How to programmatically get the number of users logged in on a linux machine?

Question

I was wondering if it was possible to programmatically get the number of users logged in on a Linux machine in C? I did some researching and found out about utmp.h but since not all programs use utmp logging, I did not think it would be accurate enough. Thanks in advance to anyone willing to help.

EDIT: I apologize guys for not being more specific but when I say logged in users, I am referring to any logged in via shell. Basically what you get when you run the who command with no command line arguments.

Answer 1

#include <utmp.h>
#include <err.h>

#define NAME_WIDTH  8

    FILE *ufp;
    int numberOfUsers = 0;
    struct utmp usr;
    ufp = file(_PATH_UTMP);
    while (fread((char *)&usr, sizeof(usr), 1, ufp) == 1) {
    if (*usr.ut_name && *usr.ut_line && *usr.ut_line != '~') {
         numberOfUsers++;
        }
    }

    FILE *file(char *name)
    {
        FILE *ufp;

        if (!(ufp = fopen(name, "r"))) {
            err(1, "%s", name);
        }
        return(ufp);
    }

After a couple of days playing around with utmp, I figured it out. Thanks for the help guys.