Reference: Molay, Understanding Unix/Linux Programming, Chapter 2.1-2.5

Our standard approach to studying a Unix system command:

1. What does who do?
2. How does who work?
3. Can I write who?

The vast majority of system commands are programs written in C and located in one of the standard system directories, such as /bin, /usr/bin, or /usr/local/bin.

Run the who command:

$ who
johnqdoe   console  Sep 17 08:59
janedoe    ttys000  Sep 22 09:24

Read the manual:

$ man who
WHO(1)                    BSD General Commands Manual                   WHO(1)

NAME
     who -- display who is logged in

SYNOPSIS
     who [-abdHlmpqrsTtu] [file]
     who am i

DESCRIPTION
     The who utility displays a list of all users currently logged on, showing
     for each user the login name, tty name, the date and time of login, and
     hostname if not local.

     Available options:

     -a    Same as -bdlprTtu.

     -b    Time of last system boot.

     -d    Print dead processes.

     -H    Write column headings above the regular output.

     -l    Print system login processes (unsupported).

     -m    Only print information about the current terminal.  This is the
           POSIX way of saying who am i.

     -p    Print active processes spawned by launchd(8) (unsupported).

     -q    ''Quick mode'': List only the names and the number of users cur-
           rently logged on.  When this option is used, all other options are
           ignored.

     -r    Print the current runlevel.  This is meaningless on Mac OS X.

     -s    List only the name, line and time fields.  This is the default.

     -T    Print a character after the user name indicating the state of the
           terminal line: '+' if the terminal is writable; '-' if it is not;
           and '?' if a bad line is encountered.

     -t    Print last system clock change (unsupported).

     -u    Print the idle time for each user, and the associated process ID.

     am I  Returns the invoker's real user name.

     file  By default, who gathers information from the file /var/run/utmpx.
           An alternative file may be specified.

FILES
     /var/run/utmpx

SEE ALSO
     last(1), mesg(1), users(1), getuid(2), utmpx(5)

STANDARDS
     The who utility conforms to IEEE Std 1003.1-2001 (''POSIX.1'').

HISTORY
     A who utility appeared in Version 6 AT&T UNIX.

BSD                            January 17, 2007                            BSD

Search the manual:

$ man -k utmp
utmp(5), wtmp(5), lastlog(5) - login records (DEPRECATED)
utmpx(5)                 - user accounting database

$ man utmp
UTMP(5)                     BSD File Formats Manual                    UTMP(5)

NAME
     utmp, wtmp, lastlog -- login records (DEPRECATED)

SYNOPSIS
     #include 

DESCRIPTION
     The interfaces in file  are all DEPRECATED and are only provided
     for compatibility with previous releases of Mac OS X.  See pututxline(3)
     and utmpx(5) for the supported interfaces.

      declares the structures used to record information about current
     users in the file utmp, logins and logouts in the file wtmp, and last
     logins in the file lastlog.  The time stamps of date changes, shutdowns
     and reboots are also logged in the wtmp file.

     These files can grow rapidly on busy systems, daily or weekly rotation is
     recommended.  If any of these files do not exist, it is not created.
     These files must be created manually and are normally maintained in
     either the script /etc/daily or the script /etc/weekly.  (See cron(8).)
           #define _PATH_UTMP      "/var/run/utmp"
           #define _PATH_WTMP      "/var/log/wtmp"
           #define _PATH_LASTLOG   "/var/log/lastlog"

           #define UT_NAMESIZE     8
           #define UT_LINESIZE     8
           #define UT_HOSTSIZE     16

           struct lastlog {
                   time_t  ll_time;
                   char    ll_line[UT_LINESIZE];
                   char    ll_host[UT_HOSTSIZE];
           };

           struct utmp {
                   char    ut_line[UT_LINESIZE];
                   char    ut_name[UT_NAMESIZE];
                   char    ut_host[UT_HOSTSIZE];
                   time_t  ut_time;
           };

     Each time a user logs in, the login program looks up the user's UID in
     the file lastlog. If it is found, the timestamp of the last time the user
     logged in, the terminal line and the hostname are written to the standard
     output. (Providing the login is not quiet, see login(1).)  The login pro-
     gram then records the new login time in the file lastlog.

     After the new lastlog record is written , the file utmp is opened and the
     utmp record for the user inserted.  This record remains there until the
     user logs out at which time it is deleted.  The utmp file is used by the
     programs rwho(1), users(1), w(1), and who(1).

     Next, the login program opens the file wtmp, and appends the user's utmp
     record.  The same utmp record, with an updated time stamp is later
     appended to the file when the user logs out. (See launchd(8).)  The wtmp
     file is used by the programs last(1) and ac(8).

     In the event of a date change, a shutdown or reboot, the following items
     are logged in the wtmp file.

     reboot
     shutdown    A system reboot or shutdown has been initiated.  The charac-
                 ter `~' is placed in the field ut_line, and reboot or
                 shutdown in the field ut_name.  (See shutdown(8) and
                 reboot(8).)
     date        The system time has been manually or automatically updated.
                 (See date(1).)  The command name date is recorded in the
                 field ut_name.  In the field ut_line, the character `|' indi-
                 cates the time prior to the change, and the character `{'
                 indicates the new time.

FILES
     (These files no longer exist in 10.5 or later.)

     /var/run/utmp     The utmp file.
     /var/log/wtmp     The wtmp file.
     /var/log/lastlog  The lastlog file.

SEE ALSO
     last(1), login(1), who(1), ac(8), launchd(8)

HISTORY
     A utmp and wtmp file format appeared in Version 6 AT&T UNIX.  The lastlog
     file format appeared in 3.0BSD.

4th Berkeley Distribution       March 17, 1994       4th Berkeley Distribution

Read the .h files

$ more /usr/include/utmp.h
...
struct utmp
{
        char ut_user[256];             /* User login name */
        char ut_id[14];                /* /etc/inittab id */
        char ut_line[64];              /* device name (console, lnxx) */
        pid_t ut_pid;                  /* process id */
        short ut_type;                 /* type of entry */
        time_t ut_time;                /* time entry was made */
        struct exit_status
        {
            short e_termination;       /* Process termination status */
            short e_exit;              /* Process exit status */
        }
        ut_exit;                       /* The exit status of a process
                                        * marked as DEAD_PROCESS.
                                        */
        char ut_host[256];             /* host name */
        int __dbl_word_pad;            /* for double word alignment */
        int __reservedA[2];
        int __reservedV[6];
};
...

The manual explains the basic operation of the who command:

open utmp
repeat
   read record
   display record
close utmp

Use the open, read, and close system calls.

int fd = open(char *name, int how);

Creates a connection (a file descriptor fd) between a process and a file named name. how indicates the mode: O_RDONLY, O_WRONLY, O_RDWR.

ssize_t numread = read(int fd, void *buf, size_t qty);

Asks the kernel to transfer qty bytes of data from the file descriptor fd to the array buf. Returns the number of bytes read on success, -1 otherwise.

int result = close(int fd);

Hangs up the connection specified by the file descriptor fd.

Note: use utmpx instead of utmp on OSX.

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

#define SHOWHOST 1

/*
 *  show info()
 *   displays contents of the utmp struct in human readable form
 *   *note* these sizes should not be hardwired
 */
void show_info( struct utmp *utbufp )
{
   printf("%-8.8s", utbufp->ut_name);   /* the logname  */
   printf(" ");                         /* a space      */
   printf("%-8.8s", utbufp->ut_line);   /* the tty      */
   printf(" ");                         /* a space      */
   printf("%10d", utbufp->ut_time);     /* login time   */
   printf(" ");                         /* a space      */
#ifdef   SHOWHOST
   printf("(%s)", utbufp->ut_host);     /* the host     */
#endif
   printf("\n");                        /* newline      */
}

int main()
{
   struct utmp   current_record;   /* read info into here       */
   int    utmpfd;                  /* read from this descriptor */
   int    reclen = sizeof(current_record);

   if ( (utmpfd = open(UTMP_FILE, O_RDONLY)) == -1 ){
      perror( UTMP_FILE );         /* UTMP_FILE is in utmp.h    */
      return 1;
   }

   while ( read(utmpfd, &current_record, reclen) == reclen )
      show_info(&current_record);
   close(utmpfd);
   return 0;         /* went ok */
}