In this section, we will explain how define and use instrumentation backends
landmarks) so that you can enable and disable
dune-workspace files or by passing a command-line flag or
environment variable. In addition to providing an easy way to toggle
instrumentation of your code, this setup avoids creating a hard dependency on
the precise instrumentation backend in your project.
Specifying what to instrument¶
When an instrumentation backend is activated, Dune will only instrument libraries and executables for which the user has requested instrumentation.
To request instrumentation, one must add the following field to a library or executable stanza:
(library (name ...) (instrumentation (backend <name> <args>)))
<name> can be passed arguments using
This field can be repeated multiple times in order to support various backends. For instance:
(library (name foo) (modules foo) (instrumentation (backend bisect_ppx --bisect-silent yes)) (instrumentation (backend landmarks)))
This will instruct Dune that when either the
instrumentation is activated, the library should be instrumented with this
By default, these fields are simply ignored. However, when the corresponding
instrumentation backend is activated, Dune will implicitly add the relevant
rewriter to the list of
At the moment, it is not possible to instrument code that is preprocessed via an action preprocessors. As these preprocessors are quite rare nowadays, there is no plan to add support for them in the future.
Activating an instrumentation backend can be done via the command line or the
Via the command line, it is done as follows:
$ dune build --instrument-with <names>
<names> is a comma-separated list of instrumentation backends. For example:
$ dune build --instrument-with bisect_ppx,landmarks
This will instruct Dune to activate the given backend globally, i.e. in all defined build contexts.
It is also possible to enable instrumentation backends via the
dune-workspace file, either globally, or for specific builds contexts.
To enable an instrumentation backend globally, you can type in your
(lang dune 2.8) (instrument_with bisect_ppx)
or for each context individually:
(lang dune 2.8) (context default) (context (default (name coverage) (instrument_with bisect_ppx))) (context (default (name profiling) (instrument_with landmarks)))
If both the global and local fields are present, the precedence is the same as
profile field: the per-context setting takes precedence over the
command-line flag, which takes precedence over the global field.
Declaring an instrumentation backend¶
Instrumentation backends are libraries with the special field
(instrumentation.backend). This field instructs Dune that the library can be
used as an intrumentation backend and also provides the parameters that are
specific to this backend.
Currently, Dune will only support
ppx instrumentation tools, and the
instrumentation library must specify the
ppx rewriters that instruments the
code. This can be done as follows:
(library ... (instrumentation.backend (ppx <ppx-rewriter-name>)))
When such an instrumentation backend is activated, Dune will implicitly add the
ppx rewriter to the list of
ppx rewriters for libraries and
executables that specify this instrumentation backend.