Program to display all logged in users via C program on Ubuntu?

Question

I have one program called allloggedin.c, whose purpose is to display all logged in users.

Whenever I try to run it on terminal (using gcc), it gives me erro

Answer 1

The Segmentation Fault you are getting is because of the line:

printf("%c",a[i]);

Since you didn't initialize the pointer a, it can point to any address, which may cause a Segmentation Fault when you dereference it.

I'm not sure what you are trying to print in the for loop so I removed it entirely from the code.

This is a cleaned version of the code:

#include<stdio.h>
#include<sys/utsname.h>
#include<utmp.h>

int main(void)
{
  struct utmp *n;
  setutent();
  n=getutent();

  while(n) {
    if(n->ut_type==USER_PROCESS) {
      printf("%9s%12s (%s)\n", n->ut_user, n->ut_line, n->ut_host);
    }
    n=getutent();
  }
  return 0;
}

I replaced the number 7 by the proper macro USER_PROCESS. Also, you can place all the prints in a single printf call.

So what the program basically does is to get the utmp struct for each user and print it in a fancy way.

For more information about the functions used and the utmp struct you can take a look at the utmp.h reference.