Build backend

Uses a list of [[build_target_ptr]] and a valid [[fpm_model]] instance to schedule and execute the compilation and linking of package targets.

The package build process ([[build_package]]) comprises three steps:

1. Target sorting: topological sort of the target dependency graph ([[sort_target]])
2. Target scheduling: group targets into schedule regions based on the sorting ([[schedule_targets]])
3. Target building: generate targets by compilation or linking

@note Note If compiled with OpenMP, targets will be build in parallel where possible.

Incremental compilation

The backend process supports incremental compilation whereby targets are not re-compiled if their corresponding dependencies have not been modified.

• Source-based targets (i.e. objects) are not re-compiled if the corresponding source file is unmodified AND all of the target dependencies are not marked for re-compilation

• Link targets (i.e. executables and libraries) are not re-compiled if the target output file already exists AND all of the target dependencies are not marked for re-compilation

Source file modification is determined by a file digest (hash) which is calculated during the source parsing phase (fpm_source_parsing) and cached to disk after a target is successfully generated.