package b0

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B0 driver development manual

A B0 driver like b0 or d0 is an executable that provides a service on the definitions of a B0 file. This manual shows how to develop a custom driver for B0 files.


The B0 file needs to be compiled and made accessible to the driver executable. At the moment the OCaml Dynlink API is not being used. Instead the driver executable relinks its objects with a compilation of the B0 file to produce another executable that is executed to run the driver on the definitions.

With the help of the B0_driver module this all happens transparently in the .drivers directory of the _b0 directory.

The driver library

For your driver to access a B0 file you need to create an OCaml library which has all the objects of your driver and registers its main function by calling B0_driver.set.

Source wise a typical driver source structure is:


The file implements and registers the driver. It's compiled object should be part of your driver library. Here's a minimal example :

open B0_std
open Cmdliner

let driver =
  let name = "mydriver" and version = "v0.0.5" in
  let libs = ["mydriver" (* other needed libraries can be added here *) ] in
  B0_driver.create ~name ~version ~libs

let my_driver conf = "Running %s!@."; B0_driver.Exit.ok
let my_driver =
  let doc = "My driver" in
  let sdocs = Manpage.s_common_options in
  let exits = B0_driver.Exit.Info.base_cmd in
  let man = [ `S Manpage.s_description; "$(mname) does not much." ] in
  B0_driver.with_b0_file ~driver (Term.const unit_cmd), "mydriver" ~version:"v0.0.5" ~doc ~sdocs ~exits ~man

let main () = Term.eval mydriver
let () = B0_driver.set driver ~main

The file defines your driver executable when it has no B0 file linked in. It's the program that runs your driver without the B0 file linked in. It should simply be:

let () =
  let module D = Mydriver_main (* make sure we link it *) in
  if !Sys.interactive then () else ~has_b0_file:false

A few things to note:

  • Your driver library must be installed and available in the OCAMLPATH under the name you gave in to B0_driver.
  • The driver library's directory is not added to the includes for compiling the B0 file. This prevents drivers from adding declarations to the compilation environment since using them in a B0 file would then break other drivers. New declarations must always be explicitely imported in the B0 file by using the #require directive.


The driver dance seems to be a bit slow for now. Even for the up-to-date dance we get into the 43ms b0 unit list. But there's room for improvement. Here are different things to consider that could be done.

  • Cold driver compilation. I think we get hit a bit by the executable link phase (globally 500ms, but the compilation spawn takes 480ms). Investigate what makes OCaml linking faster. Maybe a mono cmo ? Alternatively consider using Dylink once it gets lib support, but then what about platforms that don't support it.
  • The various timing obtained with b0 file log --stats and b0 -v -v don't add up to what time b0 reports. Investigate, is it the execv ? June 2020: maybe it is/was that. Also see how much toplevel init takes (how ?).
  • Try to jump directly into the compiled driver executable and let it auto verify and auto recompile if needed ? This could allow to share part of the hash cache in the future.
  • Maybe go back to use an ad-hoc stamp for driver compilation rather than use B00 itself.

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