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* internals.texi (Stack-allocated Objects): Describe this feature.

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Dmitry Antipov 2014-09-30 19:35:16 +04:00
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doc/lispref/ChangeLog
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@ -1,3 +1,7 @@
2014-09-30 Dmitry Antipov <dmantipov@yandex.ru>
* internals.texi (Stack-allocated Objects): Describe this feature.
2014-09-15 Daniel Colascione <dancol@dancol.org>
* text.texi (Registers): Make `insert-register' documentation

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doc/lispref/internals.texi
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@ -14,6 +14,7 @@ internal aspects of GNU Emacs that may be of interest to C programmers.
* Building Emacs:: How the dumped Emacs is made.
* Pure Storage:: Kludge to make preloaded Lisp functions shareable.
* Garbage Collection:: Reclaiming space for Lisp objects no longer used.
* Stack-allocated Objects:: Temporary conses and strings on C stack.
* Memory Usage:: Info about total size of Lisp objects made so far.
* C Dialect:: What C variant Emacs is written in.
* Writing Emacs Primitives:: Writing C code for Emacs.
@ -529,6 +530,31 @@ during garbage collection so far in this Emacs session, as a
floating-point number.
@end defvar
@node Stack-allocated Objects
@section Stack-allocated Objects
@cindex stack allocation overview
The garbage collector described above is used to manage data visible
from Lisp program, as well as the most of data internally used by the
interpreter. But sometimes it may be useful to allocate temporary
internal (i.e. not visible for Lisp program) objects using C stack of
an interpreter. Currently conses and strings are the only objects which
can be allocated in that way. Strings are limited to ASCII characters
only and should be treated as immutable (calling @code{ASET} on them is
undefined).
@cindex stack allocation internals
In C, this is implemented as a special macros which expands to
a @code{Lisp_Object} with block-scoped semantics and lifetime (see
the code around @code{USE_STACK_LISP_OBJECTS} in @file{lisp.h}). This
means that these objects are not managed by the garbage collector;
instead, they are allocated like local variables in C and automatically
freed when an execution reaches an end of the corresponding scope. Thus,
allocation and freeing are faster than using garbage collector. But
remember that passing them out of their scope results in undefined
behavior. Think twice before using this feature and carefully debug
your code with @code{GC_CHECK_MARKED_OBJECTS} (see @file{alloc.c}).
@node Memory Usage
@section Memory Usage
@cindex memory usage