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buildpy2 Documentation

[for version 1.1.0]

How it works

buildpy2 generates files based on rules. Each rule has:

  • A set of files that it is capable of creating
  • A set of prerequisite files that are required in order to create a given file
  • A function to create ('build') the file in question
  • A 'phony' flag, which tells the system to always run the build function regardless of the state of the file or its prerequisites.

To build a given input file, the build system searches for the first rule that can build that file. Unlike Make, the system only considers the first rule that it finds. This may change in the future.

If no rule is found to build the file, the system only checks if the file already exists. If the file does not exist, the build exits with an error.

However, if a rule is found, the system checks all of the prerequisites that the rule provides for the file and attempts to build them. Then it decides whether to build the target file:

  • If the rule is marked as 'phony', it is always rebuilt.
  • If any of the prerequisites were rebuilt, the target file is also rebuilt.
  • If the target file does not exist, it is rebuilt.
  • If any of the prerequisites exist and have a newer modification time than the target file, it is rebuilt.

Rules

Rules are defined as subclasses of the Rule class. Subclasses should implement the following functions:

  • __init__: 'Phony' rules should call super().__init__ with the phony argument set to True.
  • can_build(self, file): This should return True if this rule can build the given input file regardless of its prerequisites.
  • get_prereqs(self, file): Takes an input file previously validated by can_build and returns a list of prerequisite files needed for that file.
  • build(self, file): This should run the build function for the given input file, which has previously been validated by can_build. Prerequisites for the file should exist at this point.

Built-in rules

Since rules can be fairly bulky and complex, some built-in rules have been defined for convenience:

Target

Target is a special type of rule that can only build a single specific file. Its constructor takes several arguments:

  • file: The specific target file
  • prerequisites: A list of prerequisite files required to build the target file
  • build_func: A function or lambda to build the input file. For convenience, the function is passed the input file and the list of prerequisites.
    If the build function is None, no action is taken to build the file.
  • phony: [optional] Set to True to mark this target as 'phony'. This is useful if the target file is not actually a file, or for targets such as 'all', 'clean', 'install', etc.

Example: The all target

# 'all' target builds the library and documentation
rules.append(Target('all', ['libthing.so', 'doc'], None, True))

Built-in C/C++ build rules

CppRule

This rule can be used to compile C and C++ files. It manages dependency files and file paths. Its constructor takes several arguments:

  • build_dir: where to put build files. It is not recommended to set this to '.' or the current working directory.
  • compiler: the path to the compiler executable.
  • options: Compiler options in a list
  • src_ext: [optional] Set the source file extension (.cpp by default)
  • out_ext: [optional] Set the output file extension (.o by default)

CppRule will build any files with the extension {src_ext}.{out_ext} that exist in {build_dir}. It reads prerequisites from the compiler-generated dependency file, which is generated using the -MMD compiler option.

To convert a source file into an output file, a convenience function CppRule.gen_buildfile() is provided. This function takes the source file and build directory as parameters, and it returns a build file.

Example: Converting source files to object files

sources = ['src/a.cpp', 'src/b.cpp', 'src/main.cpp']
objs = [CppRule.gen_buildfile(f) for f in sources]

Example: Creating rules for C and C++ files

# convert .cpp -> .cpp.o
rules.append(CppRule('build/', '/usr/bin/g++', cxx_opts, '.cpp', '.o'))
# convert .c -> .c.o
rules.append(CppRule('build/', '/usr/bin/gcc', c_opts,   '.c',   '.o'))

ElfRule

This rule will compile an ELF file (shared library or executable) from several object files and/or libraries. Its constructor takes the following parameters:

  • file: The output file to build
  • objs: A list of object files to link
  • libs: A list of (shared or static) library files to link. These library files must be named lib{name}.a or lib{name}.so.
  • compiler: Path to the compiler
  • options: Link options

LibRule

This rule builds a static library file from several object files. Its constructor takes the following parameters:

  • file: The output .a file to build
  • objs: A list of object files to archive
  • archiver: Path to the compiler toolchain's ar program

Functions

The build system provides functions to build rules, run external code, and check files.

  • run_build(rules, def_target): Build the file specified on the command line, or the target specified by def_target if nothing is given on the command line.
  • build(rules, rule, file): Build a file with the given rule.
  • run(args, pwd='.'): Run an external program, blocking until it finishes. If the program returns a non-zero exit code, this function will forcibly quit the Python process.
  • exists(f): Shorthand for os.path.exists(f)
  • is_newer(f1, f2): Returns True iff f1 was modified more recently than f2.