库函数daemon的实现

代码来自https://www.gnu.org/software/libc/download.html

 

$ cat misc/daemon.c/*- * Copyright (c) 1990, 1993 *      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. * 4. 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. */#if defined(LIBC_SCCS) && !defined(lint)static char sccsid[] = "@(#)daemon.c    8.1 (Berkeley) 6/4/93";#endif /* LIBC_SCCS and not lint */#include <errno.h>#include <fcntl.h>#include <paths.h>#include <unistd.h>#include <sys/stat.h>#include <device-nrs.h>#include <not-cancel.h>intdaemon(nochdir, noclose)        int nochdir, noclose;{        int fd;        switch (__fork()) {        case -1:                return (-1);        case 0:                break;        default:                _exit(0);        }        if (__setsid() == -1)                return (-1);        if (!nochdir)                (void)__chdir("/");        if (!noclose) {                struct stat64 st;                if ((fd = open_not_cancel(_PATH_DEVNULL, O_RDWR, 0)) != -1                    && (__builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0)                        == 0)) {                        if (__builtin_expect (S_ISCHR (st.st_mode), 1) != 0#if defined DEV_NULL_MAJOR && defined DEV_NULL_MINOR                            && (st.st_rdev                                == makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR))#endif                            ) {                                (void)__dup2(fd, STDIN_FILENO);                                (void)__dup2(fd, STDOUT_FILENO);                                (void)__dup2(fd, STDERR_FILENO);                                if (fd > 2)                                        (void)__close (fd);                        } else {                                /* We must set an errno value since no                                   function call actually failed.  */                                close_not_cancel_no_status (fd);                                __set_errno (ENODEV);                                return -1;                        }                } else {                        close_not_cancel_no_status (fd);                        return -1;                }        }        return (0);}