elisp: Stack-allocated Objects
E.4 Stack-allocated Objects
===========================
The garbage collector described above is used to manage data visible
from Lisp programs, as well as most of the data internally used by the
Lisp interpreter. Sometimes it may be useful to allocate temporary
internal objects using the C stack of the interpreter. This can help
performance, as stack allocation is typically faster than using heap
memory to allocate and the garbage collector to free. The downside is
that using such objects after they are freed results in undefined
behavior, so uses should be well thought out and carefully debugged by
using the ‘GC_CHECK_MARKED_OBJECTS’ feature (see ‘src/alloc.c’). In
particular, stack-allocated objects should never be made visible to user
Lisp code.
Currently, cons cells and strings can be allocated this way. This is
implemented by C macros like ‘AUTO_CONS’ and ‘AUTO_STRING’ that define a
named ‘Lisp_Object’ with block lifetime. These objects are not freed by
the garbage collector; instead, they have automatic storage duration,
i.e., they are allocated like local variables and are automatically
freed at the end of execution of the C block that defined the object.
For performance reasons, stack-allocated strings are limited to ASCII
characters, and many of these strings are immutable, i.e., calling
‘ASET’ on them produces undefined behavior.
Info Catalog
elisp: Garbage Collection
elisp: GNU Emacs Internals
elisp: Memory Usage