Copyright © 2002-2018,2021 by Thomas E. Dickey
Berkeley Yacc (byacc) is generally conceded to be the best yacc variant available. In contrast to bison, it is written to avoid dependencies upon a particular compiler.
Byacc was written around 1990 by Robert
Corbett who is the original author of bison.
Byacc is noted in Lex & Yacc by
John Levine et al (O'Reilly, 1992) for its compatibility with the
Later, in Flex & Bison (O'Reilly, 2009), John
Levine states that because Corbett's implementation was faster
than Bell (AT&T)
yacc, and because it was
distributed under the BSD license, "it quickly became the most
popular version of yacc". See below for
comments on licensing.
Originally written in K&R C, I have modified it to conform to ANSI C, and made other improvements.
See the changelog for details:
Early (when byacc was first released), Alex
Blakemore told me that byacc was "the cleanest
piece of code that (he) had ever seen". At the time, I was not
interested in byacc in particular, although it
was around the time that I started working on cproto. Since byacc
was compatible with
yacc, there was nothing
Later, I was reminded of the differences between
bison, when the latter
dumped core attempting to build the
Since that point, I have used byacc except for
those programs that have been modified to rely upon
My motivation for working on byacc was to use it in a port of the tin newsreader on VMS, early in 2000. The compiler reminded me that the source was K&R, so I converted it to ANSI C, and submitted it along with the other programs I was working on at that point to the VMS freeware cdrom. For instance, here is an abstract referencing that port.
ANSI C of course refers to
gcc. The latter have features
which are useful in a few specific applications, but
none of those include programs such as byacc. In
bison in its different versions has
I started by modifying the original byacc 1.9 distribution. For reference, there is an unmodified copy on my ftp site. That done, I took a look around for a maintainer, to "own" the changes. There was none. Alternatively, I could continue to maintain it. So I looked further, found a different variant with some possibly useful changes for MS-DOS. (I set that aside and did not return to it).
Initially, I ignored the source repositories for FreeBSD, NetBSD and the like, for the usual reasons:
However, I did make a note in 2005 to review the FreeBSD vs
bison use of
YYPARSE_PARAM (a similar idea
implemented in two incompatible ways). Eventually I wrote my own
version, which bridges the two.
The original distribution of byacc states in
Berkeley Yacc is in the public domain. The data structures and algorithms used in Berkeley Yacc are all either taken from documents available to the general public or are inventions of the author. Anyone may freely distribute source or binary forms of Berkeley Yacc whether unchanged or modified. Distributers may charge whatever fees they can obtain for Berkeley Yacc. Programs generated by Berkeley Yacc may be distributed freely.
As a rule, I'm license-neutral unless someone offers some reason to change it. For instance, I relicensed dialog to address user requests. Byacc is public domain. If I were to change it, e.g., to reflect my improvements, then it would likely be to the same MIT X11 license which I use the most.
Perry Metzger mentioned in correspondence during 2008 that having it remain public domain would suit his goals. On the other hand, Xin LI (a FreeBSD packager) asked me late in 2009 to add a copyright notice (license), and that making it on the same terms as FreeBSD's would suit his goals.
After discussing it with Perry, I did investigate further. Here is a recap:
But BSD 4.4 Lite was released June 1994 or Lite2 in 1995.
I've found no reliable source which states that byacc must bear a UCB copyright.
Here is a summary of the amount of change prompted by the discussion of licensing in 2009. For context, see this mailing list discussion.
Some additional amount of compatibility with
bison would be useful (to me, and others), with the
bison itself is no longer compatible
yacc. The most obvious difference was the
support for reentrant code, which has evolved in
byacc to the point where it can be compared and
However, even after factoring out that feature, there are a
few of the half-dozen or so programs that I have seen using
bison's reentrancy feature which are also relying
upon inferences which do not appear to match
(and some investigation is needed to isolate the exact problem).
Ironically, one of
bison's maintainers advised
the developers of "new" flex to avoid making their program
incompatible with the POSIX lex. Again, my motivation for working
on relocalizable flex was that
"new" flex was incompatible with both "old" flex and POSIX
Another area to explore would be integrating the
Perl feature, which appears to have no
There are numerous references on the net to byacc. Here are a few of the more interesting ones:
Some of the history of Perl-Byacc is covered in Computer Science & Perl Programming, by Jon Orwant (O'Reilly Media Inc., 2002).