Module Ocb_stubblr

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

Utilities

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.

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)