USING_BSD_HOOK
    if (ttyhook_hashook(tp, getc_capture))
        ttyhook_getc_capture(tp, buf, len);
#endif

    ttydisc_wakeup_watermark(tp);
    rt_atomic_add(&tty_nout, len);

    return (len);
}

int ttydisc_getc_uio(struct lwp_tty *tp, struct uio *uio)
{
    int error = 0;
    ssize_t obytes = uio->uio_resid;
    size_t len;
    char buf[TTY_STACKBUF];

    tty_assert_locked(tp);

    if (tp->t_flags & TF_STOPPED)
        return 0;

    /*
     * When a TTY hook is attached, we cannot perform unbuffered
     * copying to userspace. Just call ttydisc_getc() and
     * temporarily store data in a shadow buffer.
     */
    if (ttyhook_hashook(tp, getc_capture) || ttyhook_hashook(tp, getc_inject))
    {
        while (uio->uio_resid > 0)
        {
            /* Read to shadow buffer. */
            len = ttydisc_getc(tp, buf, MIN(uio->uio_resid, sizeof buf));
            if (len == 0)
                break;

            /* Copy to userspace. */
            tty_unlock(tp);
            error = uiomove(buf, len, uio);
            tty_lock(tp);

            if (error != 0)
                break;
        }
    }
    else
    {
        error = ttyoutq_read_uio(&tp->t_outq, tp, uio);

        ttydisc_wakeup_watermark(tp);
        rt_atomic_add(&tty_nout, obytes - uio->uio_resid);
    }

    return error;
}

size_t ttydisc_getc_poll(struct lwp_tty *tp)
{
    tty_assert_locked(tp);

    if (tp->t_flags & TF_STOPPED)
        return 0;

#if USING_BSD_HOOK
    if (ttyhook_hashook(tp, getc_poll))
        return ttyhook_getc_poll(tp);
#endif

    return ttyoutq_bytesused(&tp->t_outq);
}