Ways to define C constants

Discussion in 'C Programming' started by James Harris, Jun 2, 2014.

  1. James Harris

    James Harris Guest

    I don't agree with either of you! I cannot see the point of redefining ui16
    to be an alias of uint16_t just to alter the name, if that's what you are
    suggesting when you say 'defined in terms of'.

    I use ui16 because it gives me independence from the compilers I use, one
    which includes definitions for stdints and one which does not.

    James
     
    James Harris, Jun 2, 2014
    #21
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  2. James Harris

    James Harris Guest

    I like the way people are going off on to their own assumptions about the
    OP's motives!

    In fact, it is nothing of the sort. (Have just posted the reasons in two
    other posts so won't repeat them here.)

    James
     
    James Harris, Jun 2, 2014
    #22
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  3. James Harris

    James Harris Guest

    To check, would you use all caps for names in enums as well as those in
    #defines? Would you use all caps for all such names whether they were in a
    header file (and thus intended to be used in other modules) or in a .c file
    (and intended to be private to the .c file)?

    James
     
    James Harris, Jun 2, 2014
    #23
  4. James Harris

    James Kuyper Guest

    On 06/02/2014 01:18 PM, BartC wrote:
    ....
    "no need" would follow from that argument, only if C code is never read
    by C experts. If it were true that this would never happen, that would
    be a rather appalling indictment of the programming industry.

    Being able to recognize uint_least16_t and uint_fast16_t strikes me as a
    pretty low standard to set for identifying C experts. They've been
    standard types for 14 years now, and were among the first of the C99
    features to be widely implemented (they were fairly widely implemented
    as extensions to C even before the standard was approved).
     
    James Kuyper, Jun 2, 2014
    #24
  5. This is the irony.
    As soon as something is standardised, you can't use it. Not everything will
    support the standard, and you can't provide it without confusing everyone.
     
    Malcolm McLean, Jun 2, 2014
    #25
  6. James Harris

    James Kuyper Guest

    Yes. If I could have implemented it using a macro expanding to an
    integer constant expression, the fact that I chose instead to make it an
    enumeration constant doesn't affect my feeling that it should be all-caps.
     
    James Kuyper, Jun 2, 2014
    #26
  7. [...]

    That will break if INT_MAX < 65535, since enumeration constants are
    always of type (signed) int.

    If you can assume at least 32-bit int (as POSIX requires, for example),
    then there's no real need to limit it to 65535.
     
    Keith Thompson, Jun 2, 2014
    #27
  8. In that situation, I'd write my own "stdint.h" header that implements as
    much of the standard <stdint.h> as possible, using the same names
    (perhaps I'd give the header a different name). I'd then use #ifdef,
    perhaps with some build-time configuration mechanism, to decide whether
    to use my own header or <stdint.h>.

    If I define my own typedef named "uint16_t", I see no great benefit in
    distinguishing between it and the typedef of the same name provided by
    <stdint.h>.

    Eventually, with any luck, all the implementations I need to worry about
    will properly implement <stdint.h>, and my own header can quietly go
    away.

    See, for example, http://www.lysator.liu.se/(nobg)/c/q8/index.html
     
    Keith Thompson, Jun 2, 2014
    #28
  9. James Harris

    James Kuyper Guest

    I was thinking that the underlying type could be "unsigned int", so
    65535 would be acceptable, but you're right, that doesn't work. I doubt
    that I've ever defined an enumeration constant with a value greater than
    16384, so the issue has never actually come up.
     
    James Kuyper, Jun 2, 2014
    #29
  10. James Harris

    James Harris Guest

    That reminds me about a comment by Richard H in C Unleashed.... goes to
    check .... these names being prohibited as externals:

    strength
    memorandum
    isotope
    tolerance

    along with names such as E4, EX, town and isthmus. All reserved because of
    their prefixes.

    James
     
    James Harris, Jun 2, 2014
    #30
  11. I disagree, at least in this case: you *can* provide it yourself without
    confusing everyone. If `uint16_t` meets the standard's requirements, it
    doesn't matter much whether it's provided by <stdint.h> or by
    "my_stdint.h".

    "my_stdint.h" can even be implemented by a simple "#include <stdint.h>"
    when it's available.
     
    Keith Thompson, Jun 2, 2014
    #31
  12. Yes, the underlying type could be unsigned int, but that doesn't
    help. The enumeration type (which, if you're just using it to define
    constants, is likely to be anonymous and never used) is compatible
    with some implementation-defined integer type, but the enumeration
    constants themselves are always of type int. (Yes, that's ugly.)
     
    Keith Thompson, Jun 2, 2014
    #32
  13. James Harris

    James Harris Guest

    We discussed this here some time ago when I was thinking to set up what
    became a types.h that would provide common definitions and work for 16-, 32-
    and 64-bit compilers. Some people suggested using the standard names as you
    do above. I can see merit in that but my reasons for using my own names are
    unchanged, i.e.:

    The source is clearly using custom definitions. No chance of maintenance
    programmer assuming that code has access to stdint defs which might differ.
    Isolation from bugs in one definition or the other. Control, especially as
    one compiler has been found to be a little imperfect. And stability of
    definitions over compiler upgrades. The silent migration from self-defined
    to compiler-defined that you suggest makes me especially uneasy. I would
    rather know what was going on even if it looks a little awkward - which in
    this case I don't think it does, as it happens. Call it control freakery if
    you like but I would rather see what's going on than cover it over. :)

    OK, I know that paragraph is a little overstated(!) but it does explain a
    bit where I am coming from.

    James
     
    James Harris, Jun 2, 2014
    #33
  14. James Harris

    David Brown Guest

    That makes no sense at all. I have used plenty of pre-C99 compilers -
    and since some of the embedded software I write occasionally needs
    maintenance, I sometimes still need to run them. Most of these pre-C99
    compilers had a <stdint.h> defining at least the basic uintXX_t and
    intXX_t types (some skipped the "least" or "fast" types). If I have to
    write code that is portable across such ancient tools that they don't
    have <stdint.h>, then I make my own headers (as Keith and others
    suggest) that define these standard types for that particular compiler.

    Then the source is clearly using standard definitions (even if I wrote
    them myself) and standard types, that everyone can understand. There is
    no chance of a maintenance programmer getting it wrong, unlike if I have
    my own special types.

    It's your choice, of course, but making up your own types to replace
    standard ones is seldom a good idea.
     
    David Brown, Jun 2, 2014
    #34
  15. James Harris

    Richard Bos Guest

    And note the crucial point here: if you need portability across
    platforms which may or may not have <stdint.h>, you _can_ legally and
    safely #include "my_stdint.h", with quote marks instead of angle
    brackets. In fact, if this hasn't changed in C11[*], you can even
    #include "stdint.h", and if you haven't provided a "stdint.h" elsewhere,
    it will reliably fall back on the implementation's <stdint.h>.

    In other words: use
    #include "stdint.h"
    in your program.
    - If the implementation provides a <stdint.h>, you shouldn't provide
    one, and the implementation will use its own.
    - If the implementation doesn't provide <stdint.h>, you can provide one
    with the necessary #definitions, and the implementation will use that.
    - The same code will compile correctly in either case.

    Richard

    [*] Which I don't have - is there a link anywhere?
     
    Richard Bos, Jun 2, 2014
    #35
  16. James Harris

    Richard Bos Guest

    If they're stdint definitions, they may _not_ differ. Any implementation
    which supplies <stdint.h> _must_ use the Standard definitions; any
    programmer who supplies his own "stdint.h" really, really ought to, and
    if he doesn't, smack him.

    By contrast, if they are "Harrisesintegertypes.h" definitions, goodness
    only knows what they mean.
    I call it control over-freakery. You're trying to control something
    which _is already controlled_, and replacing it with something which
    isn't.

    Richard
     
    Richard Bos, Jun 2, 2014
    #36
  17. No, it hasn't changed in C11.

    The latest pre-C11 draft (which is very close to the published standard)
    is N1570:

    http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
     
    Keith Thompson, Jun 3, 2014
    #37
  18. James Harris

    Ian Collins Guest

    It is if you care about clarity.
    Using #define in this context is a bad idea (as it usually is). Use a
    const! Macros don't respect scope.
     
    Ian Collins, Jun 3, 2014
    #38
  19. Were's setting it, once, in main().

    setupIOprocedures is passed the address of the port to use for "Port a". It then
    sets one or more variables to 0x74, which get passed down to subroutines.

    So we can use a #define,a const int, or a hardcoded literal in main(), but
    the value 0x74 appears in one place in the binary, the #define/ const int
    is used in one place in main(). You can argue that a definition is a bit
    clearer than a literal, but it's a very small point.
     
    Malcolm McLean, Jun 3, 2014
    #39
  20. James Harris

    BartC Guest

    What *is* the address or number of port A in your example, or doesn't it
    mention it? Because your code is very confusing (the OP was talking about a
    constant defining a port number).
     
    BartC, Jun 3, 2014
    #40
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