package ocb-stubblr

  1. Overview
  2. Docs

OCamlbuild plugin for C stubs

See the Intro, Interface or Examples.

Intro

ocb-stubblr helps dealing with C libraries that are built as part of an OCaml project. It is especially useful for libraries of OCaml primitives (stubs).

Most of the plugin consists of new tags that can be applied to files, and new rules that are activated when OCamlbuild tries to build certain targets. In order to enable these, init needs to be called from Ocamlbuild_plugin.dispatch:

let () = Ocamlbuild_plugin.dispatch Ocb_stubblr.init

The plugin helps with three aspects of building C stubs:

1. Using .clib files

stubblr modifies .clib build rules to automatically search for, and require, project-local headers that the C source files #include.

Furthermore, it provides the tag link_stubs(). This tag acts on OCaml archives, and records the link flags needed for linking with the given C library. The parameter is assumed to be a .clib file in the same project.

For example, adding

<foo.cm{,x}a>: link_stubs(path/libbar)

records the link flag -lbar in foo.cm{,x}a. Assuming that the C libraries described by path/libbar.clib are installed, this causes any final executables that use the archive foo.cmxa (resp. foo.cma) to link to libbar.a (resp dllbar.so). This is useful if Foo provides the interface to the C primitives in libbar.

Another feature is the automatic addition of use_<lib> tags for every <lib>.mllib. OCaml sources tagged with this tag are built against the archive <lib>.cm{,x}a, instead of using its constituent cm{o,x} through include tags. This means that the in-tree executables inherit the link flags (as introduced, for example, above), and are correctly linked against the in-tree C libraries.

Finally, the tags ccopt and cclib, which were introduced in OCamlbuild 0.9.3, are added if the plugin is used with an older version. This allows setting these options directly from _tags with any OCamlbuild version.

2. pkg-config

stubblr provides the tag pkg-config().

Tagging objects with pkg-config(package) will query pkg-config for the package, and:

  1. add the C flags (pkg-config --cflags) to the compilation of tagged C sources;
  2. add the link flags (pkg-config --libs) to the linking step of tagged C libraries; and
  3. record those link flags in the tagged OCaml archives.

For example

<src/*.{c,cma,cmxa}>: pkg-config(sdl2)

will add the flags needed to compile against SDL2 when compiling C sources, and record the flags needed to link the final executables against libSDL2.so to the native and bytecode archives.

The full syntax of the tag is pkg-config(package[ relax][ <params>]) where <params> is a space-separated combination of any of: cflags, libs, static.

relax will ignore the package if it is not found. Otherwise, the build will abort.

cflags will query --cflags.

libs will query --libs.

static will query --libs --static (and has precedence over libs).

If none of cflags, libs and static are present, cflags libs is assumed. Thus pkg-config(pkg relax) will query --cflags and --libs, and ignore the error if pkg is not installed; pkg-config(pkg cflags) will query only the --cflags; while pkg-config(pkg static) will query only --libs --static.

Note .pc files in the current Opam switch take precedence; see Pkg_config.

3. Multi-lib

Sometimes it can be desirable to compile a C library in several ways, e.g. with different compilation options, and install all of the versions.

For any file path/libstub.clib, stubblr introduces the rules to build X/<TARGET>/path/libstub+<TARGET>.clib, where <TARGET> is an arbitrary name. The new library is built from the same sources, but it doesn't inherit any of the tags directly applied to the original .clib and its products. Instead, the files X/<TARGET>/**/* can be marked with a separate set of tags, causing them to be compiled and/or linked with different options.

As a special case, there are pre-defined targets for different MirageOS runtimes (currently mirage-xen and mirage-freestanding). These are automatically tagged with the required compilation options.

Note If your paths already contain +, OCamlbuild solver is likely to get confused. Assume that the meaning of + in paths has been hijacked by ocb-stubblr. The new semantics of + is accessible only through transcendental hermenautics.

Interface

type ocb_hook = Ocamlbuild_plugin.hook -> unit
val init : ?incdirs:bool -> ?mllibs:path list -> ocb_hook

init ?incdirs ?paths initializes the plugin.

incdirs causes include_include_dirs to be called on initialisation. Defaults to true.

mllibs are passed to ocaml_libs, to detect any <lib>.mllib files and enable their corresponding use_<lib> tags. Use [] to disable. Defaults to ["."].

Utilities

val ocaml_libs : ?mllibs:path list -> ocb_hook

ocaml_libs ~mllibs calls Ocamlbuild_plugin.ocaml_lib on every .mllib found in mllibs. It's a shortcut to enable use_<lib> tag for every <lib>.mllib in the project.

mllibs is a list of files or directories. Directories in the list are searched recursively. mllibs defaults to ["."].

val include_include_dirs : ocb_hook

include_include_dirs will add -I dir when linking OCaml programs and cmxs for every dir marked as include.

val ccopt : ?tags:string list -> string -> ocb_hook

ccopt tags options adds -ccopt options when compiling the C sources tagged with ~tags.

tags defaults to [].

val cclib : ?tags:string list -> string -> ocb_hook

cclib tags options adds -cclib options when linking the C libraries tagged with ~tags.

tags defaults to [].

val ldopt : ?tags:string list -> string -> ocb_hook

ldopt tags options adds -ldopt options when linking the C libraries tagged with ~tags.

tags defaults to [].

val after_rules : (unit -> unit) -> ocb_hook

after_rules f is function After_rules -> f () | _ -> ().

val dispatchv : ocb_hook list -> unit

dispatchv hooks is a shortcut for registering several ocb_hooks.

It is equivalent to Ocamlbuild_plugin.dispatch hookf where hookf is a function that applies each hook from hooks in order.

val (&) : ocb_hook -> ocb_hook -> ocb_hook

h1 & h2 is a hook combining h1 and h2.

module Pkg_config : sig ... end

Query pkg-config.

OS and machine detection

These utilities are included because it is sometimes necessary to change options for building C libraries depending on the host OS and architecture.

type os = [
  1. | `Linux
  2. | `Hurd
  3. | `Darwin
  4. | `FreeBSD
  5. | `OpenBSD
  6. | `NetBSD
  7. | `DragonFly
  8. | `KFreeBSD
  9. | `Haiku
  10. | `HP_UX
  11. | `AIX
  12. | `Interix
  13. | `Minix
  14. | `QNX
  15. | `SunOS
  16. | `Cygwin of string
  17. | `Mingw of string
  18. | `Uwin of string
  19. | `UNKNOWN of string
]

A selection of popular operating systems.

type machine = [
  1. | `x86_64
  2. | `x86
  3. | `ARMv6
  4. | `ARMv7
  5. | `UNKNOWN of string
]

A selection of machine architectures supported by OCaml.

val os : unit -> os

os () is the normalized result of uname -s.

val machine : unit -> machine

machine () is the normalized result of uname -m.

Examples

Assume a project laid out like the following:

Project dir:

./myocamlbuild.ml
./_tags
./src/foo.ml
./src/stubs.c
./src/extra/defs.h
./src/libstubs.clib
./src/foo.mllib
./exe/demo.ml

The content of src/foo.mllib:

Foo

The content of src/libstubs.clib:

stubs.o

The content of _tags:

<src>: include

Basic integration

Initialize the plugin from myocamlbuild.ml:

let () = Ocamlbuild_plugin.dispatch Ocb_stubblr.init

The file src/extra/defs.h will be automatically used when compiling src/stubs.c, if needed.

Adding the tag

<src/*.cm{,x}a>: link_stubs(src/libstubs)

will record the link flag -lstubs in foo.cm{,x}a, causing executables that use them to link against libstubs.a/dllstubs.so.

pkg-config

Adding the tag

<src/*.{c,cma,cmxa}>: pkg-config(sdl2 relax)

will cause stubs.c to be compiled with C flags from sdl2.pc, and foo.cm{,x}a to record the link flags.

If SDL2 is not installed, relax will cause it to be ignored.

In-tree executables

To build demo.native and/or demo.byte, _tags needs to contain

<exe/*>: use_foo

causing demo.{native,byte} to build against foo.cm{,x}a and inherit its link flags. The archive, in turn, contains flags for linking to the stub library (link_stubs()), and linking to SDL2 (pkg-config()).

Multi-lib

Invoking ocamlbuild X/fnord/src/libstubs+fnord.a will build libstubs+fnord.a.

If _tags contains

<src/*.c>: ccopt(-flub)
<X/fnord/**/*.c>: ccopt(-DA)

then libstubs+fnord.a will not be compiled with -flub. Instead, it will be compiled with the pre-processor symbol A defined.

Mirage

If using Topkg, register the .clib file using Ocb_stubblr_topkg.mirage.

Pkg.describe ... @ fun c ->
...
Ok [ Pkg.clib "path/to/libstubs.clib";
     Ocb_stubblr_topkg.mirage "path/to/libstubs.clib"]

Otherwise, arrange for building and installation of X/<TARGET>/path/to/libstubs+<TARGET>.a for all MirageOS TARGETs.

Use of these alternate archives is a matter of MirageOS.

Composition

let myhook = function
| After_rules -> ...
| ...
let () = Ocb_stubblr.(dispatchv [init; myhook])
let () = dispatch Ocb_stubblr.(init & myhook)
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