linux 守护进程创建流程

#include <sys/stat.h>
#include <fcntl.h>
/* Bit-mask values for 'flags' argument of becomeDaemon() */

#define BD_NO_CHDIR           01    /* Don't chdir("/") */
#define BD_NO_CLOSE_FILES     02    /* Don't close all open files */
#define BD_NO_REOPEN_STD_FDS  04    /* Don't reopen stdin, stdout, and
                                       stderr to /dev/null */
#define BD_NO_UMASK0         010    /* Don't do a umask(0) */

#define BD_MAX_CLOSE  8192          /* Maximum file descriptors to close if
                                       sysconf(_SC_OPEN_MAX) is indeterminate */
int becomeDaemon(int flags)         /* Returns 0 on success, -1 on error */

{
    int maxfd, fd;

    switch (fork())                        /* Become background process */
    {                  
    case -1: return -1;
    case 0:  break;                     /* Child falls through... */
    default: _exit(EXIT_SUCCESS);       /* while parent terminates */
    }

    if (setsid() == -1)                 /* Become leader of new session */
        return -1;

    switch (fork()) {                   /* Ensure we are not session leader */
    case -1: return -1;
    case 0:  break;
    default: _exit(EXIT_SUCCESS);
    }

    if (!(flags & BD_NO_UMASK0))
        umask(0);                       /* Clear file mode creation mask */

    if (!(flags & BD_NO_CHDIR))
        chdir("/");                     /* Change to root directory */

    if (!(flags & BD_NO_CLOSE_FILES)) { /* Close all open files */
        maxfd = sysconf(_SC_OPEN_MAX);
        if (maxfd == -1)                /* Limit is indeterminate... */
            maxfd = BD_MAX_CLOSE;       /* so take a guess */

        for (fd = 0; fd < maxfd; fd++)
            close(fd);
    }

    if (!(flags & BD_NO_REOPEN_STD_FDS)) {
        close(STDIN_FILENO);            /* Reopen standard fd's to /dev/null */

        fd = open("/dev/null", O_RDWR);

        if (fd != STDIN_FILENO)         /* 'fd' should be 0 */
            return -1;
        if (dup2(STDIN_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
            return -1;
        if (dup2(STDIN_FILENO, STDERR_FILENO) != STDERR_FILENO)
            return -1;
    }

    return 0;
}