Contents of an exception object

Discussion in 'C Programming' started by James Harris, May 28, 2014.

  1. Kiki Thompson <> showed once again that he isn't happy with
    our friend Malcolm. Why he hasn't just killfiled him and moved on remains
    a mystery:
    Boom goes the hammer. I guess Malcolm is banned for life now.

    "The anti-regulation business ethos is based on the charmingly naive notion
    that people will not do unspeakable things for money." - Dana Carpender

    Quoted by Paul Ciszek (pciszek at panix dot com). But what I want to know
    is why is this diet/low-carb food author doing making pithy political/economic

    Nevertheless, the above quote is dead-on, because, the thing is - business
    in one breath tells us they don't need to be regulated (which is to say:
    that they can morally self-regulate), then in the next breath tells us that
    corporations are amoral entities which have no obligations to anyone except
    their officers and shareholders, then in the next breath they tell us they
    don't need to be regulated (that they can morally self-regulate) ...
    Kenny McCormack, Jun 1, 2014
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  2. I have no idea what you're discussing here or what relevance this topic has
    to any other topic, but...

    For the benefit of those of us in the peanut gallery, I'm going to venture
    a guess that what you are getting at in this example is that MS now has
    this thing about banning the normal functions (especially, string
    functions, like strcpy) in favor of their silly new "s_" versions of those
    functions. Is that right? Is that why the above code won't compile on MS?

    But the Bush apologists hope that you won't remember all that. And they
    also have a theory, which I've been hearing more and more - namely,
    that President Obama, though not yet in office or even elected, caused the
    2008 slump. You see, people were worried in advance about his future
    policies, and that's what caused the economy to tank. Seriously.

    (Paul Krugman - Addicted to Bush)
    Kenny McCormack, Jun 1, 2014
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  3. James Harris

    Ian Collins Guest

    Is that an advantage? Functions should be called, no cut and pasted.
    So can the subroutines.

    Extracting functionality into functions is one of the fundamentals of
    good software design.
    That should be categorised as an anti-pattern.
    Ian Collins, Jun 1, 2014
  4. James Harris

    Ian Collins Guest

    Which is why exception aren't a good fit in C.
    Ian Collins, Jun 1, 2014
  5. foo() is undefined for a bit state where b = 0. So if b = 0 there
    is a programming error in caller.
    Exactly. We've got to tighten up bit shuffling functions so that sort of
    thing can't happen.
    Cut and pasted by a person editing the source.
    They can see a leaf function, take it, and then call it from anywhere. But
    not if it calls subroutines. Then it can only be called from an environment
    in which the subroutines are available, it can only be used at all
    in an environment in which the subroutines are known.
    That's often a practical problem. Maybe it shouldn't be. The reality
    is that subroutines often become detached from the main body of a function,
    maybe because the subroutines are in a library, maybe because of
    clerical errors by someone transcribing the function but not fully
    understanding what he was required to do. That's then a headache for
    anyone trying to use that function.
    The difference seem to me IO. If you're grabbing a value from an external
    source, you can apply function to that value, but you can't define a
    function in terms of that value. At least as mathematicians understand
    the term "function". Correct me if I'm wrong.
    Malcolm McLean, Jun 1, 2014
  6. James Harris

    James Harris Guest

    A longjmp approach has that issue (but can still be written to cope with
    it). It is not a problem at all for the exception word proposal.

    James Harris, Jun 1, 2014
  7. James Harris

    BartC Guest

    Yes, I understand that's the main mechanism available in C, apparently it's
    not always so straightforward because of the need to end up with registers
    in a stable state after the jump.

    (I've never used it for myself apart from trying it out; normally I use
    inline assembler, to restore the frame and stack pointers, and jump to the
    required return point. That's another difference with this, as it is
    necessary for A to first set up the data structure necessary for it to
    Then B, C and/or D would need to cooperate, if they're the only places where
    those resources can be freed.

    Sometimes there are ways for A to control the resources that could be used
    by B, C and D (requests for memory or file handles for example), then it can
    take care of that too (similar to what happens when A is part of an OS while
    B, C etc comprise a separate application which A has to invoke. Here A can't
    rely on B etc doing the right thing).
    BartC, Jun 1, 2014
  8. It's C.
    OK, I see what you mean now, but it's not obvious. This meaning
    suggests that I can only tell if something is a bit-shuffling function
    by guessing determining if it can be written in pure, portable C.
    That's a very odd definition (to my ears).
    Ah, right. Function calls must be eliminable. I don't like that at all
    because of what it means for functional data. A change in one corner of
    a program can suddenly alter the classification of a seeming innocent
    function. (Of course, since I am not that interested in this
    classification, calling this a problem is rhetorical).

    For example, part of an expression evaluator:

    typedef double operator(double, double);
    struct exp { operator *op; struct exp *l, *r; };

    double eval(struct exp *exp)
    return exp ? exp->op(eval(exp->l), eval(exp->r)) : 0;

    Is or is not a bit-shuffler depending on data definitions and
    constructions that might be far away. It may suddenly switch from being
    one to not being one when a non bit-shuffling operator gets added at
    some point. But why would I care? I can reason about this function and
    determine what it guarantees and what is does not guarantee without
    caring about its status as bit-shuffler.
    (type there: return c; }
    But I can re-write it. I don't see why I should care that I can't avoid
    some external routine.

    Why would I want run a program the generates output on a device that
    can't do output? If it can, and it supports C, using C's stdio will
    make it work.
    Yes. C is under-specified in that sense. How does your classification
    How does banning subroutines help with a broken implementation? Whether
    I can re-write a function is not tied to what you have banned subroutine
    calls or not.
    The details are not important. I want to be able to argue about all C
    functions, so I don't care if they are bit-shufflers or not.
    I think you misread the code. It was designed in response to a specific
    statement about what a callback may do whilst still allowing the caller
    to be tested independently of the callback.
    No, the type of n is int. The two loops are quite different.
    My objection was not about the examples. You focus on variability of a
    subset of functions (bit-shufflers) with restrictions on the parameters
    (when they are functions). I want to do it as widely as possible. Your
    focus on bit-state means that higher-level idea just get lost:

    function comp(f, g) { return function (x) { return g(f(x)); }; }

    (ECMAscript). I don't know if it is a bit-shuffler or not, and I have
    no idea if it meets the bit-state rules of input and output form a
    function, but I can reason about it, and it is useful (though a trivial
    example of a more general genre). The arguments about what it does are
    based on higher-level reasoning than bit-state.
    It's too huge a topic. You can reason about IO very well (particularly
    well in Haskell) but you can't do it in terms of bit-state.
    I have no ideas, sorry. It seems to me to be not the property of the
    function, but rather of the function+arguments (or maybe
    function+possible argument set) so any name based on xxx-function seems
    No. You can't guarantee any output from either version. I call that
    "equivalent" in that they are both as useless as each other (in this
    context). I'd also say that they are both as useful as each other in
    contexts where one of them is useful.

    What's more, if I can reason about IO, I can re-write IO functions
    without keeping them identical. They simply must meet the
    specification, but that's all.
    That's just good code organisation. I can re-use code that does IO too,
    if I separate it out properly. The trick is to pick the right
    components and to abstract out the right parts of them. Your
    distinction inhibits (or at least does not help) that.
    And that's worth saying that "map" belongs in different places depending
    on what function it's passed?

    I think you get the same benefits from normal software engineering
    principles. These usually operate at a finer level of granularity than
    the bit-shuffling/IO distinction.
    Ben Bacarisse, Jun 1, 2014
  9. James Harris

    Ian Collins Guest

    Exceptions only become truly useful if callers don't have to explicitly
    check for them being thrown. You may as well just replace errno with an
    exception like object and include details of the error there.

    The cleanup handler stack approach Kaz Kylheku posted to this thread is
    the best option for C. The same technique is also used to handle
    cleaning up when threads are cancelled in pthreads, which is a similar
    situation to jumping out of a function on an exception.
    Ian Collins, Jun 2, 2014
  10. James Harris

    James Harris Guest

    I disagree about "useful". I found this immensely useful.

    Besides, using an exception word the check for an exception is no more work
    than the check for a return code.
    It is a good approach. I think you go too far in declaring it "best" though.
    It would be best in some circumstances but not all.

    James Harris, Jun 2, 2014
  11. It's more of a description than a definition. You can rewrite that
    more formally if you want, at cost of possibly losing some readers.
    It means that we can't expect to write

    void foo(void)

    Then update bar(), and have a guarantee that foo() will have updated
    in the way that we desire, as some methodologies promise. The reason
    is that that promise is hard to fulfil. Not in this trivial case, of
    No, because if op() is not a bit shuffler, you lose the strong guarantee
    from op. behaviour depends on the physical state of hardware attached
    to the machine. That might be good enough for many purposes, but not
    for all.

    Callbacks aren't eliminable, of course.
    Because printf() might be taken away on some future version of
    the hardware. Obviously you'll have to deal with this at some level,
    but it's best to isolate all the calls to printf() is sections of
    the program labelled "procedures".
    Because you may want to test and develop most of the program in a
    Unix box type environment, and only put in the hardware-specific
    calls when the hardware is actually built. You may wish to move
    the program to new hardware, making small changes in the logic,
    and to start software development before the new hardware becomes
    Because we've isolated all the calls to fgetc(). So it's easier to
    replace them with calls to internet-enabled functions which deal
    with slow connections.
    strcpy() can be taken away. It's not a disaster, if you know you can
    rewrite the function without using strcpy().
    Parallel programming is a whole different can of worms. We've plenty
    to talk about just on serial programming.
    Oh, you mean machine representation issues. Unfortunately that's
    a necessary concession. Functions can't be defined in terms of bits,
    but in terms of machine representations.
    I think you're being misled by the fact that I necessarily use
    short examples. It's to do with high-level code organisation, not
    classifying trivial functions as "bit-shufflers".
    It's a different programming paradigm. Callbacks are difficult for
    the bit-shuffling non-bit-shuffling paradigm, because it's difficult
    to say if an indirect call should count. So they need to be short
    bits of missing functionality. If you build a method on callbacks
    you're programming in a different way.
    When an argument is another function there's a question mark, do we regard
    the function as tied (it's one function) or not (it's two functions).
    Any tty can fail at any time. So printf() is useless.
    Well yes it is, as a "function". That's the whole point.
    printf("H\bHello world\n");

    is "Hello world" on the hardware I happen to have attached to my machine.
    is it always "Hello world"?
    No, it helps. It's all about good code organisation, I agree.
    No. Looks like an IO procedure which just happens to have the IO calls
    abstracted away with callbacks, so it's a bit shuffler, which is good,
    it can be tested without the IO stuff. That's the "stub function"
    method, convert an IO procedure into a function temporarily.
    So it would be written by a IO person, probably.

    Maybe it's a bit of marginal case.
    I've been accused of being too low level, now I'm accused of not
    operating at a fine enough level of granularity.
    Malcolm McLean, Jun 2, 2014
  12. By "no", do you mean that I *can't* reason about this code? if so, I
    think we'll have to agree to disagree about what that means, and about
    it's role in program development.
    I think they are in pure portable C, but it would certainly be only in
    principle. Mind you, I think that's what matters here -- that a call be
    eliminable in principle.
    They are all isolated by the fact that they are calls to a function
    called printf. I can deal with this bizarre situation by writing my own
    printf. If the specification of the program means that the output is
    optional (non-essential logging say), this might be done very easily

    Now, normal code organisation principles will mean that it's almost
    certain that all logging will done through a logging function, so there
    will be an alternative method as well.

    That IO functions are often segregated is just good design. Other, non
    IO functions will be organised and segregated, but both will be
    organised at finer grain that bit-shuffling vs. IO and, in my opinion,
    the two division won't always coincide (i.e. your two are not just
    larger aggregations of the divisions that I might choose).
    Yes, I do that kind of thing all the time. In fact the same pattern of
    work is very useful for pure logic functions too.

    What I'm missing is why this distinction of yours helps more than the
    usual process by which people develop software. It has downsides --
    some things I like are "banned", yet there's no sign that it will help
    any more than good software engineering practice has been for decades.
    The calls to fgetc are isolated already by the fact that they are calls
    to fgetc. Maybe all this comes from your using some really badly
    designed software? It's normal to isolate functional components form
    each other via well-defined interfaces, and that makes porting to new
    environments simpler, but I see no advantage of your particular rules
    over what is usually done.

    C has no proper module system, so it is easy to make a mess of this
    separation, but the solution to that is either to write in another
    language, or to structure your code better. You can make a mess of the
    pure logic bits as well as of the IO bits. Your division gives one cut
    through the software where good design needs many. Since I don't see
    that your cut is particularly useful, I would not use it. I'd start
    with proper modular design.
    I don't see that as an answer. My point was in reply to the need "to
    retain the right to ban subroutines" and so I asked how that helps. I
    can re-write function to get round broken implementations even if you
    have never had the right to ban subroutines. Case in point: you don't
    reserve the right to ban subroutines in IO functions, but you happily
    talk about re-writing them to work in IO encumbered situations.

    My point has got lost in snipping. I made it terms of a concurrent
    program, but following your objection I re-made it in more general
    terms. And now it's lost (and it was not a detail).
    Actually, no. I have already said more than I really want to, so I
    won't be continuing. Feel free to rebut whatever points remain in this
    post, but, unless I feel particularly outraged, I won't be replying.
    Eh? I just meant that you were wrong to say "this code is the same as
    that code". The two pieces of code behave differently for some values
    of the arguments. Without a specification, all I can do is conclude
    that for one bit of code to be the same as the other, they should behave
    the same in all cases.

    But the fact that your re-write did no preserve the function's behaviour
    is of no significance to your argument. I should just accept that you
    are not a fan of details, where I am detail oriented. Pointing this out
    has added nothing to the big picture, but, sadly, I obsess about details.
    There's no callbacks here. That's why I chose this example. I
    understand, but disagree with, what you said about callbacks being
    difficult. I was making a fresh point about higher-level functions. Do
    they fit into your paradigm at all? If not, fine. But if they do, how
    exactly? The talk of bit-states is alien in that sort of code.
    More to the point, do you? The use of "we" is unusual here.
    It's my whole point as well. I don't know how exactly the same facts
    can lead the two of us to make such different statements, but I think it
    reflects a fundamentally different view of what a program is.
    Unravelling that, if it is indeed the case, is beyond me right now.

    I've said my bit about IO and re-writing IO functions, but since I don't
    know what you are saying about it I am stumped. I agree with every
    low-level, factual, statement you've made about it, yet we don't seem to
    agree about the most basic higher-level remarks.

    Ben Bacarisse, Jun 2, 2014
  13. No, because we've defined a "Malcolm-function" (if you insist) as bit state on
    function exit given bit state on function entry. The address of the callback is
    part of that bit state. So the callback has to be evaluated, I don't see how it
    can't be.
    You can change the definition of a function to mean " C functions and bound
    callbacks", but that's not going to be helpful.
    No, because the hardware attached to the machine might not easily support a
    printf() type interface.
    IO and bit-shuffling is certainly not the only principle of organisation.
    It's one principle.
    It's not the only possible programming paradigm.
    The reality is that "use the stdio interface and reimplement it" method doesn't
    work very well, in practice.
    Because you've got to provide the IO subroutines, you've got to make sure they
    behave exactly as specified.
    You're putting access to /dev/random in the same source file as, say, a
    Mersenne twister. So someone wants your Mersenne twister, but the file
    breaks because /dev/random isn't available. Because he doesn't know much
    about random number generators, he's now got a difficult problem.
    Yes, if we do Io, and things change, then it's problematic. There's no way round
    that. We were happily loading in data from a file on disk, now it's on the
    internet, and it hangs. users expect to be told "this is slow" and take the decision
    themselves whether to terminate the operation or persist.
    So let's at least isolate that.
    No-one's forced to reply to me. It's an open newsgroup. I also have a policy
    of only replying where I feel I have something useful to say.
    Basically, all bits are equivalent. A set bit is a set bit, regardless of whether
    it's in RAM chip or a ferrite core store. That's not true of other devices attached
    to the computer.
    Basically we define a function in terms of bits on input versus bits on output.
    It can then perform any calculation we wish. There's nothing we can't achieve
    in logic terms with those rules.
    Then if data isn't in the right format, we write little function to put it into
    the right format. That's how we add flexibility, not be trying to pass
    functions to other functions.
    Except sometimes you have to leave out a little bit of functionality. that's
    rare, it's done reluctantly, because it introduces all the problems you've been
    It's a royal we.
    You can see it in two ways. Unles you impose some restriction on what callbacks are
    allowed to do, the function can't be tested independently of the callbacks, and it's not
    sensible to try to describe it at the "bits on entry versus bits on exit" level. But it
    is sensible to describe a bound function in those terms.
    A program is a mapping of an input state to an output state, hooked up to
    some IO devices so that user can see the results and, possibly, run several
    such mappings in sequence.
    Well yes, people disagree. That's the point of discussing things.
    If I was completely sure of my ideas, I'd just publish in a book, rather than try to
    thrash them out with other programmers.
    Malcolm McLean, Jun 2, 2014
  14. James Harris

    Ian Collins Guest

    But you still have to and can forget to check. To be truly useful
    exceptions have to be raised in such a way they can't be ignored. Then
    you can start to write clearer, uncluttered code like




    rather than

    if( doThis() != OK )
    // some error handling

    and so on.
    Ian Collins, Jun 2, 2014
  15. James Harris

    James Harris Guest

    Well, on the usefulness, whether someone thinks it is *as* useful (and neat)
    as possible or not it still *is* useful to be able to do as has been
    discussed, very much so.

    With respect to the brevity, at a minimum the proposal would have

    if (exception) return;

    after most function calls. It's not perfect but the addition could be
    shorter than normal error-checking code so it's pretty tidy.

    By contrast, what would you do to make your preference possible? It is neat
    as written but at some point you need the mechanism to make it work. I
    presume we are still talking about C. Were you thinking of setjmp/longjmp?
    Imagine that you wanted to catch exceptions at different levels. How would
    you code it? I know what I would do: use standards-compliant setjmp/longjmp
    with a stack or list of jump buffers managed by a separate library module -
    but even in a libarary that's not very nice. :-( Wouldn't that "wrapping"
    code become verbose in order to achieve the brevity of what you wrote above?

    To make an example, say you had a call stack which had on it these five

    A B C D E

    Imagine that A was the top level and caught most/all exceptions but that you
    wanted to catch some exceptions in C and that before calling E function D
    allocated resources that it had to free when the stack was unwound. Would
    you end up with something along the lines of setjmp code at A, C and D -
    that at A and C to catch exceptions and that at D to release the resources
    allocated there?

    Besides, the proposed exception word can easily work *with* setjmp/longjmp
    if required. One doesn't preclude the other because the exception word idea
    uses C code that is completely standard and doesn't do anything behind the
    scenes that could interfere with setjmp/longjmp. So it's not an either/or
    choice. The lightweight exception word can be used in some places and the
    more extensive setjmp/longjmp in others.

    James Harris, Jun 2, 2014
  16. James Harris

    Ian Collins Guest

    You really need language support. I have uses something similar to
    Kaz's library in the past as I also use that mechanism with pthreads in
    C (the platform I use has run-time support for thread cancellation in C++).
    Ian Collins, Jun 2, 2014
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