A Simple Scanner Example

A Simple Scanner Example Suppose, for example, that we want to create a simple scanner for .foo files. A .foo file contains some text that will be processed, and can include other files on lines that begin with include followed by a file name: include filename.foo Scanning a file will be handled by a Python function that you must supply. Here is a function that will use the Python re module to scan for the include lines in our example: import re include_re = re.compile(r'^include\s+(\S+)$', re.M) def kfile_scan(node, env, path, arg): contents = node.get_text_contents() return include_re.findall(contents) The scanner function must accept the four specified arguments and return a list of implicit dependencies. Presumably, these would be dependencies found from examining the contents of the file, although the function can perform any manipulation at all to generate the list of dependencies. node An &SCons; node object representing the file being scanned. The path name to the file can be used by converting the node to a string using the str() function, or an internal &SCons; get_text_contents() object method can be used to fetch the contents. env The construction environment in effect for this scan. The scanner function may choose to use construction variables from this environment to affect its behavior. path A list of directories that form the search path for included files for this scanner. This is how &SCons; handles the &cv-link-CPPPATH; and &cv-link-LIBPATH; variables. arg An optional argument that you can choose to have passed to this scanner function by various scanner instances. A Scanner object is created using the &Scanner; function, which typically takes an skeys argument to associate the type of file suffix with this scanner. The Scanner object must then be associated with the &cv-link-SCANNERS; construction variable of a construction environment, typically by using the &Append; method: kscan = Scanner(function = kfile_scan, skeys = ['.k']) env.Append(SCANNERS = kscan) When we put it all together, it looks like: import re include_re = re.compile(r'^include\s+(\S+)$', re.M) def kfile_scan(node, env, path): contents = node.get_text_contents() includes = include_re.findall(contents) return includes kscan = Scanner(function = kfile_scan, skeys = ['.k']) env = Environment(ENV = {'PATH' : '/usr/local/bin'}) env.Append(SCANNERS = kscan) env.Command('foo', 'foo.k', 'kprocess < $SOURCES > $TARGET')

Adding a search path to a scanner: &FindPathDirs; Many scanners need to search for included files or dependencies using a path variable; this is how &cv-link-CPPPATH; and &cv-link-LIBPATH; work. The path to search is passed to your scanner as the path argument. Path variables may be lists of nodes, semicolon-separated strings, or even contain SCons variables which need to be expanded. Fortunately, &SCons; provides the &FindPathDirs; function which itself returns a function to expand a given path (given as a SCons construction variable name) to a list of paths at the time the scanner is called. Deferring evaluation until that point allows, for instance, the path to contain $TARGET references which differ for each file scanned. Using &FindPathDirs; is quite easy. Continuing the above example, using KPATH as the construction variable with the search path (analogous to &cv-link-CPPPATH;), we just modify the &Scanner; constructor call to include a path keyword arg: kscan = Scanner(function = kfile_scan, skeys = ['.k'], path=FindPathDirs('KPATH')) FindPathDirs returns a callable object that, when called, will essentially expand the elements in env['KPATH'] and tell the scanner to search in those dirs. It will also properly add related repository and variant dirs to the search list. As a side note, the returned method stores the path in an efficient way so lookups are fast even when variable substitutions may be needed. This is important since many files get scanned in a typical build.