Problems Running Later Code on Vanilla V6


Backporting code from later Unix versions to V6 can be somewhat problematic.
The issues can be divided into i) language issues, where the C language has
changed; ii) system interface issues, and iii) other.

Use of the new 'Typesetter' C compiler (available for V6) gives access to a
more modern (but not fully modern) version of the language, with longs, etc
but some issues (particularly in the system interface) remain.


Language Issues

The main issue is that V6 C is pretty archaic, and there are a number of
places where the syntax is different from 'modern' C (or even later C's, like
V7). Many of these can be fixed/avoided by use of the 'Typesetter' C compiler.

Here are the ones I know of:

- Things like "a =+ 2" does not now do what it used to back then, and the new
compilers don't barf on it (as they do with things like "=|"). Many will
probably cause a compiler error on a new compiler; the ones to search for
(which _won't_ always give compiler errors) are "=+", "=-", "=*" and "=&" (I
think that list is complete).

And the vanilla V6 compiler is so old it doesn't recognize "+=", so one _has_
to write the old form. (One could write "a = a + 2", so the code would be
portable across the V6/later boundary.)

- Initializations don't have the '=' between the name and initializer. This
shouldn't be a big problem; the compiler should barf on them. Again, one
can't code around this - the V6 compiler only supports the older style.

- V6 C is pretty promiscuous about types (especially pointers). So it's
common to see things like:

	int	*a;
	char	*b;

	a = b;

Again, most later compilers will notice this and bark at you.

- V6 C doesn't support "register" storage class for arguments, so you will see

	foo(aip)
	int	*aip;

	{	register int *ip;

		ip = aip;

all over the place. This is pretty harmless, although it takes a few extra
characters.


System Interface Issues 

Anything which relies on system headers (which is not just 'system' code like
tar, etc, but also anything that does a 'stat' call) has to use the old
formats. In particular, in V6 the stat block was a plain inode structure, and
in V7, it's synthetic and different; e.g. the file size isn't a simple long
in V6.

To truly support stdio's functionality, one needs to add at least one new
system call, lseek(). [Taking a long argument isn't the problem: the problem
is that the tell() functionality just isn't available [AFAIK] in vanilla V6.]

A few system calls have changed their arguments, but most of these can be
worked around [code for many is given in the \src\libc\v6 directory on V7]:
	chown() - going to short UIDs from bytes means it's now three args
	execl() [although the arguments are the same, the V7 version uses a
	static external to find the environment],

Some system calls don't exist in V7, and there are new ones.
  Removed: sleep() [replaced by a subr which calls alarm() and pause()]
	tiu()
  Added: alarm(), pause()
	utime()
	access() [which is done in a hacky way in the v6 replacement - doing it
		right would involve another new system call]
	ftime()
	tell() [already obsolete]
	sysacct()
	sysphys()
	syslock()
	ioctl()
	mpxchan()
	exece()
	umask()
	chroot()

Some things are missing from the V6 stdio library:
	mktemp()


Other Issues 

- If switching to the 'Typesetter C', one has to replace csv.s with the newer
CSV, as the old one bashed r1 (used to return long values).