Introduction and Quick Start¶

Installation and Quick Start¶

Install one or more of the following automated theorem provers. In each case, you must ensure that the executable (z3, cvc4, cvc5, alt-ergo) is in your PATH.
- Z3
- CVC4
- cvc5
- Alt-Ergo
Install a Java SDK if you have not already done so. A package manager is the easiest way to do this. We recommend the latest long term support release, but CIVL should work fine with Java 17 or later.
The CIVL releases are published on the GitHub releases page. Choose your desired version; for most users, the latest stable version is the best choice. In any case, this will be a tar gzipped archive with a name of the form civl-x.y.tgz. Unpack this and you should have a directory named civl-x.y. The directory contains jar files, source code, examples, and other resources. You can move it wherever you like.
Use one of the following methods to create the civl executable:
- Method 1 (jlink): Change into the civl-x.y directory and type make. This should result in a directory named civl-runtime which contains a custom JVM optimized for CIVL. You can move civl-runtime wherever you like (or keep it where it is). Add the /path/to/civl-runtime/bin to your PATH, e.g., by adding a line such as export PATH=/path/to/civl-runtime/bin:$PATH to your shell startup file (.zprofile, .bash_profile, etc.). See the comments in the Makefile for more details and options.
- Method 2 (jar): Move the file in civl-x.y/lib named civl-complete.jar to wherever you like (or keep it where it is). Create an executable shell script like the following:
```
#!/bin/sh
java -Xmx16g -jar /path/to/civl-complete.jar $@
```
  Adjust the JVM arguments however you like; in the example above, the maximum heap size is set to 16GB. Move this script into a directory in your PATH.
From the command line, type civl help. You should see a help message describing the command line syntax.
From the command line, type civl config. This should find the provers in your PATH and create a file named .sarl in your home directory.

To test your installation, copy the file examples/concurrency/locksBad.cvl to your working directory. Look at the program: it is a simple 2-process program with two shared variables used as locks. The two processes try to obtain the locks in opposite order, which can lead to a deadlock if both processes obtain their first lock before either obtains the second. Type civl verify locksBad.cvl. You should see some output culminating in a message

The program MAY NOT be correct. See CIVLREP/locksBad_log.txt.

Type civl replay locksBad.cvl. You should see a step-by-step account of how the program arrived at the deadlock.

Verifying CIVL-C Programs¶

Dijkstra’s well-known Dining Philosophers system can be encoded in CIVL-C as follows:

$input int B = 4; // upper bound on number of philosophers
$input int n;     // number of philosophers
$assume(2<=n && n<=B);

_Bool forks[n]; // Each fork will be on the table ($true) or in a hand ($false).

void dine(int id) {
  int left = id;
  int right = (id + 1) % n;
  while (1) {
    $when (forks[left]) forks[left] = $false;
    $when (forks[right]) forks[right] = $false;
    forks[right] = $true;
    forks[left] = $true;
  }
}

void main() {
  $for(int i: 0..n-1) forks[i] = $true;
  $parfor(int i: 0..n-1) dine(i);
}

In this encoding, an upper bound B is placed on the number of philosophers n. When verifying this program, a concrete value will be specified for B. Hence the result of verification will apply to all n between 2 and B, inclusive.

Both B and n are declared as input variables using the type qualifier $input. An input variable may be initialized with any valid value of its type. In contrast, non-input variables declared in file scope will be initialized with a special undefined value; if such a variable is read before it is defined, an error will be reported. In addition, any input variable may have a concrete initial value specified on the command line. In this case, we will specify a concrete value for B on the command line.

An $assume statement restricts the set of executions of the program to include only those traces in which the assumptions hold. In contrast with an $assert statement, CIVL does not check that the assumed expression holds, and will not generate an error message if it fails to hold. Thus an $assume statement allows the programmer to say to CIVL “assume that this is true,” while an $assert statement allows the programmer to say to CIVL “check that this is true.”

A $when statement encodes a guarded command. The $when statement includes a boolean expression called the guard and a statement body. The $when statement is enabled if and only if the guard evaluates to true, in which case the body may be executed. The first atomic statement in the body executes atomically with the evaluation of the guard, so it is guaranteed that the guard will hold when this initial sub-statement executes. Since assignment statements are atomic in CIVL, in this example the body of each $when statement executes atomically with the guard evaluation.

The $for statement is very similar to a for loop. The main difference is that it takes a domain and loops over it.

The $parfor statement is a combination of $for and $spawn. The latter is very similar to a function call. The main difference is that the function called is invoked in a new process which runs concurrently with the existing processes.

The program may be verified for an upper bound of 5 by typing

civl verify -inputB=5 diningBad.cvl

which results in the following output:

CIVL v1.18+ of 2018-12-28 -- http://vsl.cis.udel.edu/civl

Violation 0 encountered at depth 21:
CIVL execution violation (kind: DEADLOCK, certainty: PROVEABLE)
at diningBad.cvl:20.32-32
  $parfor(int i: 0..n-1) dine(i);
                                ^

A deadlock is possible:
  Path condition: true
  Enabling predicate: false
process p0 (id=0): false
process p1 (id=1): false
process p2 (id=2): false

Call stacks:
process 0:
  main at diningBad.cvl:20.32 ";"
process 1:
  dine at diningBad.cvl:12.4-8 "$when" called from
  _par_proc0 at diningBad.cvl:20.25-28 "dine"
process 2:
  dine at diningBad.cvl:12.4-8 "$when" called from
  _par_proc0 at diningBad.cvl:20.25-28 "dine"

Logging new entry 0, writing trace to CIVLREP/diningBad_0.trace
Terminating search after finding 1 violation.

=== Source files ===
diningBad.cvl  (diningBad.cvl)


=== Command ===
civl verify -inputB=5 diningBad.cvl

=== Stats ===
   time (s)            : 1.43
   memory (bytes)      : 163053568
   max process count   : 3
   states              : 32
   states saved        : 26
   state matches       : 1
   transitions         : 30
   trace steps         : 21
   valid calls         : 106
   provers             : cvc4, z3
   prover calls        : 4

=== Result ===
The program MAY NOT be correct.  See CIVLREP/diningBad_log.txt

The output indicates that a deadlock has been found and a counterexample has been produced and saved. We can examine the counterexample, but it is more helpful to work with a minimal counterexample, i.e., a deadlocking trace of minimal length. To find a minimal counterexample, we issue the command

civl verify -inputB=5 -min diningBad.cvl

which results in the output

CIVL v1.18+ of 2018-12-28 -- http://vsl.cis.udel.edu/civl

Violation 0 encountered at depth 21:
CIVL execution violation (kind: DEADLOCK, certainty: PROVEABLE)
at diningBad.cvl:20.32-32
  $parfor(int i: 0..n-1) dine(i);
                                ^

A deadlock is possible:
  Path condition: true
  Enabling predicate: false
process p0 (id=0): false
process p1 (id=1): false
process p2 (id=2): false

Call stacks:
process 0:
  main at diningBad.cvl:20.32 ";"
process 1:
  dine at diningBad.cvl:12.4-8 "$when" called from
  _par_proc0 at diningBad.cvl:20.25-28 "dine"
process 2:
  dine at diningBad.cvl:12.4-8 "$when" called from
  _par_proc0 at diningBad.cvl:20.25-28 "dine"

Logging new entry 0, writing trace to CIVLREP/diningBad_0.trace
Restricting search depth to 20

Violation 1 encountered at depth 16:
CIVL execution violation (kind: DEADLOCK, certainty: PROVEABLE)
at diningBad.cvl:20.32-32
  $parfor(int i: 0..n-1) dine(i);
                                ^

A deadlock is possible:
  Path condition: true
  Enabling predicate: false
process p0 (id=0): false
process p1 (id=1): false
process p2 (id=2): false

Call stacks:
process 0:
  main at diningBad.cvl:20.32 ";"
process 1:
  dine at diningBad.cvl:12.4-8 "$when" called from
  _par_proc0 at diningBad.cvl:20.25-28 "dine"
process 2:
  dine at diningBad.cvl:12.4-8 "$when" called from
  _par_proc0 at diningBad.cvl:20.25-28 "dine"

New log entry is equivalent to previously encountered entry 0
Length of new trace (16) is less than length of old (21): replacing old with new...
Restricting search depth to 15

=== Source files ===
diningBad.cvl  (diningBad.cvl)


=== Command ===
civl verify -inputB=5 -min diningBad.cvl

=== Stats ===
   time (s)            : 1.46
   memory (bytes)      : 163053568
   max process count   : 6
   states              : 96
   states saved        : 77
   state matches       : 2
   transitions         : 91
   trace steps         : 64
   valid calls         : 334
   provers             : cvc4, z3
   prover calls        : 4

=== Result ===
The program MAY NOT be correct.  See CIVLREP/diningBad_log.txt

The output indicates that a minimal counterexample consists of 16 execution steps. It was the second and shortest trace found. It was deemed equivalent to the earlier traces and hence the earlier ones were discarded and only this one saved. We can replay the trace with the command

civl replay -showTransitions diningBad.cvl

which results in the output

CIVL v1.18+ of 2018-12-28 -- http://vsl.cis.udel.edu/civl

Initial state:

State (id=9)
| Path condition
| | true
| Dynamic scopes
| | dyscope d0 (parent=NULL, static=1)
| | | variables
| | | | B = NULL
| | | | n = NULL
| | | | forks = NULL
| Process states
| | process 0
| | | call stack
| | | | Frame[function=main, location=0, diningBad.cvl:1.15 "4", dyscope=d0]

Executed by p0 from State (id=9)
  0->1: B=5 at diningBad.cvl:1.0-15 "$input int B = 4"
  1->2: n=InitialValue(n) [n:=X_n] at diningBad.cvl:2.0-11 "$input int n"
  2->3: $assume((2<=X_n)&&(X_n<=5)) at diningBad.cvl:3.0-20 "$assume(2<=n && n ... )"
  3->4: forks=InitialValue(forks) [forks:=(boolean[X_n]) ($lambda i: int. false)] at diningBad.cvl:5.0-13 "_Bool forks[n]"
--> State (id=18)

Step 1: Executed by p0 from State (id=18)
  4->5: $elaborate_domain(($domain(1))(0..X_n-1#1)) [$assume(0==(X_n - 2))] at diningBad.cvl:19.14-19 "0..n-1"
--> State (id=22)

Step 2: Executed by p0 from State (id=22)
  5->6: LOOP_BODY_ENTER (guard: ($domain(1))(0..1#1) has next for (NULL)) at diningBad.cvl:19.14-19 "0..n-1"
  6->7: NEXT of (NULL) in ($domain(1))(0..1#1) [i:=0] at diningBad.cvl:19.2-5 "$for"
--> State (id=26)

Step 3: Executed by p0 from State (id=26)
  7->5: forks[0]=true at diningBad.cvl:19.22-civlc.cvh:10.14 "forks[i] = 1"
--> State (id=29)

Step 4: Executed by p0 from State (id=29)
  5->6: LOOP_BODY_ENTER (guard: ($domain(1))(0..1#1) has next for (0)) at diningBad.cvl:19.14-19 "0..n-1"
  6->7: NEXT of (0) in ($domain(1))(0..1#1) [i:=1] at diningBad.cvl:19.2-5 "$for"
--> State (id=33)

Step 5: Executed by p0 from State (id=33)
  7->5: forks[1]=true at diningBad.cvl:19.22-civlc.cvh:10.14 "forks[i] = 1"
--> State (id=36)

Step 6: Executed by p0 from State (id=36)
  5->8: LOOP_BODY_EXIT (guard: !($domain(1))(0..1#1) has next for (1)) at diningBad.cvl:19.14-19 "0..n-1"
--> State (id=38)

Step 7: Executed by p0 from State (id=38)
  8->9: $elaborate_domain(($domain(1))(0..1#1)) [$assume(true)] at diningBad.cvl:20.17-22 "0..n-1"
  9->10: $parfor(i0: ($domain(1))(0..1#1)) $spawn _par_proc0(i0) at diningBad.cvl:20.2-8 "$parfor"
  10->11: _civl_ir1=0 at diningBad.cvl:20.32 ";"
--> State (id=52)

Step 8: Executed by p0 from State (id=52)
  11->12: LOOP_BODY_ENTER (guard: 0<2) at diningBad.cvl:20.32 ";"
--> State (id=54)

Step 9: Executed by p1 from State (id=54)
  23->15: dine(0) at diningBad.cvl:20.25-31 "dine(i)"
  15->16: left=0 at diningBad.cvl:8.2-14 "int left = id"
  16->17: right=(0+1)%2 [right:=1] at diningBad.cvl:9.2-25 "int right = (id  ... n"
--> State (id=61)

Step 10: Executed by p1 from State (id=61)
  17->18: LOOP_BODY_ENTER (guard: 1!=0) at diningBad.cvl:10.9 "1"
--> State (id=63)

Step 11: Executed by p2 from State (id=63)
  23->15: dine(1) at diningBad.cvl:20.25-31 "dine(i)"
  15->16: left=1 at diningBad.cvl:8.2-14 "int left = id"
  16->17: right=(1+1)%2 [right:=0] at diningBad.cvl:9.2-25 "int right = (id  ... n"
--> State (id=70)

Step 12: Executed by p2 from State (id=70)
  17->18: LOOP_BODY_ENTER (guard: 1!=0) at diningBad.cvl:10.9 "1"
--> State (id=72)

Step 13: Executed by p1 from State (id=72)
  18->19: forks[0]=false at diningBad.cvl:11.24-civlc.cvh:12.15 "forks[left] = 0"
--> State (id=75)

Step 14: Executed by p2 from State (id=75)
  18->19: forks[1]=false at diningBad.cvl:11.24-civlc.cvh:12.15 "forks[left] = 0"
--> State (id=78)

Step 15:
State (id=78)
| Path condition
| | true
| Dynamic scopes
| | dyscope d0 (parent=NULL, static=1)
| | | variables
| | | | B = 5
| | | | n = 2
| | | | forks = {[0]=false, [1]=false}
| | dyscope d1 (parent=d0, static=4)
| | | variables
| | | | i = 1
| | | | __LiteralDomain_counter0__ = NULL
| | dyscope d2 (parent=d0, static=7)
| | | variables
| | | | _dom_size0 = 2
| | | | _par_procs0 = {[0]=p1, [1]=p2}
| | dyscope d3 (parent=d0, static=8)
| | | variables
| | | | i = 0
| | dyscope d4 (parent=d0, static=8)
| | | variables
| | | | i = 1
| | dyscope d5 (parent=d2, static=9)
| | | variables
| | | | _civl_ir1 = 0
| | dyscope d6 (parent=d5, static=10)
| | | variables
| | dyscope d7 (parent=d0, static=3)
| | | variables
| | | | id = 0
| | dyscope d8 (parent=d7, static=12)
| | | variables
| | | | left = 0
| | | | right = 1
| | dyscope d9 (parent=d0, static=3)
| | | variables
| | | | id = 1
| | dyscope d10 (parent=d9, static=12)
| | | variables
| | | | left = 1
| | | | right = 0
| Process states
| | process 0
| | | call stack
| | | | Frame[function=main, location=12, diningBad.cvl:20.32 ";", dyscope=d6]
| | process 1
| | | call stack
| | | | Frame[function=dine, location=19, diningBad.cvl:12.4-8 "$when", dyscope=d8]
| | | | Frame[function=_par_proc0, location=23, diningBad.cvl:20.25-28 "dine", dyscope=d3]
| | process 2
| | | call stack
| | | | Frame[function=dine, location=19, diningBad.cvl:12.4-8 "$when", dyscope=d10]
| | | | Frame[function=_par_proc0, location=23, diningBad.cvl:20.25-28 "dine", dyscope=d4]

Violation of Deadlock found in (id=78):
A deadlock is possible:
  Path condition: true
  Enabling predicate: false
process p0 (id=0): false
process p1 (id=1): false
process p2 (id=2): false

Trace ends after 15 trace steps.
Violation(s) found.

=== Source files ===
diningBad.cvl  (diningBad.cvl)


=== Command ===
civl replay -showTransitions diningBad.cvl

=== Stats ===
   time (s)            : 1.44
   memory (bytes)      : 163053568
   max process count   : 3
   states              : 27
   valid calls         : 100
   provers             : cvc4, z3
   prover calls        : 4

The output indicates that a deadlock has been found involving 2 philosophers. After the initialization sequence, each philosopher picks up her left fork.

Verifying Sequential C Programs¶

Since almost anything you can do in sequential C is also legal CIVL-C, there is not much you have to do to apply the verifier to C programs.

The verifier requires a complete program — i.e., there must be a main function — and there is usually some set-up that you want to do for CIVL that is different than what you want the program to do in normal use. For this reason, there is a preprocessor object-like macro _CIVL which is defined when using the CIVL verifier. This allows you to insert some CIVL-C code that will be used for verification, without interfering with the normal compilation and use of the program. Consider the following example, sum.c:

#include <assert.h>
#include <stdio.h>
#ifdef _CIVL
#include <civlc.cvh>
$input int B=5, N;
$assume(1<=N && N<=B);
#else
#define N 100
#endif
int sum=0;
int main() {
  for (int i = 1; i <= N; i++) sum += i;
  printf("N=%d, sum = %d\n", N, sum);
  assert(sum == (N+1)*N/2);
}

The program can be compiled and executed as usual…

$ cc -o sum sum.c
$ ./sum
N=100, sum = 5050
$

…and it can be verified using CIVL:

$ civl verify sum.c
CIVL v1.18+ of 2018-12-28 -- http://vsl.cis.udel.edu/civl
N=5, sum = 15
N=4, sum = 10
N=3, sum = 6
N=2, sum = 3
N=1, sum = 1

=== Source files ===
sum.c  (sum.c)


=== Command ===
civl verify sum.c

=== Stats ===
   time (s)            : 2.26
   memory (bytes)      : 163053568
   max process count   : 1
   states              : 51
   states saved        : 31
   state matches       : 0
   transitions         : 50
   trace steps         : 16
   valid calls         : 54
   provers             : cvc4, z3
   prover calls        : 13

=== Result ===
The standard properties hold for all executions.
$

Another approach for separating the CIVL driver code from the “real” program is to place these in separate translation units. In the following example, a toy library “sumlib” has been implemented using a header file sumlib.h and an implementation sumlib.c:

sumlib.h

int sum(int n);

sumlib.c

#include "sumlib.h"
int sum(int n) {
  int result = 0;
  for (int i=1; i<=n; i++) result += i;
  return result;
}

A simple test has been implemented in a separate translation unit named sumlib_test.c. The translation units can be compiled, linked, and executed, in the usual way.

sumlib_test.c

#include <stdio.h>
#include <assert.h>
#include "sumlib.h"
#define N 100
int main() {
  int result = sum(N);
  printf("N=%d, sum = %d\n", N, result);
  assert(result == (N+1)*N/2);
}

$ cc sumlib_test.c sumlib.c
$ ./a.out
N=100, sum = 5050

Finally, a CIVL verification driver is provided in another translation unit, sumlib_driver.cvl. The CIVL verifier can be applied to the whole program composed of the two translation units sumlib_driver.cvl and sumlib.c:

sumlib_driver.cvl

#include <stdio.h>
#include "sumlib.h"
$input int B=5, N;
$assume(1<=N && N<=B);
int main() {
  int result = sum(N);
  printf("N=%d, sum = %d\n", N, result);
  $assert(result == (N+1)*N/2);
}

$ civl verify sumlib_driver.cvl sumlib.c
CIVL v1.18+ of 2018-12-28 -- http://vsl.cis.udel.edu/civl
N=5, sum = 15
N=4, sum = 10
N=3, sum = 6
N=2, sum = 3
N=1, sum = 1

=== Source files ===
sumlib_driver.cvl  (sumlib_driver.cvl)
sumlib.h  (sumlib.h)
sumlib.c  (sumlib.c)


=== Command ===
civl verify sumlib_driver.cvl sumlib.c

=== Stats ===
   time (s)            : 2.78
   memory (bytes)      : 163053568
   max process count   : 1
   states              : 47
   states saved        : 32
   state matches       : 0
   transitions         : 46
   trace steps         : 16
   valid calls         : 49
   provers             : cvc4, z3
   prover calls        : 13

=== Result ===
The standard properties hold for all executions.

There are limitations to the application of CIVL to C programs. Support for the standard library is only partial. Small bounds will have to be placed on many parameters in order for CIVL verification to terminate (or terminate in a reasonable amount of time).

Verifying C/MPI Programs¶

CIVL can verify C/MPI programs that use a subset of MPI. The instructions for sequential programs apply equally to MPI programs. In addition, one must specify either (1) the number of processes for the MPI program, or (2) an upper and a lower bound on the number of processes for the MPI program.

In the following example, the C/MPI program ring.c is verified for exactly 5 processes:

civl verify -input_mpi_nprocs=5 ring.c

In the following example, ring.c is verified for any number of processes between 2 and 5, inclusive:

civl verify -input_mpi_nprocs_lo=2 -input_mpi_nprocs_hi=5 ring.c

Verifying C/OpenMP Programs¶

CIVL uses an input variable omp_thread_max for verifying OpenMP programs. It must be specified on the command line, e.g.,

civl verify -input_omp_thread_max=3 sum_omp.c

Upon entering an OpenMP parallel region, CIVL will nondeterministically choose an integer between 1 and omp_thread_max, and create a thread team consisting of that number of threads. If omp_thread_max is not specified, then the program must explicitly specify the number of threads for each parallel region.

By default, CIVL attempts to simplify an OpenMP program by replacing parallel code with sequential code when it can determine that the two are equivalent. In the best case, this can remove all of the OpenMP, resulting in a sequential program. The option -ompNoSimplify can be used to disable such simplification. Another option, -ompLoopDecomp=X can be used to specify the loop decomposition strategy, where X is one ALL (the default), ROUND_ROBIN, or RANDOM.