Create a daemon

Reference resources 《 The background and development ： Core technology and application 》 The first 10.4 Section

If you want a process not to be affected by user or terminal changes or other changes , Then we must turn this process into a daemon

One 、 Basic steps of creating a daemon ：

1、 Create child process , The parent process exits

2、 call setid():

（1） Let the process get rid of the original session control ;
（2） Let the process out of the control of the original process group ;
（3） Let the process get rid of the control of the original control terminal

3、 Change the current directory to root

4、 Reset file permission mask

5、 Close the file descriptor

All the above steps are to make the child process get rid of the control of the original parent process .

Two 、 Implementation code

/* Implement an instance of a daemon （ every other  10s  stay ／tmp/dameon.log  Write a sentence ）*/

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<fcntl.h>
#include<unistd.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<sys/stat.h>
#define MAXFILE 65535

int main()
{
    
	pid_t pc;
	int i,fd,len;
	char *buf="this is a Dameon\n";
	len = strlen(buf);
	
	pc = fork(); /* First step :  Create child process , The parent process exits */
	if(pc<0)
	{
    
		printf("error fork\n");
		exit(1);
	}
	else if(pc>0)// Child process number =0, Parent process number is greater than 0
	{
    
		exit(0);// The parent process exits , Child process becomes orphan process 
	}
	
	setsid(); /* The second step :setsid()  Function to create a new session , And serve as the leader of the conversation group   call setid effect :1、 Let the process get rid of the original session control ; 2、 Let the process out of the control of the original process group ; 3、 Let the process get rid of the control of the original control terminal */
	
	chdir("/"); /* The third step : Change the current directory to root */
	
	umask(0); /* Step four : Reset file permission mask */
	
	for(i=0;i<MAXFILE;i++) /* Step five : Close the file descriptor */
	    close(i);
		
    while(1)
	{
    
		if((fd=open("/tmp/dameon.log",O_CREAT|O_WRONLY|O_APPEND,0600))<0)
		{
    
			perror("open");
			exit(1);
		}
		write(fd,buf,len+1);
		close(fd);
		sleep(10);
    }
    return 0;
}