关闭描述符是否影响创建的守护进程

Question

I wanna create a daemon process, which runs the top command of Linux in the background. If I closed all the descriptor, I can't find that top was created, because there is nothing about top when I use the ps -aux command to check. If I removed the lines about closing the descriptor, the top will run at terminal.

What's wrong with my code? If I wanna the program run correctly when closing the descriptor, what should I do?

This is the code.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <fcntl.h>

void main(void)
{
    int pid;
    int i;
    int fd;
    char *d[2];
    char a[5] = "top";
    d[0] = a;
    a[3] = '\0';
    d[1] = NULL;
    signal(SIGCHLD, SIG_IGN);
    if ((pid = fork()) == -1)
        perror("fork error");
    else if (pid > 0)
        exit(0);
    else {
        setsid();
        if ((pid = fork()) == -1)
            perror("fork error");
        else if (pid > 0)
            exit(0);
        else {
            for (i = 0;i < 255;i++)
                close(i);
            fd = open("/dev/null", O_RDWR);
            dup2(fd, STDIN_FILENO);
            dup2(fd, STDOUT_FILENO);
            dup2(fd, STDERR_FILENO);
            close(fd);
            chdir("/");
            umask(0);
            printf("%s\n", d[0]);
            execvp(d[0], d);
            perror("execvp failed");
            exit(1);
        }
    }
}

Answer 1

I think I find the stupid mistake I have made.I close all the descriptors and point the stdin,stdout,stdout to /dev/null.But the top command need the system information coming from the input.So the execvp faild, the top command doesn't execute.

If I don't close the descriptor 0 (stdin), the top command will execute in the background.If I close the descriptor 0 and point the descriptor 2 (stderr)to a log, the error will be print in the log.

It is too stubborn to think that the daemon process must close all descripotrs first .In case of wasting resource, we should just close the unecessary descripotrs.

From the top manpage: "-b :Batch-mode operation Starts top in 'Batch' mode, which could be useful for sending output from top to other programs or to a file. In this mode, top will not accept input"

The top command can execute without input ,using argument -b, and this works in my code .

Thanks very much.