Syntax for union parameter

Discussion in 'C Programming' started by Rick C. Hodgin, Jan 29, 2014.

  1. I appreciate everyone's input. I don't expect anyone to use RDC. As
    I have said in another thread, people will come to RDC from the inside
    out, not the outside in. I will not be able to convince people to use
    RDC, but those people who do use it will have been drawn to use it by
    its many features and design, and will in no small part be drawn by
    their foundational relationship with Jesus Christ, as I am doing what
    I am doing for Him, and for those He brings to the project to contribute
    and/or use. This extends out to all of my projects at:

    https://github.com/RickCHodgin/

    As for the rest of the people on the Earth, there are many many compilers
    to choose from that work as you are used to programming. I wish each
    of you well on one of those.

    May the Lord grant you peace in your hearts, love abounding, and joy
    overflowing, as He replaces the angst of a life lived apart from Him
    with the new hope found only as citizens of His everlasting Kingdom.

    If anyone would like to contact me, click the "main site" link on this
    page, and my email is on the upper-left:

    http://www.visual-freepro.org/

    Or on this page, click "main site" and I'm on the main page:

    http://www.libsf.org/

    Take care.

    Best regards,
    Rick C. Hodgin
     
    Rick C. Hodgin, Jan 30, 2014
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  2. Ok, that invalidates a lot of of what I wrote.
    If function declarations were the only things needed in header files,
    that would be great. What about type definitions? Macro definitions?

    Suppose you have a file "stdio.c" that has definitions for all the C
    standard I/O functions. How would your tool know to add a typedef for
    `FILE` when it generates "stdio.h"?
    So I recompile recompile gmp.c, and gmp.h is automatically updated. Now
    gmp.h has a new timestamp, even though it hasn't actually changed, and
    when I type "make" it unnecessarily recompiles every source file that
    depends on that header.

    The update would have to first regenerate gmp.h as a temporary file,
    then update the actual gmp.h only if it doesn't match the temporary
    file. Which is certainly doable.
    I certainly have no problem using automated tools to (help) generate
    header files. Such a tool can't do 100% of the job (unless perhaps it
    pays attention to extra annotations in the input source file).

    I don't think changing the language to support this would be a good
    idea; as long as your tool can work within the language as it's defined
    now, why bother?

    For most C projects, header files are maintained manually (perhaps with
    some automated assistance), and any mismatches between a function
    definition and the corresponding declaration are caught immediately on
    recompilation. I don't find the copy-and-paste step of just a line or
    so for each function to be enough of a burden to worry about.

    And if I were going to change the language to handle this better, I'd
    probably want to define an actual module system of some sort rather than
    adding a layer on top of the existing convention of .h and .c files.
     
    Keith Thompson, Jan 30, 2014
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  3. Rick C. Hodgin

    Seebs Guest

    There's two separate issues. One is the utility of prototypes for functions
    which you aren't going to define in this file, one is the need to have the
    prototypes declared prior to the calls. These aren't directly connected, but
    the existence of the former created a reason for there to be a mechanism for
    prototyping, and it's not obvious that it's worth much if any effort to have
    a way to bypass that mechanism that's only usable in a special case.

    The other thing, from my own experience, is: Prototype and function
    mismatches are ECC bits for me.

    -s
     
    Seebs, Jan 30, 2014
  4. Rick C. Hodgin

    BartC Guest

    I have an advantage in that my scheme works on a somewhat different
    language, so that I can change a few things in a way that doesn't interfere
    with how C works now. You'd need to tread more carefully in changing C
    itself because header files and so on work a certain way (see below)/

    But anyway, type definitions, enums, variable definitions (and named
    constants) can all have a global attribute and be exported in the same way
    as functions. (My macros are a little different, they are expanded
    internally and never exported to C.)
    ? How does FILE get created now?
    Yes, it's very easy. A couple lines of in a scripting language.
    A module system would be better (although I suspect it would have the same
    problems of mutual imported files as I've encountered).

    But what would be feasible now would be:

    (1) Allow function definitions in any order in a source file; there is no
    real reason not to have this.

    (2) Have a compiler option to write out exported functions to a file
    (perhaps something like this already exists in some compiler or other).

    This could be a ".h" header file, and it could decide to concern itself
    solely with functions; variables, types, macros that the declarations might
    also need, can be dealt with conventionally (in a manual header that might
    include the automatic one).

    (3) Perhaps go a bit further and write out exported file-scope variables
    too. This can have the advantage that a definition like this:

    int table[] = {10,20,30,40,50};

    might be generated in the header file as:

    extern int table[5];

    which provides useful array dimension info that is otherwise awkward to
    impart to another module.

    Typedefs and macros, which do not involve actual code or data, can anyway
    already be shared without writing two versions, by putting the one
    definition in the right header. (However this is not quite as tidy as my
    scheme where you share a type defined anywhere in a module, just by putting
    'global' in front.)
     
    BartC, Jan 30, 2014
  5. Probably manually. On my system, /usr/include/stdio.h is 944 lines
    long; the definition of FILE is

    typedef struct _IO_FILE FILE;

    where _IO_FILE is defined in some other header that it includes,
    directly or indirectly. <stdio.h> is a particulary nasty case; it's
    full of system-specific stuff that's not intended for human readers.

    [snip]
    Of course function definitions can already be in any order in a source
    file. To restate what I think you mean, you'd keep the current rule
    that a function definition but you'd change the scope of a function
    declaration (whether it's part of a definition or separate). Currently
    the scope extends from the point of the declaration to the end of the
    translation unit. You'd have the scope extend to the entire translation
    unit.

    That's for declarations at file scope. For local function declarations
    (which IMHO are rarely useful, but they're certainly valid) I presume
    the scope would cover the entire enclosing compound statement -- or I
    suppose they could be left the way they are.

    That would require a compiler to do some sort of pre-pass over the
    translation unit to collect function declarations. I don't know how
    much work that would impose on compiler developers; it *might* be
    substantial.
    I don't know that that needs to be a compiler option. A tool that can
    parse C source could do the job. And a separate tool could be used with
    any compiler.
     
    Keith Thompson, Jan 30, 2014
  6. Rick C. Hodgin

    Ike Naar Guest

    It's not the end of the world, cpu time is cheap nowadays;
    and it will happen only once per update of gmp.c (or other .c file),
    given a decent build tool.

    Manually updating gmp.h (when necessary) also takes time, perhaps even
    more time than time that is wasted on automatic rebuilds.
     
    Ike Naar, Jan 30, 2014
  7. When you recompile a module, it updates the associated header file.
    The solution is a comparison against the existing file contents. The newly
    generated file is only written if anything's changed.

    Best regards,
    Rick C. Hodgin
     
    Rick C. Hodgin, Jan 31, 2014
  8. Which I said in the following paragraph, which you snipped.

    And please don't delete attribution lines.
     
    Keith Thompson, Jan 31, 2014
  9. Keith, why did you include the part about unnecessary recompiles when
    you also saw the immediate and obvious solution which completely
    negates any need for unnecessary recompiles in the first place?

    Best regards,
    Rick C. Hodgin
     
    Rick C. Hodgin, Jan 31, 2014
  10. Rick C. Hodgin

    BartC Guest

    Sorry, I don't get any of that.
    Nor that (clearly we're at cross-purposes).
    That's starting to make sense. Let me illustrate the problem:

    void fna(void); /* forward declaration */

    int main (void){
    fna();
    }

    void fna(void){
    }

    To compile this without errors, I need to include the forward declaration.
    I'm saying this should not be necessary (nor should it be necessary for me
    to arrange the functions in some convoluted order so that a function is
    always defined ahead of any calls to it. I should have the freedom to put
    functions anywhere, to copy & paste to and from anywhere else, etc, without
    worrying about relative ordering or having to drag forward declarations
    along with them).

    And to clarify further, I'm concerned here with local (ie. non-exported)
    functions, so imagine there's a 'static' in front of those declarations.
    (I'm still trying to get my head around C exporting any file-scope function
    or variable by default, instead of the other way around.)

    As for the extra compiler effort involved: I'm fairly certain they already
    do enough passes to make this possible, and they're just adopting this
    annoying behaviour for compatibility reasons.
    Yes, it can, but should it? A tool that parses C is already half a compiler,
    including implementing a full pre-processor, but would need to recognise any
    special attributes used by the main compiler, as well duplicating the same
    set of options passed to the main one.

    Effectively you're saying someone not happy with a compiler, should use
    their own better one! So why not just fix the original?
     
    BartC, Jan 31, 2014
  11. Rick C. Hodgin

    James Kuyper Guest

    That's too vague a complaint to allow a useful response. I've tried to
    figure out an alternate way of explaining what Keith just explained -
    all I've been able to come up with are minor variations in the wording.
    You should explain more precisely the aspects of what he's saying that
    confuse you.
     
    James Kuyper, Jan 31, 2014
  12. Rick C. Hodgin

    BartC Guest

    Reading it more carefully, I can now see what is being said. Except my issue
    didn't seem to have anything to do with scope, since we're talking about
    file-scope which you might expect spans the entire file.

    I can't see any advantage in limiting scope to only after the point of
    declaration, and not before (if we're talking about exactly the same scope
    level). And that doesn't seem to affect goto labels, otherwise you could
    only jump backwards!

    Also, it's not just calls to functions that could refer to forward ones; you
    might want some file-scope data which is initialised to function-pointers;
    you don't want this stuff at the end of a file or hidden between two
    functions.
     
    BartC, Jan 31, 2014
  13. Rick C. Hodgin

    Eric Sosman Guest

    ... which it doesn't. In C as it stands, an file-scope
    identifier's scope is not the entire file but the tail of the
    translation unit, from the point of declaration to the end.
    Your proposal amounts to making a "late" declaration visible
    "early" -- which is to say, you suggest that the identifier's
    scope should be the entire T.U. and not just the tail. Your
    proposal, in other words, is *entirely* about scope.
    An interesting thing about labels is that their nature can
    be deduced without the need for a declaration. When the compiler
    sees `goto blazes;' it knows immediately that `blazes' can only
    be a statement label; you needn't say so. Also, a label's sole
    attributes are its label-ness and the statement it designates;
    there's no need to specify whether it's `unsigned' or `const'
    (or `volatile' -- now, *there's* a dizzying idea!). In short,
    labels aren't declared at all (the Standard does not use the
    words "declare" or "declaration" in connection with labels),
    so analogies between labels and declarable items are suspect.
     
    Eric Sosman, Jan 31, 2014
  14. Rick C. Hodgin

    James Kuyper Guest

    File scope currently does not span the entire file; it starts at the
    point of declaration and continues to the end of the translation unit.
    It is precisely that rule which would have to be modified to make what
    you're talking about work.
    I imagine that the original reason would have been to minimize the
    number of passes through the file that are needed. It's been argued that
    this is no longer as important with modern compilers as it used to be 30
    years ago, and I have no quarrel with that argument.

    It's removing the ability to declare an identifier that identifies
    something which is defined in a different translation units that worries
    me. Automatic extraction of a declaration from the definition requires
    that the definition already exist, and be in a form that the compiler
    can understand; neither restriction applies under the current rules.
    Currently, you can declare an identifier for a function or variable
    whose definition has not even been written yet, and which might end up
    being written is an entirely different language from C, so long as that
    language provides some mechanism for creating C-compatible interfaces.
    The scope of a label includes the entire function body, as explained in
    6.2.1p3. 6.2.1p4 starts off "Every other identifier has scope determined
    by the placement of its declaration ...". There are four different rules
    for how the placement determines where the scope starts - they are
    provide by 6.2.1p7 and p8. For all four rules, the scope always starts
    somewhere near the place where that identifier is declared/defined.
    Identifiers for functions and identifiers for function pointer objects
    are both covered by the "Any other identifier ..." part of 6.2.1p7; a
    change to that clause would therefore affect both (unless the change
    itself made a distinction between them).
     
    James Kuyper, Jan 31, 2014
  15. Rick C. Hodgin

    Eric Sosman Guest

    ... but it does use "is declared implicitly" in 6.2.1p3.
    Sorry for the mis-statement.
    The point stands, though: It's something of a stretch to
    argue that there is (or should be) a lot of similarity between
    declarations that are explicit and those that cannot be so.
     
    Eric Sosman, Jan 31, 2014
  16. Rick C. Hodgin

    Noob Guest



    I think a[0] contains 'U' and a[1] contains 'B'.
     
    Noob, Feb 5, 2014
  17. Rick C. Hodgin

    Robbie Brown Guest



    Well I'm new here so I fully expect I'm setting myself up for some kind
    of 'humorous aside' but ...

    [i++] no change, postfix increment operator

    however

    [++i] also no change. prefix increment operator ... but ... but ... %-}
     
    Robbie Brown, Feb 6, 2014
  18. Rick C. Hodgin

    James Kuyper Guest



    Your answer leaves me uncertain whether or not you understood the point
    Kaz was making. Explaining a joke ruins the joke, so I apologize for the
    following.

    "If a side effect on a scalar object is unsequenced relative to either a
    different side effect on the same scalar object or a value computation
    using the value of the same scalar object, the behavior is undefined."
    (6.5p2)

    i++ has the side-effect of incrementing the value of the scalar object
    i. Determining the value of i in a requires performing a value
    computation using the value of that same scalar object. There are no
    sequence points in that expression, and neither of the two
    sub-expressions involving i is a sub-expression of the other, so they
    are unsequenced relative to each other. Therefore, the behavior is
    undefined.

    As a result, there is no such thing as any particular values that
    "should be" in a[]. It could contain any values whatsoever, include 3.21
    or "'a' no longer has the type int[3]".
     
    James Kuyper, Feb 6, 2014
  19. Rick C. Hodgin

    Robbie Brown Guest



    Of course I didn't understand it, obviously (!)

    Interesting 'sense of humor' though.

    In Java

    public static void main(String args[]){
    int a[] = {1, 2, 3};
    int i = 0;

    a = a[i++];
    System.out.print(a[0]);
    }

    prints 1

    whereas

    a = a[++i];
    System.out.print(a[0]);

    prints ... yep, 2.

    Now why, given the semantics of the pre and postfix increment operators
    should I have expected anything different in C. The semantics are the
    same but in C the results are 'undefined'


    One can only assume that this apparent reluctance to cause a compiler to
    indicate when something is or maybe undefined is a result of the
    apparent ethos of the C community that doing such a thing would impinge
    on the freedom of the individual to do anything they like regardless of
    how stupid it may be.

    Fascinating stuff...
     
    Robbie Brown, Feb 6, 2014
  20. Rick C. Hodgin

    Robbie Brown Guest



    Here's another thing.

    Let's imagine that I didn't understand what a sequence point is.
    Actually I didn't but wikipedia explained it in common English rather
    than some incomprehensible technobabble and now I do.

    The reason that i=i++ is undefined in C is because the language
    definition doesn't (apparently) specify one of the possible behaviors
    preferring instead to leave it undefined. What possible reason could
    there be for this? It's a well known and understood paradigm in other
    languages but apparently not in C.

    But I've learned something, which is the whole object of the exercise so
    all I can say is 'result'
     
    Robbie Brown, Feb 6, 2014
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