Commit c95a4266 authored by Eli Zaretskii's avatar Eli Zaretskii

Add comments with instructions for creating new Lisp data types.

parent c664f463
......@@ -222,7 +222,9 @@ enum enum_USE_LSB_TAG { USE_LSB_TAG = 0 };
/* Define the fundamental Lisp data structures. */
/* This is the set of Lisp data types. */
/* This is the set of Lisp data types. If you want to define a new
data type, read the comments after Lisp_Fwd_Type definition
below. */
/* Lisp integers use 2 tags, to give them one extra bit, thus
extending their range from, e.g., -2^28..2^28-1 to -2^29..2^29-1. */
......@@ -298,6 +300,53 @@ enum Lisp_Fwd_Type
Lisp_Fwd_Kboard_Obj, /* Fwd to a Lisp_Object field of kboards. */
};
/* If you want to define a new Lisp data type, here are some
instructions. See the thread at
http://lists.gnu.org/archive/html/emacs-devel/2012-10/msg00561.html
for more info.
First, there are already a couple of Lisp types that can be used if
your new type does not need to be exposed to Lisp programs nor
displayed to users. These are Lisp_Save_Value, a Lisp_Misc
subtype, and PVEC_OTHER, a kind of vectorlike object. The former
is suitable for temporarily stashing away pointers and integers in
a Lisp object (see the existing uses of make_save_value and
XSAVE_VALUE). The latter is useful for vector-like Lisp objects
that need to be used as part of other objects, but which are never
shown to users or Lisp code (search for PVEC_OTHER in xterm.c for
an example).
These two types don't look pretty when printed, so they are
unsuitable for Lisp objects that can be exposed to users.
To define a new data type, add one more Lisp_Misc subtype or one
more pseudovector subtype. Pseudovectors are more suitable for
objects with several slots that need to support fast random access,
whil Lisp_Misc types are foreverything else. A pseudovector object
provides one or more slots for Lisp objects, followed by struct
members that are accessible only from C. A Lisp_Misc object is a
wrapper for a C struct that can contain anything you like.
To add a new pseudovector type, extend the pvec_type enumeration;
to add a new Lisp_Misc, extend the Lisp_Misc_Type enumeration.
For a Lisp_Misc, you will also need to add your entry to union
Lisp_Misc (but make sure the first word has the same structure as
the others, starting with a 16-bit member of the Lisp_Misc_Type
enumeration and a 1-bit GC markbit) and make sure the overall size
of the union is not increased by your addition.
Then you will need to add switch branches in print.c (in
print_object, to print your object, and possibly also in
print_preprocess) and to alloc.c, to mark your object (in
mark_object) and to free it (in gc_sweep). The latter is also the
right place to call any code specific to your data type that needs
to run when the object is recycled -- e.g., free any additional
resources allocated for it that are not Lisp objects. You can even
make a pointer to the function that frees the resources a slot in
your object -- this way, the same object could be used to represent
several disparate C structures. */
#ifdef CHECK_LISP_OBJECT_TYPE
typedef struct { EMACS_INT i; } Lisp_Object;
......
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