Nginx主进程切换为后台守护进程

/*
 * Copyright (C) Igor Sysoev
 * Copyright (C) Nginx, Inc.
 */

#include <ngx_config.h>
#include <ngx_core.h>

/*
spawn a new process and kill parent process, 
redirect io to /dev/null and run background ...
*/
ngx_int_t
ngx_daemon(ngx_log_t *log)
{
    int  fd;

    switch (fork()) {
    case -1:
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "fork() failed");
        return NGX_ERROR;

    case 0:				//for child process, return val is: 0
        break;

    default:				//child process id, exit original main process elegantly
        exit(0);
    }

    ngx_pid = ngx_getpid();			//get current process id

    if (setsid() == -1) {			//for parent process exit, here produce another session for child process
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "setsid() failed");
        return NGX_ERROR;
    }

    umask(0);				//set new file access mode

	/*
		redirect IO to /dev/null, and all the messages will not occurs on the screen
	*/
    fd = open("/dev/null", O_RDWR);
    if (fd == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,
                      "open(\"/dev/null\") failed");
        return NGX_ERROR;
    }

    if (dup2(fd, STDIN_FILENO) == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "dup2(STDIN) failed");
        return NGX_ERROR;
    }

    if (dup2(fd, STDOUT_FILENO) == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "dup2(STDOUT) failed");
        return NGX_ERROR;
    }

#if 0
    if (dup2(fd, STDERR_FILENO) == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "dup2(STDERR) failed");
        return NGX_ERROR;
    }
#endif

    if (fd > STDERR_FILENO) {
        if (close(fd) == -1) {
            ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "close() failed");
            return NGX_ERROR;
        }
    }

    return NGX_OK;
}