Alternative Provers

Installed with SPARK Pro

The provers Alt-Ergo, CVC4 and Z3 are installed with the SPARK tool. By default, GNATprove uses prover CVC4 only. Switch --level changes the default to use one or more provers depending on the chosen level (see Running GNATprove from the Command Line). Switch --prover allows to use another prover, or a list of provers. Prover names altergo, cvc4 and z3 are used to refer to the versions of provers Alt-Ergo, CVC4 and Z3 that are installed with the SPARK toolset. The string alt-ergo can also be used to refer to Alt-Ergo. More information on Alt-Ergo, CVC4 and Z3 can be found on their respective websites:

Installed with SPARK Discovery

In this case, only prover Alt-Ergo is installed with the SPARK tool. Hence, by default GNATprove only uses prover Alt-Ergo. In particular, switch --level has no impact on the use of different provers. If provers CVC4 and Z3 are separately installed by the user and available on the execution path, then GNATprove will use them as documented in this User’s Guide. In particular, CVC4 will then become the default prover instead of Alt-Ergo. Sources and binaries for provers CVC4 and Z3 can be found on their respective websites:

Installing CVC4 and Z3 for SPARK Discovery

  • Download the latest version of CVC4 or Z3 from the websites specified above.
  • Rename the executable file of the download to cvc4 or z3 (with .exe suffix on Windows). In the case of CVC4, you can simply rename the file you downloaded to cvc4 (or cvc4.exe on Windows). In the case of Z3, you need to unzip the downloaded file. The executable file is in bin/z3.
  • Add the location of the executable file to your PATH variable.


If you are using SPARK Discovery GPL 2017, a known problem with CVC4 is that its options have changed. In order to use a current unstable CVC4 build, you should modify the file <spark-install>/share/spark/config/why3.conf to remove --boolean-term-conversion-mode=native at the 4 places where it occurs.

Other Automatic or Manual Provers

Updating the Why3 Configuration File

GNATprove can call other provers, as long as they are supported by the Why3 platform (see complete list on Why3 webpage). To use another prover, it must be listed in your Why3 configuration file.

To create or update automatically a Why3 configuration file, call the command <spark2014-install>/libexec/spark/bin/why3config --detect-provers. It searches your PATH for any supported provers and adds them to the default configuration file .why3.conf in your HOME, or a configuration file given in argument with switch -C <file>. This file consists of a few general settings and a section for each prover which is supported.

GNATprove reads the default configuration file .why3.conf in your HOME, or the configuration file given in argument with switch --why3-conf=<file>. Any prover name configured in this configuration file can be used as an argument to switch --prover.

Note that using this mechanism, you cannot replace the definitions provided with the SPARK tools for the provers altergo, cvc4 and z3.

If more than one prover is specified, the provers are tried in order on each VC, until one of them succeeds or all fail. Interactive provers cannot be combined with other provers, so must appear on their own.

Sharing Libraries of Theorems

When GNATprove is used with a manual prover, the user can provide libraries of theorems to use during the proof process.

To do so, the user will need to set a proof directory (see Project Attributes for more details on this directory). The user needs to create a folder with the same name as the chosen manual prover (the casing of the name is the same as the one passed to the switch --prover) and put the library sources inside this folder.

Finally, some additional fields need to be added to the prover configuration in the Why3 configuration file (a basic example of prover configuration can be found in the section on Coq):

  • configure_build: this field allows you to specify a command to configure the compilation of the library of theorems. This command will be called each time a source file is added to the library.
  • build_commands: this field allows you to specify a set of command which will be called sequentially to build your library. These commands will be called each time GNATprove runs the corresponding manual prover. (In order to define multiple commands for this field, just set the field multiple times with different values, each time the field is set it adds a new element to the set of build_commands).

Inside these commands, pattern %f refers to the name of the library file considered, and %o to the name of the main gnatprove repository generated by GNATprove. This allows referring to the path of the compiled library of theorems inside these commands with %o/user/<prover_name>.


gnatprove has support for the Coq interactive prover, even though Coq is not part of the SPARK distribution. If you want to use Coq with SPARK, you need to install it yourself on your system and put it in your PATH environment variable. Then, you can simply provide --prover=coq to gnatprove. Note that the only supported version currently is Coq 8.5.