C remains central to our computing infrastructure but still lacks a
clear and complete semantics. Programmers lack tools to explore the
range of behaviours they should expect; compiler development lacks
test oracles; and formal verification and analysis must make
(explicitly or implicitly) many choices about the specific C they
target. The ISO standards for C have been developed over many years
but they remain unclear in some important respects, and in some areas
there are differences between the properties of C that are relied on
by the corpus of systems code in production, the properties that
compiler implementers aim to provide, and the properties of C as it is
specified in ISO C11.
The Cerberus project is developing semantic models for a substantial
fragment of C. It has several distinctive features:
This is a step towards a clear, consistent, and unambiguous semantics for C.
- Where the ISO C11 standard is clear and corresponds with
practice, Cerberus aims to follow that.
- Where there are ambiguities or differences, chiefly in
the memory layout model (the behaviour of pointers,
pointer arithmetic, uninitialised values, etc.), we aim to
clarify the de facto standards and understand how the ISO
standard could be reconciled with them.
- Cerberus precisely defines the range of allowed behaviour, not just
that of some specific implementation.
- It is executable, to explore either all behaviours or single
paths of small test programs.
- Its thread-local semantics is factored via an elaboration into a
simpler Core language, to make it readable and conceptually and
mathematically tractable; the dynamic semantics of Core can be
linked with various memory object models (or, in a previous
version, with an
operational concurrency model, proved equivalent to the C/C++11
axiomatic concurrency model of Batty et al.).
- Its front-end is written from scratch to closely follow the C11
standard, including a parser that follows the C11 standard
grammar, and a typechecker.
Cerberus Web Interface
The Cerberus web
interface lets one interactively, randomly, or exhaustively
explore the behaviour of small C test programs in the Cerberus
Exploring C Semantics and Pointer Provenance.
Kayvan Memarian, Victor B. F. Gomes, Brooks Davis, Stephen Kell,
Robert N. M. Watson, Peter Sewell.
In Proc. 46th ACM SIGPLAN Symposium on Principles of Programming Languages
Supplementary material (tests and test results).
Into the Depths of C: Elaborating the De Facto Standards.
Kayvan Memarian, Justus Matthiesen,
James Lingard, Kyndylan Nienhuis, David Chisnall,
Robert N. M. Watson, and Peter Sewell.
In Proc. 37th
ACM SIGPLAN Conference on Programming Language Design and
Implementation (PLDI 2016). Distinguished paper award.
- An operational semantics for C/C++11
Kyndylan Nienhuis, Kayvan Memarian, and Peter Sewell.
In OOPSLA 2016.
This gives an operational semantics for C11 concurrency, proved
equivalent (in Isabelle/HOL) to the axiomatic model of Batty et al.
The C Memory Layout Model: ISO WG14 papers
WG14 Brno meeting working drafts 2018-04
WG14 Brno pre-meeting mailing, 2018-04
WG14 Pittsburgh meeting, 2016-10
WG14 London meeting, 2016-04
Here is a tarball of our test programs, including a tool to run
them and log the compile- and run-time output:
In April-September 2015 we distributed a survey of 15 questions about
C (What is C in practice? (Cerberus survey v2)). We were asking what
C is in current mainstream practice: the behaviour that programmers
assume they can rely on, the behaviour provided by mainstream
compilers, and the idioms used in existing code, especially systems
code. We were explicitly not asking what the ISO C standard permits,
which is often more restrictive, or about obsolete or obscure hardware
or compilers. We focussed on the behaviour of memory and pointers.
We had responses from 323 people, including many compiler
and OS developers. Here
is a summary of the results and the (anonymised) detailed comments:
The current main developers are Kayvan Memarian and Victor Gomes.
Kyndylan Nienhuis worked on the operational semantics for C11
Cerberus originated with Justus Matthiesen's 2010-11 Part II project
dissertation and his 2011-12 MPhil dissertation; James Lingard's
2013-14 MPhil dissertation developed a certifying translation
validator for simple C programs for the Clang front-end, w.r.t. the
Cerberus and Vellvm semantics.
This work is funded by REMS: Rigorous Engineering of Mainstream
Systems, EPSRC Programme Grant
EPSRC grant EP/H005633 (Leadership Fellowship, Sewell),
a Gates Cambridge Scholarship (Nienhuis).
This work is also part of the CTSRD projects sponsored by the Defense
Advanced Research Projects Agency (DARPA) and the Air Force Research
Laboratory (AFRL), under contract FA8750-10-C-0237.
The views, opinions, and/or findings contained in this paper are those of the
authors and should not be interpreted as representing the official views or
policies, either expressed or implied,
of the Department of Defense or the U.S. Government.