Build Backends§

bfg9000 supports generating files for multiple build tools, called "backends". These are specified in the bfg9000.backends entry point; third parties can add support for their own backend by hooking into this.

How a build.bfg file is compiled§

Before we get started looking at how each bakend works, let's quickly go over how a build.bfg file is compiled at a high level. At its core, a build file is just a DAG (directed acyclic graph) that gets walked through by the build system, where the nodes are files and the edges are build steps. A bfg9000 build file is no different.

Snapshot the environment§

When bfg9000 is invoked, it first takes a snapshot of the current environment (the operating system, environment variables, compiler to use, etc). This is important to provide a stable state for regeneration of the build file if necessary (e.g. if build.bfg is changed).

Build an internal dependency graph§

Next, it executes the build.bfg file. Most bfg9000 functions represent build steps or other related parts of the dependency graph. When called, these build up an internal DAG structure with all the backend-agnostic data filled in (e.g. virtual filenames are resolved to real ones, such as "foo" to "libfoo.so").

Emit the final build file§

Once this is complete, the DAG is passed to the appropriate backend, which iterates over all the known edges (build steps) and emits the backend-specific code for them. Since all the backends handle walking the DAG on their own, bfg9000 can safely avoid worrying about trying to do this efficiently in Python.

Public API for backends§

backend.write(env, build_inputs)§

Called when bfg9000 has finished building its internal dependency graph and it's time to generate the backend-specific build files. env is an Environment object and build_inputs is a BuildInputs object containing the internal dependency graph. build_inputs.edges() returns a list of Edges that correspond to each build step defined in the build.bfg file.

backend.priority§

The priority of this build backend. This helps determine the default backend. The default is the backend with the highest priority that's also "valid" (i.e. backend.version() returns a non-None value).

backend.version()§

Return the version (as a verspec.loose.Version object -- also aliased as bfg9000.versioning.Version) of this build backend's underlying tool. If the tool can't be found (or is otherwise broken), this returns None.