list_users.c

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

int main()
{
	setutent();

	struct utmp* entry;
	for ( entry = getutent(); entry; entry = getutent() )
	{
		if ( entry->ut_type == USER_PROCESS )
			printf( "User: %s\n", entry->ut_user );
	}

	return 0;
}