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Cerberus

Cerberus

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 a semantic model for a substantial fragment of C. It has several distinctive features:

Where the ISO C11 standard is clear and corresponds with practice, Cerberus aims to follow that.
Where there are 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 a range of allowed behaviour, not just that of some specific implementation.
It is executable, to explore all behaviours or single paths of small test programs.
It is designed to be integrated with an operational model for C11 concurrency, with a mechanised proof of equivalence to the axiomatic C11 model of Batty et al.
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 our sequential memory layout model or with that operational concurrency model.
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.

This is a step towards a clear, consistent, and unambiguous semantics for C.

Papers

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 '16). Distinguished paper award.

The C Memory Layout Model

We have assembled a collection of questions about the C memory layout semantics, supported by small test-case programs and discussion with respect to the ISO and de facto standards. This note summarises the most important questions, from an ISO-standard point of view:

Clarifying the C memory object model, Kayvan Memarian and Peter Sewell (this is a revision of WG14 N2012)

We discussed some of these in the WG14 London meeting, 2016-04.

The following discusses the questions in more detail, including the test programs and some experimental data:

C memory object and value semantics: the space of de facto and ISO standards, David Chisnall, Justus Matthiesen, Kayvan Memarian, Kyndylan Nienhuis, Peter Sewell, Robert N.M. Watson (this is a revision of WG14 N2013), and
a longer version, with more experimental data

Here is a tarball of our test programs, including a tool to run them and log the compile- and run-time output:

defacto_tarball.tar

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:

What is C in practice? (Cerberus survey v2): Analysis of Responses, Kayvan Memarian and Peter Sewell. (WG14 N2014)
What is C in practice? (Cerberus survey v2): Analysis of Responses - with Comments, Kayvan Memarian and Peter Sewell. (WG14 N2015)

In the WG14 Pittsburgh meeting, 2016-10, we presented the following:

N2089: Clarifying Unspecified Values (Draft Defect Report or Proposal for C2x) Kayvan Memarian and Peter Sewell, 2016-09-22
N2090: Clarifying Pointer Provenance (Draft Defect Report or Proposal for C2x) Kayvan Memarian and Peter Sewell, 2016-09-22
N2091: Clarifying Trap Representations (Draft Defect Report or Proposal for C2x) Kayvan Memarian and Peter Sewell, 2016-09-22

The C Memory Concurrency Model

We have developed an operational semantics for C11 concurrency, proved equivalent (in Isabelle/HOL) to the axiomatic model of Batty et al.

An operational semantics for C/C++11 concurrency. Kyndylan Nienhuis, Kayvan Memarian, Peter Sewell. Draft.

People

Contributors:

The current main developers are Kayvan Memarian, Victor Gomes, and Kyndylan Nienhuis. 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.

Funding

This work is funded by REMS: Rigorous Engineering of Mainstream Systems, EPSRC Programme Grant EP/K008528/1, EPSRC grant EP/H005633 (Leadership Fellowship, Sewell), and 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.