[HN Gopher] Scandalous weird old things about the C preprocessor
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Scandalous weird old things about the C preprocessor
Author : fanf2
Score : 57 points
Date : 2020-05-25 10:43 UTC (1 days ago)
(HTM) web link (blog.robertelder.org)
(TXT) w3m dump (blog.robertelder.org)
| WalterBright wrote:
| He missed one important thing - the C preprocessor is completely
| unaware of C types. Its expressions follow different rules.
|
| A major goal of D was to make the language expressive enough to
| not leave room for the preprocessor:
|
| 1. modules
|
| 2. manifest constants
|
| 3. nested functions
|
| 4. lambdas
|
| 5. static if
|
| 6. compile time function execution
|
| 7. string mixins
| TheSoftwareGuy wrote:
| I don't think he did miss that, rather it simply wasn't on-
| topic for the article he was writing. Anybody who has used the
| C preprocessor knows it works at the level of text/tokens and
| no further. The fact that it knows nothing of types would
| hardly be "scandalous"
|
| But you can go ahead and use this thread to promote D anyways.
| WalterBright wrote:
| The #if does more than just text and tokens, it has an
| expression grammar (constant-expression). It's
| implementation-defined whether the following two produce the
| same value: #if 'z' - 'a' == 25 if
| ('z' - 'a' == 25)
|
| Another difference is all constants are typed as intmax_t and
| uintmax_t, not the usual C types. Yes, it does have types.
|
| This difference in behavior from C expressions is not
| necessary.
| cafard wrote:
| Hmm. The _Intermediate Greek Lexicon_ gives the original meaning
| of "skandalon" as "a trap or a snare laid for an enemy":
| http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%...
| bla3 wrote:
| (2015)
| Ididntdothis wrote:
| All I can say is that I really miss the preprocessor. There are
| so many things that could easily be done with the preprocessor
| but instead have to be done with reflection or code generation in
| a more complex way.
|
| Sure, it can be be abused/misused but I am not sure that's a good
| reason to take it away completely like for example in C#.
| ygra wrote:
| C# has other mechanisms to achieve what macros are commonly
| used for, though. In a bit, with Roslyn's source generators
| there will be an even better way for some of them.
| rwmj wrote:
| One place where preprocessors shine is where the language
| itself has changed in an incompatible way, where "language"
| can be interpreted broadly including calls to external
| libraries. For example some function you need to call added
| an extra parameter: #if LIB_VERSION_GE_3
| lib_f (1, NULL); #else lib_f (1); #endif
|
| It can be difficult to do this with other mechanisms where
| the compiler is actually compiling and therefore type
| checking both branches.
|
| OCaml is a pretty rich language with many features, but we
| fall back on preprocessors like cppo (or even cpp) to deal
| with these kinds of cases.
| Ididntdothis wrote:
| It has a lot of mechanisms but they are usually way more
| complex with more code. For example when I do data binding in
| WPF I could write a five line macro that fires change
| notification events as needed. There are other approaches but
| they are way more complicated.
| joosters wrote:
| Which of these are scandalous, exactly?
|
| If the OP is aggrieved by the operation of cpp, perhaps they
| could have explained in each of their points what cpp is doing
| wrong, and how they would improve the standard. Instead, they
| just list 'odd' behaviour, with no comment about why it is wrong
| and how it could be better.
| klyrs wrote:
| This document immensely valuable in enumerating pitfalls in the
| spec and in how the spec is interpreted by various
| implementations. I would wager that most good-faith* C
| programmers are weary of overusing cpp, and use a limited
| subset of its capabilities. This is a great resource to throw
| at a junior who gets too excited about macros, for example.
|
| * where I'm defining "bad-faith" C programmers as using it for
| sadistic purposes like IOCCC, golf, etc
|
| > perhaps they could have explained in each of their points
| what cpp is doing wrong, and how they would improve the
| standard.
|
| This is a fairly toxic attitude, and possibly the reason that
| your comment is unpopular. A child can correctly observe that
| an emperor is not wearing clothes, and bring that up without
| possessing the skill to make the emperor a suitable outfit.
| joosters wrote:
| I just found the article odd; it starts off by declaring the
| behaviour of cpp to be objectionable, but then it just lists
| different aspects of its actions. At no point does the OP
| complain that cpp is doing the _wrong_ thing - indeed, they
| point out in several places that it couldn 't really be
| changed to do anything else because either it would break
| existing code, or would be just as odd if it behaved in
| another way. So I'm wondering just what it was they were
| scandalized about? I'm not demanding that they fix cpp
| themselves!
|
| At least the child told people that the emperor was wearing
| no clothes; a more apt analogy with this article would be as
| if the child merely said "that's scandalous!" without telling
| anyone about the missing clothes...
| klyrs wrote:
| > 1) No Comprehensive Standard
|
| Here the "scandal" is that the "standard" is poorly
| specified, and therefore isn't much of a standard. This
| seems to be the root of the scandal, and all else appears
| to flow from here.
|
| > 2) Context Free, Just Kidding!
|
| Here the "scandal" is regarding to context sensitivity --
| the author vacillates about the status of the "scandal" but
| still, it seems clear that they're identifying context-
| sensitivity as scandalous.
|
| > 3) Whitespace Insensitive, Just Kidding!
|
| Here the author considers irregularities in how "whitespace
| sensitive" cpp is -- both in the "spec" and in
| implementations, with an example of non-portability between
| gcc and clang.
|
| et cetera
| asjw wrote:
| > A child can correctly observe that an emperor is not
| wearing clothes
|
| Not wearing clothes is what CPP does by default
|
| And it's hardly an emperor, it's more the guy who makes
| copies
| asveikau wrote:
| A bunch of these can't really be changed without breaking
| massive amounts of existing code, either.
|
| Other languages whose designers were not pleased with C macros
| either avoided use of a preprocesor entirely, or, like c++,
| came up with alternative means of compile time flexibility.
| (Like templates)
| coliveira wrote:
| The author tries to compare the preprocessor with programming
| languages, however he seems to forget that the processor is,
| above all, not a language! There is no parse tree for a
| preprocessor, since it is just putting text strings together,
| with little regard for the resulting syntax. That's why it is
| stupid to try to use the Cpp as a programming language, it won't
| work. Think of it as a text replacement engine, like an
| integrated sed, for example.
| raverbashing wrote:
| Ahem https://www.ioccc.org/years.html#2001_herrmann1
| WalterBright wrote:
| > There is no parse tree for a preprocessor
| #if constant-expression new-line group-opt
|
| C99 6.10.1.2
| tgv wrote:
| Next you'll be telling me that PHP isn't a programming
| language.
| ScottBurson wrote:
| While I love to hate on the preprocessor -- and did so publicly
| once in _The Unix Hater 's Handbook_ (p. 211) -- item 4 is silly.
| The _whole point_ of many macros is to circumvent referential
| transparency! This is true even in a language with real macros,
| like Lisp. ( "Real macros" operate on a syntax tree
| representation, not on character strings.)
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