
**Xref**: Cross-referencing source code
***************************************

When manipulating source code, programmers needs to know where the
various symbols are defined, where they are used, and so on. This is
generally available directly from their IDE and editors. But computing
this information in the first place is tricky, especially for
languages that support overloading of subprograms.

The best tool to resolve these reference is a compiler, since by
definition it understands the program and where each part comes from.
Some compilers like GNAT and gcc can then generate extra output files
which contain this information. For instance, GNAT will generate
".ali" files when it compiles Ada source code, and gcc will generate
".gli" files when it compiles C or C++ code and is run with *-fdump-
xref* switch.

"GNATCOLL.Xref" can then be used to parse and aggregate all those
files into a single sqlite database, which can be queries conveniently
to answer queries such as "give me the declaration for this entity",
"list all places where this entity is used", "Show all subprograms
that could be called in practice at this dispatching call", "What
files does this file depend on", "Show me the call graph for this
application",...

To us this package, some initialization needs to be performed first:

   with GNATCOLL.Xref;     use GNATCOLL.Xref;
   with GNATCOLL.SQL.Sqlite;
   with GNATCOLL.Projects; use GNATCOLL.Projects;   --  1

   procedure Support is
      DB : Xref_Database;
      Tree : Project_Tree;

   begin
      Tree.Load ("prj.gpr");  --  2
      Setup_DB (DB, GNATCOLL.SQL.Sqlite.Setup (Database => "testdb.db"));  -- 3
      Parse_All_LI_Files (DB, Tree, Tree.Root_Project);   --  4
   end Support;

GNATCOLL needs to be able to find the "*li" files. For this, it
depends on project files (as supported by "GNATCOLL.Projects"). So the
first thing to do is to parse the project (step 2).

We then need to tell GNATCOLL where the cross-reference information
need to be aggregated. In this example, it will be stored in a sqlite
database on the disk. By using a name ":memory:" instead, we would
create a temporary in-memory database. This is in general faster, but
uses more memory and needs to be recreated every time the program is
restarted. We could also decide to store the information in any other
database supported by "GNATCOLL.SQL.Exec", for instance PostgreSQL.

Finally, on step 4 we let GNATCOLL parse all the "*li" files that are
relevant for this project. This operation can take a while, depending
on the size of the project. However, if the database already exists on
the disk, it will simply be updated by parsing the files that are not
already up-to-date. When all files are up-to-date, this operation is
almost immediate.

At this point, we now have a database that we can start querying. Here
are a few examples, but see the documentation "gnatcoll-xref.ads" for
more types of queries. All these queries have a similar API: they
return a **cursor** which iterates over the result returned by a SQL
query. There are various kinds of cursors, depending on whether they
return files, entities, or references to entities. But they all
support the *Has_Element*, *Element* and *Next* operation, so all
loops will look similar:

   pragma Ada_05;   --  use object-dotted-notation
   with GNATCOLL.VFS;   use GNATCOLL.VFS;

   declare
       Entity : Entity_Information;
       Ref    : Entity_Reference;
       File   : Virtual_File;
       Refs   : References_Cursor;
   begin
       File := Tree.Create ("source.ads");   --  5
       Ref := DB.Get_Entity ("Method", File, Line => 2);   --  6
       Entity := Ref.Entity;

       DB.References (Entity, Refs);   --  7
       while Refs.Has_Element loop
          Ref := Refs.Element;
          Put_Line (" at " & Ref.File.Display_Full_Name & ':'
             & Ref.Line'Img & ':' & Ref.Column'Img);
          Refs.Next;
       end loop;
   end;

This example will print all the references to the entity that is
referenced in file source.ads at line 2 (the column is unspecified).

Step 5 gets a handle on the source file. Here, we depend on the
project to find the precise directory in which the source file is
found. We can of course use an absolute file name instead.

Step 6 gets handle on the entity referenced on line 2 in this file.
Such an entity is the starting point for most queries defined in
*GNATCOLL.Xref*.

Finally, on step 7 and the loop below we iterate over all references,
and print their location on the standard output.

Let's do a much more complex query: we want to see all references to
that entity, but also places where the entity might be called through
a *renames* statement, or called through a dispatching call via an
overriding method defined on a child tagged type (assuming this is a
primitive operation of a tagged type in the first place). We also want
to see all locations where a method that overrides "Method" is called:

   declare
      Refs : Recursive_References_Cursor;
   begin
      DB.Recursive (Entity, GNATCOLL.Xref.References'Access,
                    From_Overriding => True, From_Overridden => True,
                    From_Renames => True);
      while Refs.Has_Element loop
           ... same as before
           Refs.Next;
      end loop;
   end;

As shown above, the programing pattern is always the same.

GNATCOLL.Xref provides many more subprogram to get information like
the list of fields for a record type (or a C structure), the list of
primitive operations or methods for a tagged object or a class, the
call graph for a subprogram,...

It is also able to extract documentation for an entity from the source
code, by looking at the lines of code just before or just after the
declaration or the body of the entity.


**Xref**: gnatinspect
*********************

As discussed in the previous section, GNATCOLL provides an Ada API to
perform cross-references queries.

There exist a few alternatives when you want to reuse that cross-
reference information from other tools, or command line scripts.

You can of course access the sqlite database directly. Most
programming languages have an interface to sqlite. For instance python
does.

But GNATCOLL provides a command line tool dedicated to that purpose,
named **gnatinspect**.

When it is first started on a project, this tool will refresh the xref
database by parsing all the ALI files from the project. This might
take a while (up to several minutes) the first time, unless of course
the xref were already up-to-date because you had loaded the project in
GPS first, or already run gnatinspect.

gnatinspect then displays an interactive prompt that lets you perform
various queries on the database. The full list of queries is available
by typing "help" at the prompt, but this documentation will
demonstrate some of them.

Let's first look at a number of command line switches that might be
useful:

* *--db=ARG*: this switch can be used to specify the name of the
  database. By default, this will be a file named 'gnatinspect.db' in
  the project's object directory. If there is no such directory
  defined in the project, the file is created in the project's
  directory itself. You can however specify any name, including an
  absolute path, or a path relative to the project's object directory.

  An alternative is to specify ':memory:', which creates the database
  in memory. This is of course a temporary database which will
  disappear when gnatinspect exists, and cannot be shared with other
  tools.

* *--nightlydb=ARG*: this switch can help speed up the initial
  startup of gnatinspect. The idea is that in a lot of cases, the
  software on which a team works is build nightly in a common setup.
  Running gnatinspect in that setup will create or update an xref
  database. Individual developers can then create their own copy of
  the database by starting from the contents of the nightly database
  (which is pointed to by the --nightlydb switch), and then
  gnatinspect will parse the ALI files in the user's setup that are
  different from the nightly ones.

* *--runtime*: by default, gnatinspect will only parse the ALI files
  from your project (and of course the ones from imported projects).
  It will not however parse the ALI files found in predefined
  directories, like for instance the GNAT runtime. This saves time in
  general. If you click on a call to one of the runtime subprograms in
  your own code, gnatinspect will be able to point you to its
  declaration. However, you will not have access to the body, because
  the link from declaration to body is found in the ALI files of the
  runtime.

* *--command=ARG*: gnatinspect will update the xref database as
  usual, then execute a command, display its result, and exit. This
  can be convenient when calling gnatinspect from another tool, like
  Emacs or vi.

* *--file=ARG*: similar to --command, but reads the commands to
  execute from a file. The file can contain comments (starting with
  '--'. See also the --lead switch.

* *--lead=ARG* should be used in coordination with --file, and
  specify lines to ignore from the file. All lines starting with the
  given prefix will be ignored.

* *--basenames*: controls the display of file names in the output.
  By default, gnatinspect outputs full path information.

* *--exit*: if this switch is specified, gnatinspect updates the
  xref database and exits immediately.

* *--project=ARG* or *-P ARG* specifes the name of the project to
  load. This switch is mandatory.

* *-X VAR=VALUE* is used to specify the value of scenario variables
  used in your project. This is similar to the homonym switch in
  gprbuild.

* *--symlinks* should be specified if your projet uses symbolic
  links for files. This will ensure that the links are fully resolved
  as stored in the database, and thus that when a file is visible
  through different links, the information is appropriately coalesced
  in the database for that file.

* *--subdirs=ARG* is similar to the homonym switch in gprbuild

* *--tracefile=ARG* is used to point to a file compatible with
  GNATCOLL.Traces that controls the debug information generated by
  gnatinspect. By default, gnatinspect parses a file called
  '.gnatdebug' in the current directory.

* *--encoding=ARG* is the character encoding used for source and ALI
  files. By default, gnatinspect assumes they are encoded in UTF-8.

Once it has finished parsing the xref information, gnatinspect
displays an interactive prompt, where a number of commands can be used
to perform queries. In a lot of cases, these commands take some file
information as argument (either just the file, or an entity name and
the file in which it is defined).

The file names can be given as either a base name, or relative to the
current directory, or even a full name.  But file names are ambiguous
(even when a full path is specified) when aggregate projects are used.
It is valid for a given file to be part of multiple aggregate
projects, and depending on the project we are considering the result
of the xref queries might vary).

To remove the ambiguity, it is possible to specify the project to
which the file belongs. The project is specified either as a project
name (which itself could be ambiguous with aggregate projects), or as
a full path.

In all commands below, whenever the parameter specifies ":file", you
can use instead ":file:project" if there are ambiguities.

Here is the full list of commands supported by gnatinspect:

* *decl name:file:line:column* is probably the most useful command.
  Given a reference to an entity, it will indicate where the entity is
  declared. The line and column informations are optional:

     >>> decl Func:file.adb:12
     Func:/some/path/file2.adb:20:9

* *body name:file:line:column* is similar to *decl*, but will return
  the location of the body of the entity. When the entity is an Ada
  private type, its body is in fact the location of the full
  declaration for that type.

* *refs name:file:line:column* displays all known references to the
  entity.

* *refs_overriding name:file:line:column* displays all known
  references to the entity or one of its overriding entities

* *doc name:file:line:column* will display documentation for the
  entity. The exact format for the entity might change in future
  versions of gnatinspect, but will in general include the type of
  entity, the location of its declaration, and any comment associated
  with it in the source code:

     >>> doc Func:file.adb
     procedure declared at /some/path/file2.adb:20:9

     And the comments written below Func in file2.adb

* *fields name:file:line:column* displays the fields of an Ada
  record type or a C struct:

     >>> fields Rec:file.ads:20
     A:/some/path/file.ads:21
     B:/some/path/file.ads:22

* *child_types name:file:line:column* lists all child types for this
  entity, for instance classes that inherit from the entity. This is
  the opposite of *parent_types*.

* *child_types_recursive name:file:line:column* is similar to
  *child_types* but will also list the child types of the children.
  This query can be used to find a whole tagged type hierarchy (or
  class hierarchy in C++).

* *parent_types name:file:lin:column* returns the parent types for
  the entity, for instance the classes or interfaces from which it
  derives. See also *child_types*.

* *methods name:file:line:column* returns the list of methods (or
  primitive operations) for the entity.

* *method_of name:file:line:column* returns the class or tagged type
  for which the entity is a method.

* *calls name:file:line:column* lists all entities called by the
  entity. This includes all entities defined within the scope of the
  entity (so for a subprogram this will be the list of local
  variables, but for a package this includes all subprograms and
  nested packages defined within that package).

* *callers name:file:line:column* lists all entities that call the
  entity. This information is also available from a call to 'refs',
  but 'callers' return the callers directly, instead of references to
  the original entity.

* *overrides name:file:line:column* returns the entity that is
  overridden by the entity (generally a method from a parent class).

* *overridden name:file:line:column* returns the list of entities
  that override the parameter (generally methods from children
  classes).

* *overridden_recursive name:file:line:column* returns the list of
  entities that override the parameter (generally methods from
  children classes). This is recursive.

* *type name:file:line:column* returns the type of the entity
  (variable or constant). For an enumeration literal, this returns the
  corresponding enumeration.

* *component name:file:line:column* returns the component type of
  the entity (for arrays for instance).

* *literals name:file:line:column* returns the valid literal values
  for an enumeration.

* *pointed name:file:line:column* returns the type pointed to by the
  entity.

* *qname name:file:line:column* returns the fully qualified name for
  the entity.

* *params name:file:line:column* returns the list of parameters for
  the subprogram.

A number of queries are related to the source files of the project:

* *importing filename* lists the files that import the file (via
  with statements in Ada or #include in C for instance)

* *imports filename* lists the files that the file imports (via with
  statements in Ada or #include in C for instance). See also
  *depends_on*.

* *depends filename* lists the files that the file depends on
  (recursively calling *imports*)

* *entities file* lists all entities referenced or declared in the
  file.

Finally, some commands are not related to entities or source files:

* *refresh* refreshes the contents of the xref database, by parsing
  all ALI files that have been changed.

* *shell* Execute a shell command (an alternative is to use '!' as
  the command).

* *scenario VARIABLE VALUE* changes the value of a scenario
  variable, and reparse the project.

* *time command arguments* executes the command as usual, and report
  the time it took to execute it.
