I'm learning about gcc's cleanup attribute, and learning how it calls a function to be run when a variable goes out of scope, and I don't understand why you can use the word "cleanup" with or without underscores. Where is the documentation for, or documentation of, the version with underscores?
The gcc documentation above shows it like this:
__attribute__ ((cleanup(cleanup_function)))
However, most code samples I read, show it like this:
__attribute__ ((__cleanup__(cleanup_function)))
Ex:
http://echorand.me/site/notes/articles/c_cleanup/cleanup_attribute_c.html
http://www.nongnu.org/avr-libc/user-manual/atomic_8h_source.html
Note that the first example link states they are identical, and of course coding it proves this, but how did he know this originally? Where did this come from?
Why the difference? Where is __cleanup__ defined or documented, as opposed to cleanup?
My fundamental problem lies in the fact that I don't know what I don't know, therefore I am trying to expose some of my unknown unknowns so they become known unknowns, until I can study them and make them known knowns.
My thinking is that perhaps there is some globally-applied principle to gcc preprocessor directives, where you can arbitrarily add underscores before or after any of them? -- Or perhaps only some of them? -- Or perhaps it modifies the preprocessor directive or attribute somehow and there are cases where one method, with or without the extra underscores, is preferred over the other?
You are allowed to define a macro cleanup, as it is not a name that is reserved to the compiler. You are not allowed to define one named __cleanup__. This guarantees that your code using __cleanup__ is unaffected by other code (provided that other code behaves, of course).
As https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax explains:
You may optionally specify attribute names with __ preceding and following the name. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use the attribute name __noreturn__ instead of noreturn.
(But note that attributes are not preprocessor directives.)
Related
I cannot understand where lazy-static's #TAIL and #MAKE have been defined and their particular use cases.
If I've understood internal rules correctly, the primary usage of #as_expr in the example is to hide as_expr! (or in general, previously defined macros) from being exported i.e. it's a way of altering the global macro namespace. Following that, then #TAIL or #MAKE should already be a macro while I cannot find them in the lazy_static source.
You linked to the definitions. #TAIL is right there three lines down on 137, #MAKE is on 162.
#name is not special in any way whatsoever. There is absolutely no special behaviour. It's just a sequence of tokens that cannot show up in "normal" code, and is thus unlikely to be accidentally matched to other rules. #as_expr does not hide an as_expr! macro, it's used instead of defining a publicly visible as_expr! macro.
Some people use double dash to indicate that the function is subject to change:
What does the double minus (--) convention in function names mean in Emacs Lisp
Does including internal in function names mean similar things?
Two examples
where-is-internal
internal-make-var-non-special
The function where-is-internal has a detailed docstring and is mentioned in the manual as well. Is where-is-internal an exception?
Is there a difference between having -internal as suffix and having internal- as prefix?
Adding to confusion, there are also function names with internal-- (with double dash) as prefix.
The confusion is not just in the naming convention (variability due to history and perhaps sometimes whim). The confusion is in the very notion of "internal" in free software, where the source code is open to everyone to use or modify (even fork) as they please.
To answer your question from (what I think is) the point of view of Emacs Dev, and thus in terms of the underlying intention: "internal" means that someone using such a function is perhaps more likely to encounter future changes in the Emacs-Dev implementation and use of that function than might be the case for a non-"internal" function. IOW, you might not want to count on it remaining as it is now. That's all.
But there's a lot of "perhaps", "more likely", and "might" in there. In practice, some non-"internal" functions change more radically or more quickly than some "internal" functions. It might be the case that for the former there will be a deprecation grace period, during which the pre-change situation is tolerated, i.e., still works. That might not be the case for something "internal". But again, in practice there is some gray between the black of "internal" and the white of non-"internal".
Someone from Emacs Dev (e.g. #Stefan) will perhaps put this differently or correct my interpretation.
My own take: there have sometimes (often) been functions and variables that the author did not expect users to make use of directly, and thus naturally thought of as "internal", which users have nevertheless put to good use, or even "had" to use (modulo rewriting lots of code). Some such have had their "internal" status removed (no, I don't have examples memorized). Or sometimes a new, non-"internal" function has been added to make the behavior available - e.g., a wrapper or function-valued argument has been added (again, I have no offhand examples to give).
IOW, for Emacs Dev too it is not always clear what should be considered "internal". Just take the label as a flag that you might not want to count too much on that function or variable.
Wrt the various notations: My impression is that the -- convention seems recently to be used more (though there is also some old code that uses it); using internal is an older convention, for the most part.
The "internal" and the "--" conventions are similar. Basically "internal" is used when there's no prefix after which to put a double dash (which is usually the case for functions implemented in C).
And yes, as Drew explains, the intention behind the notion of something being "internal" is just to recommend people not use it directly. IOW if they need the corresponding functionality, they should report a bug requesting to promote its status to "non-internal".
The following preprocessor macro:
#define _VARIANT_BOOL /##/
is not actually valid C; roughly speaking, the reason is that the preprocessor is defined as working on a stream of tokens, whereas the above assumes that it works on a stream of characters.
On the other hand, unfortunately the above actually occurs in a Microsoft header file, so I have to handle it anyway. (I'm working on a preprocessor implementation.)
What other cases have people encountered in the wild, be it in legacy code however old as long as that code may be still in use, of preprocessor macros that are not actually valid, but work anyway because they were written under compilers that use a character oriented preprocessor implementation?
(Rationale: I'm trying to get some idea in advance how many special cases I'm going to have to hack, if I write a proper clean standard-conforming token oriented implementation.)
The relevant part of the standard (ยง6.10.3.3 The ## operator) says:
If the result is not a valid preprocessing token, the behavior is undefined.
This means that your preprocessor can do anything it likes and still be standard conforming, including emulating the common behaviour.
I think you can still have a "token-based" implementation and support this behaviour, by specifying that when the result of the ## operator is not a valid preprocessing token, the result is the two operand tokens unchanged. You may also want to have your preprocessor emit a warning about the invalid code.
I have seen one answer of How does Lisp let you redefine the language itself?
Stack Overflow question (answered by Noah Lavine):
Macros aren't quite a complete redefinition of the language, at least as far as I know (I'm actually a Schemer; I could be wrong), because there is a restriction. A macro can only take a single subtree of your code, and generate a single subtree to replace it. Therefore you can't write whole-program-transforming macros, as cool as that would be.
After reading this I am curious about whether there are "whole-program-transforming macros" in Lisp or Scheme (or some other language).
If not then why?
It is not useful and never required?
Same thing could be achieved by some other ways?
It is not possible to implement it even in Lisp?
It is possible, but not tried or implemented ever?
Update
One kind of use case
e.g.
As in stumpwm code
here are some functions all in different lisp source files
uses a dynamic/global defvar variable *screen-list* that is defined in primitives.lisp , but used in screen.lisp, user.lisp, window.lisp.
(Here each files have functions, class, vars related to one aspect or object)
Now I wanted to define these functions under the closure where
*screen-list* variable available by let form, it should not be
dynamic/global variable, But without moving these all functions into
one place (because I do not want these functions to lose place from their
related file)
So that this variable will be accessible to only these functions.
Above e.g. equally apply to label and flet, so that it will further possible
that we could make it like that only required variable, function will be available,
to those who require it.
Note one way might be
implement and use some macro defun_with_context for defun where first argument is
context where let, flet variables definend.
But apart from it could it be achieved by reader-macro as
Vatine and Gareth Rees answered.
You quoted Noah Lavine as saying:
A macro can only take a single subtree of your code, and generate a single subtree to replace it
This is the case for ordinary macros, but reader macros get access to the input stream and can do whatever they like with it.
See the Hyperspec section 2.2 and the set-macro-character function.
In Racket, you can implement whole-program-transforming macros. See the section in the documentation about defining new languages. There are many examples of this in Racket, for example the lazy language and Typed Racket.
Off the top of my head, a few approaches:
First, you can. Norvig points out that:
We can write a compiler as a set of macros.
so you can transform an entire program, if you want to. I've only seen it done rarely, because typically the intersection between "things you want to do to every part of your program" and "things that you need macro/AST-type transformations for" is a pretty small set. One example is Parenscript, which transforms your Lisp code ("an extended subset of CL") into Javascript. I've used it to compile entire files of Lisp code into Javascript which is served directly to web clients. It's not my favorite environment, but it does what it advertises.
Another related feature is "advice", which Yegge describes as:
Great systems also have advice. There's no universally accepted name for this feature. Sometimes it's called hooks, or filters, or aspect-oriented programming. As far as I know, Lisp had it first, and it's called advice in Lisp. Advice is a mini-framework that provides before, around, and after hooks by which you can programmatically modify the behavior of some action or function call in the system.
Another is special variables. Typically macros (and other constructs) apply to lexical scope. By declaring a variable to be special, you're telling it to apply to dynamic scope (I think of it as "temporal scope"). I can't think of any other language that lets you (the programmer) choose between these two. And, apart from the compiler case, these two really span the space that I'm interested in as a programmer.
A typical approach is to write your own module system. If you just want access to all the code, you can have some sort of pre-processor or reader extension wrap source files with your own module annotation. If you then write your own require or import form, you will ultimately be able to see all the code in scope.
To get started, you could write your own module form that lets you define several functions which you then compile in some clever way before emitting optimized code.
There's always the choice of using compiler macros (they can do whole-function transformation based on a lew of criteria, but shouldn't change the value returned, as that would be confusing).
There's reader macros, they transform the input "as it is read" (or "before it is read", if you prefer). I haven't done much large-scale reader-macro hacking, but I have written some code to allow elisp sourec to be (mostly) read in Common Lisp, with quite a few subtle differences in syntactic sugar between the two.
I believe those sorts of macros are called code-walking macros. I haven't implemented a code walker myself, so I am not familiar with the limits.
In Common LISP, at least, you may wrap top-level forms in PROGN and they still retain their status as top-level forms (see CLTL2, section 5.3). Therefore, the limitation of a macro generating a single subtree is not much of a limitation since it could wrap any number of resulting subtrees within PROGN. This makes whole-program macros quite possible.
E.g.
(my-whole-program-macro ...)
= expands to =>
(progn
(load-system ...)
(defvar ...)
(defconstant ...)
(defmacro ...)
(defclass ...)
(defstruct ...)
(defun ...)
(defun ...)
...
)
I can read the documentation, so I'm not asking for a cut-and-paste of that.
I'm trying to understand the motivation for this function.
When would I want to use it?
The documentation in the Emacs lisp manual does have some example situations that seem to answer your question (as opposed to the doc string).
From looking at the Emacs source code, eval-and-compile is used to quiet the compiler, to make macros/functions available during compilation (and evaluation), or to make feature/version specific variants of macros/functions available during compilation.
One usage I found helpful to see was in ezimage.el. In there, an if statement was put inside the eval-and-compile to conditionally define macros depending on whether the package was compiled/eval'ed in Emacs or XEmacs, and additionally whether a particular feature was present. By wrapping that conditional inside the eval-and-compile you enable the appropriate macro usage during compilation. A similar situation can be found in mwheel.el.
Similarly, if you want to define a function via fset and have it available during compilation, you need to have the call to fset wrapped with eval-and-compile because otherwise the symbol -> function association isn't available until the file is evaluated (because compilation of a call to fset just optimizes the assignment, it doesn't actually do the assignment). Why would you want this assignment during compilation? To quiet the compiler. Note: this is just my re-wording of what is in the elisp documentation.
I did notice a lot of uses in Emacs code which just wrapped calls to require, which sounds redundant when you read the documentation. I'm at a loss as to how to explain those.