Lec15) Design and Code Of UNIX who Utility , Buffering

default behavior of who is to display a list of currently logged in users, 

 

 

if there are no argument given when executing "who" command. it is basically reading from /var/run/utmp and write on terminal with appropriate format.

 

who process

1. open the file /var/run/utmp
2. read a utmp strcture untill end of file
3. display the requried fields
4. go to step2
5. close file /var/run/utmp

 

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <utmp.h>
#include <fcntl.h> 
#include <time.h>

void show_info(struct utmp*);
int main(){
   int fd = open("/var/run/utmp", O_RDONLY);	
   if (fd == -1 ){
	   perror("Error in opening utmp file");
	   exit(1);
   }
   struct utmp rec;
   int reclen = sizeof(rec);
   while (read(fd, &rec, reclen) == reclen)
	   show_info(&rec);
   close(fd);
	return 0;	
}

void show_info(struct utmp *rec){
   if(rec->ut_type != 7)
      return;
	printf("%-10.10s   ", rec->ut_user);
	printf("%-10.10s   ", rec->ut_line);
   long time = rec->ut_time;
   char* dtg = ctime(&time);
	printf("%-15.12s", dtg+4);
   printf("   (%s)", rec->ut_host);
	printf("\n");		
}

work same as "who" command.

 

Buffering

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define BUFSIZE 1024
int main(int argc, char *argv[]){
   if (argc != 3)	{
      printf("Invalid number of arguments.\n");
      exit(1);
   }
   int srcfd = open (argv[1], O_RDONLY);
   int destfd = open(argv[2],O_CREAT|O_RDWR|O_TRUNC, 00600);
   char buff[BUFSIZE];
   int n = 0;
   for(;;){
   n = read (srcfd, buff, BUFSIZE);
   if (n <= 0){
      close(srcfd); 
      close(destfd); 
      return 0;
   }
   write(destfd, buff, n);
   } 
   close(srcfd);
   close(destfd);
   return 0;
}

we can change Buffer size by changing marco define above.

with buffer we can read data with chunk. this case is user-space buffer

but kernel can also have buffer.