4 Copyright © 2013 Regents of the University of California. All rights reserved.
6 CDSChecker is distributed under the GPL v2. See the LICENSE file for details.
12 CDSChecker is a model checker for C11/C++11 which exhaustively explores the
13 behaviors of code under the C/C++ memory model. It uses partial order reduction
14 as well as a few other novel techniques to eliminate time spent on redundant
15 execution behaviors and to significantly shrink the state space. The model
16 checking algorithm is described in more detail in this paper (published in
19 <http://demsky.eecs.uci.edu/publications/c11modelcheck.pdf>
21 It is designed to support unit tests on concurrent data structure written using
24 CDSChecker is constructed as a dynamically-linked shared library which
25 implements the C and C++ atomic types and portions of the other thread-support
26 libraries of C/C++ (e.g., std::atomic, std::mutex, etc.). Notably, we only
27 support the C version of threads (i.e., `thrd_t` and similar, from `<threads.h>`),
28 because C++ threads require features which are only available to a C++11
29 compiler (and we want to support others, at least for now).
31 CDSChecker should compile on Linux and Mac OSX with no dependencies and has been
32 tested with LLVM (clang/clang++) and GCC. It likely can be ported to other \*NIX
33 flavors. We have not attempted to port to Windows.
35 You may also refer to the CDSChecker project page:
37 <http://demsky.eecs.uci.edu/c11modelchecker.php>
43 If you haven't done so already, you may download CDSChecker using
44 [git](http://git-scm.com/):
46 git clone git://demsky.eecs.uci.edu/model-checker.git
48 Get the benchmarks (not required; distributed separately):
51 git clone git://demsky.eecs.uci.edu/model-checker-benchmarks.git benchmarks
53 Compile the model checker:
57 Compile the benchmarks:
61 Run a simple example (the `run.sh` script does some very minimal processing for
64 ./run.sh test/userprog.o
66 To see the help message on how to run CDSChecker, execute:
76 > Controls the liveness of the memory system. Note that multithreaded programs
77 > often rely on memory liveness for termination, so this parameter is
78 > necessary for such programs.
80 > Liveness is controlled by `num`: the number of times a load is allowed to
81 > see the same store when a newer store exists---one that is ordered later in
82 > the modification order.
86 > Turns on CHESS-like yield-based fairness support (requires `thrd_yield()`
87 > instrumentation in test program).
91 > Turns on alternative fairness support (less desirable than `-y`).
95 > Verbose: show all executions and not just buggy ones.
99 > Constrain how long we will run to wait for a future value past when it is
104 > Value to provide to atomics loads from uninitialized memory locations. The
105 > default is 0, but this may cause some programs to throw exceptions
106 > (segfault) before the model checker prints a trace.
120 Many simple tests are located in the `tests/` directory. You may also want to
121 try the larger benchmarks (distributed separately), which can be placed under
122 the `benchmarks/` directory. After building CDSChecker, you can build and run
123 the benchmarks as follows:
127 > ./run.sh barrier/barrier -y -m 2 # runs barrier test with fairness/memory liveness
128 > ./bench.sh # run all benchmarks and provide timing results
131 Running your own code
132 ---------------------
134 You likely want to test your own code, not just our simple tests. To do so, you
135 need to perform a few steps.
137 First, because CDSChecker executes your program dozens (if not hundreds or
138 thousands) of times, you will have the most success if your code is written as a
139 unit test and not as a full-blown program.
141 Second, because CDSChecker must be able to manage your program for you, your
142 program should declare its main entry point as `user_main(int, char**)` rather
143 than `main(int, char**)`.
145 Third, test programs should use the standard C11/C++11 library headers
146 (`<atomic>`/`<stdatomic.h>`, `<mutex>`, `<condition_variable>`, `<thread.h>`).
147 As of now, we only support C11 thread syntax (`thrd_t`, etc. from
150 Test programs may also use our included happens-before race detector by
151 including <librace.h> and utilizing the appropriate functions
152 (`store_{8,16,32,64}()` and `load_{8,16,32,64}()`) for loading/storing data from/to
153 non-atomic shared memory.
155 CDSChecker can also check boolean assertions in your test programs. Just
156 include `<model-assert.h>` and use the `MODEL_ASSERT()` macro in your test program.
157 CDSChecker will report a bug in any possible execution in which the argument to
158 `MODEL_ASSERT()` evaluates to false (that is, 0).
160 Test programs should be compiled against our shared library (libmodel.so) using
161 the headers in the `include/` directory. Then the shared library must be made
162 available to the dynamic linker, using the `LD_LIBRARY_PATH` environment
163 variable, for instance.
166 Reading an execution trace
167 --------------------------
169 When CDSChecker detects a bug in your program (or when run with the `--verbose`
170 flag), it prints the output of the program run (STDOUT) along with some summary
171 trace information for the execution in question. The trace is given as a
172 sequence of lines, where each line represents an operation in the execution
173 trace. These lines are ordered by the order in which they were run by CDSChecker
174 (i.e., the "execution order"), which does not necessarily align with the "order"
175 of the values observed (i.e., the modification order or the reads-from
178 The following list describes each of the columns in the execution trace output:
180 * \#: The sequence number within the execution. That is, sequence number "9"
181 means the operation was the 9th operation executed by CDSChecker. Note that
182 this represents the execution order, not necessarily any other order (e.g.,
183 modification order or reads-from).
185 * t: The thread number
187 * Action type: The type of operation performed
189 * MO: The memory-order for this operation (i.e., `memory_order_XXX`, where `XXX` is
190 `relaxed`, `release`, `acquire`, `rel_acq`, or `seq_cst`)
192 * Location: The memory location on which this operation is operating. This is
193 well-defined for atomic write/read/RMW, but other operations are subject to
194 CDSChecker implementation details.
196 * Value: For reads/writes/RMW, the value returned by the operation. Note that
197 for RMW, this is the value that is *read*, not the value that was *written*.
198 For other operations, 'value' may have some CDSChecker-internal meaning, or
199 it may simply be a don't-care (such as `0xdeadbeef`).
201 * Rf: For reads, the sequence number of the operation from which it reads.
202 [Note: If the execution is a partial, infeasible trace (labeled INFEASIBLE),
203 as printed during `--verbose` execution, reads may not be resolved and so may
204 have Rf=? or Rf=Px, where x is a promised future value.]
206 * CV: The clock vector, encapsulating the happens-before relation (see our
207 paper, or the C/C++ memory model itself). We use a Lamport-style clock vector
208 similar to [1]. The "clock" is just the sequence number (#). The clock vector
209 can be read as follows:
211 Each entry is indexed as CV[i], where
213 i = 0, 1, 2, ..., <number of threads>
215 So for any thread i, we say CV[i] is the sequence number of the most recent
216 operation in thread i such that operation i happens-before this operation.
217 Notably, thread 0 is reserved as a dummy thread for certain CDSChecker
220 See the following example trace:
223 ------------------------------------------------------------------------------------
224 # t Action type MO Location Value Rf CV
225 ------------------------------------------------------------------------------------
226 1 1 thread start seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 1)
227 2 1 init atomic relaxed 0x601068 0 ( 0, 2)
228 3 1 init atomic relaxed 0x60106c 0 ( 0, 3)
229 4 1 thread create seq_cst 0x7f68fe51c710 0x7f68fe51c6e0 ( 0, 4)
230 5 2 thread start seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 5)
231 6 2 atomic read relaxed 0x60106c 0 3 ( 0, 4, 6)
232 7 1 thread create seq_cst 0x7f68fe51c720 0x7f68fe51c6e0 ( 0, 7)
233 8 3 thread start seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 8)
234 9 2 atomic write relaxed 0x601068 0 ( 0, 4, 9)
235 10 3 atomic read relaxed 0x601068 0 2 ( 0, 7, 0, 10)
236 11 2 thread finish seq_cst 0x7f68ff11ebc0 0xdeadbeef ( 0, 4, 11)
237 12 3 atomic write relaxed 0x60106c 0x2a ( 0, 7, 0, 12)
238 13 1 thread join seq_cst 0x7f68ff11ebc0 0x2 ( 0, 13, 11)
239 14 3 thread finish seq_cst 0x7f68ff11efc0 0xdeadbeef ( 0, 7, 0, 14)
240 15 1 thread join seq_cst 0x7f68ff11efc0 0x3 ( 0, 15, 11, 14)
241 16 1 thread finish seq_cst 0x7f68ff11e7c0 0xdeadbeef ( 0, 16, 11, 14)
243 ------------------------------------------------------------------------------------
246 Now consider, for example, operation 10:
248 This is the 10th operation in the execution order. It is an atomic read-relaxed
249 operation performed by thread 3 at memory address `0x601068`. It reads the value
250 "0", which was written by the 2nd operation in the execution order. Its clock
251 vector consists of the following values:
253 CV[0] = 0, CV[1] = 7, CV[2] = 0, CV[3] = 10
259 * Deadlock detection: CDSChecker can detect deadlocks. For instance, try the
260 following test program.
262 > ./run.sh test/deadlock.o
264 Deadlock detection currently detects when a thread is about to step into a
265 deadlock, without actually including the final step in the trace. But you can
266 examine the program to see the next step.
268 * CDSChecker has to speculatively explore many execution behaviors due to the
269 relaxed memory model, and many of these turn out to be infeasible (that is,
270 they cannot be legally produced by the memory model). CDSChecker discards
271 these executions as soon as it identifies them (see the "Number of infeasible
272 executions" statistic); however, the speculation can occasionally cause
273 CDSChecker to hit unexpected parts of the unit test program (causing a
274 division by 0, for instance). In such programs, you might consider running
275 CDSChecker with the `-u num` option.
281 Please feel free to contact us for more information. Bug reports are welcome,
282 and we are happy to hear from our users. We are also very interested to know if
283 CDSChecker catches bugs in your programs.
285 Contact Brian Norris at <banorris@uci.edu> or Brian Demsky at <bdemsky@uci.edu>.
291 [1] L. Lamport. Time, clocks, and the ordering of events in a distributed
292 system. CACM, 21(7):558-565, July 1978.