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Do not leak a file descriptor when opening nohup.out.  Make sure
that stdout and stderr are not closed.
from Nan Xiao

/*	$OpenBSD: nohup.c,v 1.18 2018/09/14 18:17:46 bluhm Exp $	*/
/*	$NetBSD: nohup.c,v 1.6 1995/08/31 23:35:25 jtc Exp $	*/

/*
 * Copyright (c) 1989 The Regents of the University of California.
 * All rights reserved.
 *
 * 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.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
 */

#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <limits.h>
#include <err.h>

static void dofile(void);
__dead static void usage(void);

/*
 * nohup shall exit with one of the following values:
 * 126 - The utility was found but could not be invoked.
 * 127 - An error occurred in the nohup utility, or the utility could
 *       not be found.
 */
#define EXIT_NOEXEC	126
#define EXIT_NOTFOUND	127
#define EXIT_MISC	127

/*
 * If the standard output is a terminal, all output written to
 * its standard output shall be appended to the end of the file
 * nohup.out in the current directory.  If nohup.out cannot be
 * created or opened for appending, the output shall be appended
 * to the end of the file nohup.out in the directory specified
 * by the HOME environment variable.
 *
 * If a file is created, the file's permission bits shall be
 * set to S_IRUSR | S_IWUSR.
 */
#define	FILENAME	"nohup.out"

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

	if (pledge("stdio rpath wpath cpath exec", NULL) == -1)
		err(1, "pledge");

	if (argc < 2)
		usage();

	if (isatty(STDOUT_FILENO) || errno == EBADF)
		dofile();

	if (pledge("stdio exec", NULL) == -1)
		err(1, "pledge");

	if ((isatty(STDERR_FILENO) || errno == EBADF) &&
	    dup2(STDOUT_FILENO, STDERR_FILENO) == -1) {
		/* may have just closed stderr */
		(void)fprintf(stdin, "nohup: %s\n", strerror(errno));
		exit(EXIT_MISC);
	}

	/*
	 * The nohup utility shall take the standard action for all signals
	 * except that SIGHUP shall be ignored.
	 */
	(void)signal(SIGHUP, SIG_IGN);

	execvp(argv[1], &argv[1]);
	exit_status = (errno == ENOENT) ? EXIT_NOTFOUND : EXIT_NOEXEC;
	err(exit_status, "%s", argv[1]);
}

static void
dofile(void)
{
	int fd;
	const char *p;
	char path[PATH_MAX];

	p = FILENAME;
	if ((fd = open(p, O_RDWR|O_CREAT|O_APPEND, S_IRUSR|S_IWUSR)) >= 0)
		goto dupit;
	if ((p = getenv("HOME")) != NULL && *p != '\0' &&
	    (strlen(p) + strlen(FILENAME) + 1) < sizeof(path)) {
		(void)strlcpy(path, p, sizeof(path));
		(void)strlcat(path, "/", sizeof(path));
		(void)strlcat(path, FILENAME, sizeof(path));
		if ((fd = open(p = path, O_RDWR|O_CREAT|O_APPEND, S_IRUSR|S_IWUSR)) >= 0)
			goto dupit;
	}
	errx(EXIT_MISC, "can't open a nohup.out file");

dupit:
	(void)lseek(fd, (off_t)0, SEEK_END);
	if (dup2(fd, STDOUT_FILENO) == -1)
		err(EXIT_MISC, NULL);
	if (fd > STDERR_FILENO)
		(void)close(fd);
	(void)fprintf(stderr, "sending output to %s\n", p);
}

__dead static void
usage(void)
{
	(void)fprintf(stderr, "usage: nohup utility [arg ...]\n");
	exit(EXIT_MISC);
}