hnel.c in git
HNEL(1)                 FreeBSD General Commands Manual                HNEL(1)

NAME
     hnel – PTY input remapper

SYNOPSIS
     hnel string1 string2 command ...

DESCRIPTION
     hnel runs the command in a new PTY, remapping input characters in string1
     to the corresponding characters in string2.  Type ^S to toggle remapping.

EXAMPLES
           hnel '[]{}' '{}[]' vim

SEE ALSO
     tr(1), forkpty(3)

FreeBSD 12.0-RELEASE-p4        September 7, 2018       FreeBSD 12.0-RELEASE-p4
/* Copyright (C) 2017  C. McEnroe <june@causal.agency>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <err.h>
#include <poll.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/wait.h>
#include <sysexits.h>
#include <termios.h>
#include <unistd.h>

#if defined __FreeBSD__
#include <libutil.h>
#elif defined __linux__
#include <pty.h>
#else
#include <util.h>
#endif

typedef unsigned char byte;

static struct termios saveTerm;
static void restoreTerm(void) {
	tcsetattr(STDIN_FILENO, TCSADRAIN, &saveTerm);
}

int main(int argc, char *argv[]) {
	int error;

	if (argc < 4) return EX_USAGE;

	byte table[256] = {0};
	if (strlen(argv[1]) != strlen(argv[2])) return EX_USAGE;
	for (const char *from = argv[1], *to = argv[2]; *from; ++from, ++to) {
		table[(byte)*from] = *to;
	}

	error = tcgetattr(STDERR_FILENO, &saveTerm);
	if (error) err(EX_IOERR, "tcgetattr");
	atexit(restoreTerm);

	struct termios raw = saveTerm;
	cfmakeraw(&raw);
	error = tcsetattr(STDERR_FILENO, TCSADRAIN, &raw);
	if (error) err(EX_IOERR, "tcsetattr");

	struct winsize window;
	error = ioctl(STDERR_FILENO, TIOCGWINSZ, &window);
	if (error) err(EX_IOERR, "TIOCGWINSZ");

	int pty;
	pid_t pid = forkpty(&pty, NULL, NULL, &window);
	if (pid < 0) err(EX_OSERR, "forkpty");

	if (!pid) {
		execvp(argv[3], &argv[3]);
		err(EX_NOINPUT, "%s", argv[3]);
	}

	bool enable = true;

	byte buf[4096];
	struct pollfd fds[2] = {
		{ .fd = STDIN_FILENO, .events = POLLIN },
		{ .fd = pty, .events = POLLIN },
	};
	while (0 < poll(fds, 2, -1)) {
		if (fds[0].revents & POLLIN) {
			ssize_t readSize = read(STDIN_FILENO, buf, sizeof(buf));
			if (readSize < 0) err(EX_IOERR, "read(%d)", STDIN_FILENO);

			if (readSize == 1) {
				if (buf[0] == CTRL('S')) {
					enable ^= true;
					continue;
				}

				byte c = buf[0];
				if (enable && table[c]) buf[0] = table[c];
			}

			ssize_t writeSize = write(pty, buf, readSize);
			if (writeSize < 0) err(EX_IOERR, "write(%d)", pty);
			if (writeSize < readSize) errx(EX_IOERR, "short write(%d)", pty);
		}

		if (fds[1].revents & POLLIN) {
			ssize_t readSize = read(pty, buf, sizeof(buf));
			if (readSize < 0) err(EX_IOERR, "read(%d)", pty);

			ssize_t writeSize = write(STDOUT_FILENO, buf, readSize);
			if (writeSize < 0) err(EX_IOERR, "write(%d)", STDOUT_FILENO);
			if (writeSize < readSize) {
				errx(EX_IOERR, "short write(%d)", STDOUT_FILENO);
			}
		}

		int status;
		pid_t dead = waitpid(pid, &status, WNOHANG);
		if (dead < 0) err(EX_OSERR, "waitpid(%d)", pid);
		if (dead) return WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE;
	}
	err(EX_IOERR, "poll");
}