Add "buildtool" to generate C source for cffi-based modules.

setup.py

@@ -229,6 +229,9 @@ def has_ext_modules(self):
             "distutils.setup_keywords": [
                 "cffi_modules = cffi.setuptools_ext:cffi_modules",
             ],
+            'console_scripts': [
+                'cffi-buildtool = cffi.buildtool:main',
+            ]
         },
 
         classifiers=[

src/cffi/buildtool.py

@@ -0,0 +1,71 @@
+import argparse
+import pathlib
+
+from cffi.api import FFI
+from cffi.recompiler import Recompiler, NativeIO
+
+parser = argparse.ArgumentParser(
+    description="Generate source for a C module from the cffi build script."
+)
+parser.add_argument("--ffi_var_name", default="ffibuilder")
+parser.add_argument("infile", type=pathlib.Path)
+parser.add_argument("outfile", type=pathlib.Path)
+
+
+class BuildError(Exception):
+    __module__ = "cffi"
+
+
+def execfile(filename, glob):
+    # We use execfile() (here rewritten for Python 3) instead of
+    # __import__() to load the build script.  The problem with
+    # a normal import is that in some packages, the intermediate
+    # __init__.py files may already try to import the file that
+    # we are generating.
+    with open(filename) as f:
+        src = f.read()
+    src += "\n"  # Python 2.6 compatibility
+    code = compile(src, filename, "exec")
+    exec(code, glob, glob)
+
+
+def get_ffi(filename, ffi_var_name):
+    globs = {}
+    execfile(filename, globs)
+    if ffi_var_name not in globs:
+        raise BuildError("%r: object %r not found in module" % (filename, ffi_var_name))
+    ffi = globs[ffi_var_name]
+    if not isinstance(ffi, FFI) and callable(ffi):
+        # Maybe it's a callable that returns a FFI
+        ffi = ffi()
+    if not isinstance(ffi, FFI):
+        raise TypeError(
+            "%r is not an FFI instance (got %r)" % (filename, type(ffi).__name__)
+        )
+    return ffi
+
+
+def generate_c_source(ffi):
+    """Generate C module source from a FFI instance.
+
+      Example of use:
+        if __name__ == "__main__":
+          from cffi.buildtool import generate_c_source
+          print(generate_c_source(ffibuilder))
+    """
+    # TODO: improve this; https://github.com/python-cffi/cffi/issues/47
+    module_name, source, source_extension, kwds = ffi._assigned_source
+    recompiler = Recompiler(ffi, module_name)
+    recompiler.collect_type_table()
+    recompiler.collect_step_tables()
+    f = NativeIO()
+    recompiler.write_source_to_f(f, source)
+    return f.getvalue()
+
+
+def main():
+    args = parser.parse_args()
+    ffi = get_ffi(args.infile, args.ffi_var_name)
+    output = generate_c_source(ffi)
+    with args.outfile.open("w") as f:
+        f.write(output)