CALLS

pty uses a bunch of syscalls (42 at last count) and library routines.
Calls marked with * are BSD-only or otherwise not available on all UNIX
systems. Calls marked with + are widely available but have different
behavior on different systems---behavior different enough that pty might
not work.

_exit: used in ptyerr.c for almost all exits. Note that pty does not use
	stdio in any way.
*accept: used in ptycomm.c`comm_accept, to accept a connection for the
	master on a descriptor created by comm_read
access: used with F_OK in ptyget.c to check if a pty bank (e.g., ttyq*)
	exists
*bind: used in ptycomm.c`comm_read to create the master's comm socket
*bzero: used inside the system's FD_ZERO macro before select
chdir: used once in the master and once in the sigler, both times in
	ptymain.c, to switch permanently to PTYDIR.
close: used throughout the program to get rid of a file descriptor.
	Note that the return value from close is not checked---there's
	absolutely no way pty could do anything about the descriptor in
	case of an error, and it's far more important that the
	descriptor be freed up than that NFS's perverted idea of UNIX
	I/O be dealt with.
*connect: used in ptycomm.c`comm_write to connect to a running master
dup: used (1) in ptytty.c to try to find the controlling tty descriptor;
	(2) in ptyslave.c to move the /dev/ttyxx descriptor to 0/1/2;
	(3) in ptysecure.c to set up descriptors for /bin/ps
*dup2: used in ptymisc.c to move a /dev/null descriptor around
execve: used in ptysecure.c to invoke /bin/ps
execvp: used in ptyslave.c to invoke the user's program, after setreuid
*fchmod: used in ptysecure.c upon a slave tty descriptor
+fcntl: used with F_GETFL in ptymisc.c (and sigsched.c) to see if a
	descriptor is open
flock: used in ptymisc.c for generic locking routine (used on log files)
fork: used in ptymain.c (twice) to create master and slave processes;
	used in ptysecure.c to invoke /bin/ps for security check
free: used in ralloc.c`rfree
*fstat: used in ptysecure.c upon a slave tty descriptor
*getdtablesize: used in slave under -E to close all open descriptors;
	used in ptymaster.c`closeallbut (which is a royal kludge)
*getpgrp: used in ptytty.c`force_fg to plug into TIOCSPGRP; used in
	ptymaster.c`contchild in preparation for killpg
getpid: used (1) in the master to fill in a sesslog structure; (2) in
	ptytty.c`force_fg to plug into getpgrp; (3) in the signaller to
	send a WINCH to itself after reconnect; (4) in the slave to set
	its pgrp before exec; (5) in the slave to pass to tctpgrp; (6)
	to set up numbers for random tty searching; (7) in the master,
	when fdctrlr asks for its pid; (8) in sigdfl.c to send a signal
	to itself; (9) in ptysecure.c for various purposes.
*getpwnam: used only in username.c
*getpwuid: used only in username.c
*geteuid: used in ptysecure.c as a secure pseudo-tty owner
getuid: used once in ptytty.c to initialize a uid variable
+ioctl: used in ptytty.c`ioc to control a tty descriptor; about two
	dozen BSD-specific ioctls are used in ptytty.c
+kill: used (1) in the master to tell the child to continue after the
	sigler says so; (2) in the sigler to tell itself to WINCH after
	a reconnect; (3) in sigdfl.c to send a signal to itself.
*killpg: used in ptymaster.c`contchild to continue the child's process
	group if a straight kill() failed
*listen: used in ptycomm.c to prepare the master's comm socket for
	connections
lseek: used in ptylogs.c, sessconnlog.c, and sesslog.c to place a record
	at a particular spot in a log file
malloc: used in ralloc.c`ralloc
+open: used (1) in ptyget.c with O_RDWR to open the master and slave
	sides of a tty; (2) in ptylogs.c with O_RDWR, O_RDWR | O_APPEND,
	and O_WRONLY | O_APPEND to open UTMP_FILE and WTMP_FILE; (3) in
	ptymisc.c`forceopen upon /dev/null and /, to attempt to make
	sure a descriptor is in use; (4) in ptyslave.c (twice) to open
	/dev/tty with O_RDWR; (5) in ptytty.c`tty_getctrl to open
	/dev/tty; (6) in sessconnlog.c and sesslog.c, with O_WRONLY |
	O_CREAT and O_WRONLY | O_APPEND | O_CREAT (both mode 0644), to
	open the sessnow, sesslog, scnow, and sclog system files for
	update; (7) in ptysecure.c for various purposes.
pipe: used (1) in ptymain.c to create pipes from the master to the
	sigler and slave, so that there's no race between the master
	setting up files and the sigler and slave starting; (2) in
	ptysigler.c to create the master->sigler and sigler->master
	pipes just before reconnect; (3) in ptysecure.c to create an
	output pipe for /bin/ps.
read
*recvmsg: used in ptycomm.c to receive a descriptor from a UNIX-domain
	socket. Note that recvmsg() is buggy in most BSD variants.
*select: used (1) in ptymisc.c to pause briefly; (2) in sigsched.c to
	check for usable descriptors.
*sendmsg: used in ptycomm.c to send a descriptor down a UNIX-domain
	socket. Note that sendmsg() is buggy in some BSD variants.
*setpgrp: used in ptyslave.c to set up pgrp of slave
*setreuid: used in ptyslave.c to make sure that slave runs as real uid;
	used in ptysecure.c to make sure that /bin/ps runs as real uid;
	used in ptymain.c to make sure that master is safe from user
*sigblock: used in sigdfl.c and sigsched.c for various purposes
+signal: used in various spots to set handlers to SIG_{IGN,DFL}
*sigsetmask: used in sigdfl.c and sigsched.c for various purposes
*sigvec: used in sigdfl.c and sigsched.c to install signal handlers
*socket: used in ptycomm.c`comm_{read,write} to set up comm socket
strcpy: used in env.c for various purposes
strlen: used throughout the code for various purposes
strncmp: used in env.c and ptylogs.c
strncpy: used in ptylogs.c, sessconnlog.c, sesslog.c
time: used in ptymisc.c`now
unlink: used only in ptycomm.c, upon a restricted pathname, to remove
	an obsolete communications socket.
*wait3: used in both master and sigler to wait for children (in the
	latter case, simply to get rid of zombies), with NOHANG and
	UNTRACED.
write

Note that pty also depends on environ and errno.

The BSD-specific calls can be divided into several groups:

A (pty allocation). Lots of vendors have added pseudo-ttys. In the BSD
world, everyone simply copied what Berkeley did. Unfortunately, System V
is graced with at least nine pseudo-tty allocation mechanisms; there are
probably many more which I haven't seen. ptyget.c and ptysecure.c have
to be completely rewritten for each new allocation design. On the other
hand, there must be code which uses the ptys on that system, so this is
certainly a doable job.

B (select). select (and the associated FD_ZERO [using bzero], FD_ISSET,
and FD_SET macros) is the foundation of the sigsched library, which
literally runs everything. select also uses the largest part of pty's
run time. It would be incredibly difficult to simulate select() with a
less powerful mechanism, such as poll(). I would have to be exceedingly
bored before attempting to port pty to a system without select().

C (communication, including fd passing). accept, bind, connect, listen,
recvmsg, sendmsg, and socket are isolated inside ptycomm.c. You can rip
out the entire ptycomm.c mechanism and replace it with anything which
(1) allows client-server connections with some security; (2) lets
read(), write(), and select() work; (3) provides file descriptor
passing; (4) allows a two-way connection on a single descriptor. Most
streams facilities can handle (3). Note that if the file descriptor
passing mechanism isn't as buggy as BSD's, you can get rid of
ptymisc.c`gaargh.

D (job control). getpgrp(), setpgrp(), the TIOCSPGRP and TIOCGPGRP
ioctls, various signals, kill(,SIGCONT), killpg, and wait3 are all part
of the BSD job control system. pty puts quite a bit of effort into
handling job control, but not nearly as much as the previous version. I
really can't guess what will go wrong if you simply rip out all the job
control code and replace wait3() with an appropriate wait().

E (tty handling). SIGWINCH and some two dozen tty ioctls all depend on
the BSD tty system. SIGWINCH is easy to take care of---it's already
#ifdef'ed for TTY_WINDOWS. (pty works under BSD 4.2, which doesn't have
SIGWINCH.) The other tty ioctls are isolated inside ptytty.c, which I
will probably rewrite for POSIX if and when somebody asks me to.
January 1992 update: Okay, I've now implemented TTY_TERMIO. POSIX, for
some absolutely idiotic reason, doesn't stick to the traditional header
files, but TTY_TERMIO should be good enough for systems with System V
tty handling.

F (signal management). sigblock, sigsetmask, sigvec, and good old signal
are all used throughout the pty code. POSIX can handle all of them, and
I will do this port soon. System V cannot---it's very important that pty
have reliable signal handling.

G (easy stuff). dup2, fcntl(,F_GETFL,0), getdtablesize, getpwnam,
getpwuid, fchmod, fstat, and setreuid are all reasonably easy to handle.
dup2() can be written in terms of F_DUPFD for System V; in fact, pty
only uses it in a situation where it should be a no-op! (See
ptymisc`forceopen and the calls to forceopen in ptymain.c.) F_GETFL is,
I believe, available on all modern UNIX-like systems. getdtablesize can
be replaced by NOFILE. setreuid(uid,uid) is essentially equivalent to
setuid(uid). getpwnam() and getpwuid() appear on most systems, and when
they don't appear, they're easy enough to write (just scan /etc/passwd).
fchmod and fstat can be replaced by chmod and stat---they're only used
in situations where the pathname is known (viz., a slave tty).