Path: utzoo!attcan!uunet!mcvax!ukc!reading!cf-cm!sme
From: sme@computing-maths.cardiff.ac.uk (Simon Elliott)
Newsgroups: comp.unix.wizards
Subject: Re: script & lock
Summary: getlogin only returns NULL when not on tty, even psuedo-tty
Message-ID: <596@cf-cm.UUCP>
Date: 9 Jan 89 10:16:19 GMT
References: <17972@adm.BRL.MIL>
Organization: University of Wales College of Cardiff, Cardiff, WALES, UK.
Lines: 48


   Getlogin returns a pointer to the login name as found in /etc/utmp.
   /etc/utmp contains a record for each terminal on the system (including
   pseudo-terminals (ttyp?).  The login name part is blank or null until
   login fills it it.  When you run script, your controlling terminal is a
   pseudo-terminal (eg. /dev/ttyp0).  Your login name is not put into
   /etc/utmp for this pseudo-terminal so the entry is still null.   A program
   run in script is attached to the pseudo terminal so getlogin() returns a
   pointer to the null string entry from utmp.    Getlogin returns the NULL
   pointer only if it called from a process which is NOT attached to any
   terminal (even a pseudo one).

-------cut here-------

#include	<stdio.h>
#include	<pwd.h>

main(ac,av)
	char	*av[];
{
	char	*uname, getlogin();
	struct	passwd	*pwd, getpwnam(), getpwuid();

	if ((uname = getlogin()) == NULL) {
		pwd = getpwuid(getuid());
fprintf(stderr, "uid: %d\n", getuid());
	}
	else {
		pwd = getpwnam(uname);
fprintf(stderr, "uname: *(char *)0x%lx = \"%s\"\n", uname, uname);
	}
	if (pwd == NULL) {
		fprintf(stderr, "%s: cannot locate passwd entry!\007\n", av[0]);
		exit(1);
	}
}

---------cut here-------
Try running this a) from shell prompt;
                 b) under 'script' (uses pseudo-tty);
                 c) using 'at' (detaches from terminal).

-- 
--------------------------------------------------------------------------
Simon Elliott            Internet: sme%v1.cm.cf.ac.uk@cunyvm.cuny.edu
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